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UNITED STATES DEPARTMENT OF AGRICULTURE
BULLETIN No. 173

Contribution from the Bureau of Entomology
L. O. HOWARD, Chief

Washington, D. C. PROFESSIONAL PAPER April 13, 1915

THE LIFE HISTORY AND HABITS OF
THE PEAR THRIPS IN
CALIFORNIA

By

S. W. FOSTER and P. R. JONES, Entomological Assistants
Deciduous Fruit Insect Investigations

CONTENTS

Systematic Position
Anatomy
Economic Importance Life History and Habits
Character of Injury Natural Enemies
Description

WASHINGTON
GOVERNMENT PRINTING OFFICE
1915

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Contribution from the Bureau of Entomology, L. O, Howard, Chief.
April 13, 1915.

(PROFESSIONAL PAPER.)

THE LIFE HISTORY AND HABITS OF THE PEAR
THRIPS IN CALIFORNIA.

By S. W. Foster! and P. R. Jones,? Entomological Assistants, Deciduous Fruit
Insect Investigations.

INTRODUCTION.’

The so-called pear thrips, (Huthrips) Teniothrips pyri Daniel, first
attracted attention durmg the spring of 1902 in a prune orchard
near San Jose, Cal. Its injuries rapidly increased in the Santa
Clara Valley, and the insect spread to other orchard sections in the
San Francisco Bay region. Its increasing destructiveness and
spread led to the establishment by the Bureau of Entomology of a
laboratory in the Santa Clara Valley to determine the life history
and habits of the pest and to determine, if possible, measures for its
control in orchards. ‘The laboratory thus started during the summer
of 1907 was continued to the fall of 1912.

Mr. Dudley Moulton, an agent of this bureau, who, as Santa Clara
County entomologist, had previously had experience with the insect,
was placed in immediate charge of the work, in which position he
continued until September, 1909. During his period of service
Mr. Moulton was assisted in the Santa Clara Valley at one time or
another by Messrs. C. T. Paine, S. W. Foster, and P. R. Jones.

In the fall of 1908 owing to the rapid dissemination of the pear
thrips to the northward an additional laboratory was established in
Contra Costa County, with headquarters at Walnut Creek. This
work was placed under the immediate direction of Mr. S. W. Foster,
who also had charge of operations in the infested counties to the north.
During the spraying season of 1909 Mr. Fred Johnson collaborated
with Mr. Foster in experimental and demonstration spraying in

1 Resigned Oct. 10, 1912.

2 Resigned Sept. 30, 1912.

3 By A. L. Quaintance, In Charge of Deciduous Fruit Insect Investigations.
73390°—Bull. 173—15——1

2 BULLETIN 173, U. S. DEPARTMENT OF AGRICULTURE.

orchards, and in July of the same year Mr. E. J. Hoddy was assigned
to the Walnut Creek laboratory and assisted in certain cultivation
experiments at Suisun in the fall of 1909, and with Mr. R. W. Braucher
assisted in the demonstration spraying operations at Suisun and
Courtland during the spring of 1910. During the spraying season
of 1911 Mr. Foster was assisted by Messrs. E. L. Jenne and R. L.
Nougaret.

Upon the resignation of Mr. Dudley Moulton Mr. P. R. Jones was
placed in charge of operations in the Santa Clara Valley and was
assisted during the spraying season of 1910 by Mr. E. L. Jenne and
during the spraying season of 1911 by Messrs. A. G. Hammar and
W. M. Davidson.

During the spraying season of 1912, owing to the absence from
California of Mr. Foster, Mr. Jones was charged with all of the pear-
thrips operations in California and was assisted in the work by Messrs.
W. M. Davidson and L. L. Scott, located at Courtland, by Mr. R. L.
Nougaret at Suisun, and by Mr. E. L. Jenne at Walnut Creek.

The manuscript for the present report has been prepared as fol-
lows: All of the data relating to Contra Costa County and counties
to the northward have been prepared by Mr. Foster, the senior author.
Report of operations in the Santa Clara Valley, as well as much of
the life-history matter, has been prepared by Mr. Jones. The re-
maining chapters were written jointly by Messrs. Foster and Jones.

Especial acknowledgment is due to the supervisors of Contra Costa
County and Santa Clara County for their assistance in furnishing
facilities for work during the season of 1909, and for supplementing
the bureau’s funds before the special appropriation from Congress
was available. The bureau desires also to acknowledge its obliga-
tions to many orchardists in the thrips-infested territory, who placed
at the disposal of the Department of Agriculture their orchards and
facilities for experimental and demonstration purposes. The suc-
cess which many orchardists have obtained in the control of the
pear thrips by the adoption of the recommendations of the bureau,
as well as the large-scale spraying demonstrations which the bureau
has conducted, has fully demonstrated the effectiveness and practi-
cability of the methods recommended. Especial acknowledgment
is made also to Mr. W.S. Ballard, of the Bureau of Plant Industry of
the United States Department of Agriculture, for much valuable
assistance and numerous courtesies rendered during the course of
the work at Suisun.

The present paper deals with the life history and habits of the
pear thrips, the results of experiments and demonstrations with
sprays and other remedial operations having been given in Circular
No. 131 of the Bureau of Entomology.

THE PEAR THRIPS IN CALIFORNIA. 8

HISTORY,

LITERATURE.

The first reference in literature to the pear thrips is the original
description of the insect by Miss M. Daniels in Entomological News
for November, 1904.1. The type specimens were taken on pear near
San Leandro, in Alameda County, Cal., for which reason it was given
the common name ‘‘pear thrips.”

Dudley Moulton,? in 1905, published the first account dealing with
the economic importance of this species. He described its different
stages and the nature and extent of injury caused by it, and included a
discussion of its life history. No advice was given as to remedial
measures, except that early winter plowing was advocated.

The third reference to the pear thrips in literature was by the same
author in Bulletin 68, Part I, of the Bureau of Entomology.*? This
contained practically all that was included in the former publication,
with additional information accumulated, making a more complete
account of the pest. It was illustrated with appropriate figures of all
stages, including the eggs and pupa, which had not theretofore been
figured. No successful remedial measures, however, had been de-
termined.

The next publication was also by Moulton, and was issued as Bul-
letin 80, Part IV, of this bureau.t It gave an extended account of
the life history of the pear thrips, with recommendations for early
fall plowing and cross-plowing, to be followed by spraying in the
spring for the adult and an application against the larve after the
falling of the petals. Tables were given showing the actual number
of thrips killed in the plowed as compared with the unplowed areas.

The next account was published as Circular 131 of the Bureau of
Entomology,* and is a concise abstract of the present paper.

The Journal of the South-Eastern Agricuitural College, Wye,
Kent County, England, No. 19, for 1910 (published in 1911), con-
tains an article by F. V. Theobald® dealing with thrips in general, in
which this species receives considerable prominence.

1 Daniel, S. M. New California Thysanoptera. ‘Jn Entomological News, v. 15, no. 9, p. 294-295, No-
vember, 1904.

2 Moulton, Dudley. The Pear Thrips (£uthrips pyri). California State Horticultural Commission,
Publication, Sacramento, 1905. 17 p., 8 figs.

8’ Moulton, Dudley. The Pear Thrips. (£uthrips pyri Daniel.) U.S. Dept. Agr., Bur. Ent., Bul. 68,
pt. 1, 16 p., 8 figs., 2 pls., June 10, 1907.

4 Moulton, Dudley. The Pear Thrips and its Control. (uthrips pyri Daniel.) U.S. Dept. Agr., Bur.
Ent., Bul. 80, pt. 4, p. 51-66, figs. 13-17, pls. 4-6, Sept. 4, 1909.

5 Foster, S. W., and Jones, P. R. How to Control the Pear Thrips. U.S. Dept. Agr., Bur. Ent., Cire.
131, 24 p., 14 figs., Jan. 9, 1911.

§ Theobald, Fred. V. Report on economic zoology for year ending Sept. 31, 1910, p. 57-67, fig. 5,
Pls. XXV-XXVIII. In Jour. Southeast. Agr. Col., Wye, no. 19, 1911.

4 BULLETIN 173, U. S. DEPARTMENT OF AGRICULTURE.

Also in 1911 Mr. P. J. Parrott * published an account of the appear-—
ance of this species in New York State, and m January, 1912, he
issued a more extended account of the pear thrips in New York.?

HISTORY IN ORCHARDS AND DISTRIBUTION.

The first reported mjury caused by the pear thrips was noticed in-
the year 1902, in an orchard owned by Judge 8. F. Leib and Mr. G. M.
Bowman. This orchard was situated in the Berryessa district of
the Santa Clara Valley, near San Jose, and consisted chiefly of the
Imperial variety of prunes. The injury was noticed at first on about
20 or 30 acres of the 200 acres of orchard, and the cause of the trouble
at that time was unknown. In the spring of 1904 every other row
of this orchard was top-worked with sugar prunes, chiefly to secure
better cross-pollination with the Inperial variety of prunes, the lack
of which was supposed to have been the cause of failure of the crops
in the past. During a drive through 100 acres of this orchard the
fruit buds were observed to be just beginning to show the white tips
of the petals, and the prospects seemed excellent for a good crop.
When revisiting the place five days later, the owner found to his
utter astonishment that the whole orchard had the appearance of
having been scorched with fire and that there was not an average of
a dozen blossoms to the tree.

The thrips were discovered this same year (1904) in the orchard
of Mr. R. K. Thomas, on Cypress Avenue, near Stevens Creek Road,
about 7 miles distant in an air line from the Leib orchard. From
these two orchards infestation has, with the exception of a few acres,
spread all over the Santa Clara Valley and into other valleys sur-
rounding the San Francisco Bay.

No exact information is available as to the first appearance of the
thrips in other counties, but many orchardists claim that it has been
in Contra Costa County since 1904 and in Solano County at least
since 1906. In addition to these centers of infestation in Santa
Clara, Contra Costa, and Solano Counties, the insect is now present
in considerable numbers in Alameda, Sacramento, Yolo, Napa,
Sonoma, San Joaquin, and San Benito Counties. The general area
of infestation in California is indicated in the accompanying map
Ghiomeals)

There have been several reported outbreaks of this species in other
parts of the State, notably from the Sierra Nevada foothills near.
Newcastle and Auburn, near Red Bluff and Anderson in the Sacra-
mento Valley, and from the fruit districts of Tulare and Fresno
Counties in the San Joaquin Valley. The species in question, how-

‘Parrott, P. J. Occurrence of Euthrips pyri Daniel in New York State. Jn Science, n. s., v. 34, no. 864,
p. 94, July 21, 1911.

2 Parrott, P. J. The Pear Thrips. N. Y. Agr. Exp. Sta., Geneva, N. Y., Bul. 343, p. 341-366, 4 figs.,
pls. 30-33 and 1 col. pl., Jan., 1912.

THE PEAR THRIPS IN CALIFORNIA. 5

ever, were found to be (Huthrips) Frankliniella occidentalis Pergande
and (Huthra Ds) Frankliniella tritict Fitch, neither of which is particu-
larly ‘jurious to deciduous fruits. Reports of mjury supposed to
have been caused by this species were received from the Rogue River
Valley im Oregon, but a critical examination, in 1909, showed no
signs of the work of the pear thrips. In the spring of 1910 many
larve of (Huthrips) Frankliniella tritici were
found, but none of the species under consid-
eration could be obtained.

Not until the year 1911 was the pear thrips
positively known to be present in the United
States outside of the infested districts of Cali-
fornia. However, in the spring of 1911 Mr.
P. J. Parrott found it in considerable numbers
around Germantown and other points
along the Hudson River in New York.
Later in the year specimens of
(Huthrips) Tenrothrips pyri
were found among some Thy-
sanoptera which had been col-
lected in the spring
by Mr. Parrott in the
vicinity of Geneva,
NY

In May, 1912, Mr.
A. LL. Quaintance sent
the authors a number
of specimens of thrips
collected in pear blos-
Fic. 1—Map showing general area of infestation by the pear thrips gogms from six differ-

in California. (Authors’ illustration. ) aa erohards by Mr.
Fred Johnson at North Fast, Pa. All proved to be the pear thrips,
(Euthrips) Teniothrips pyri.

In 1909 Bagnall? reported that numerous examples of this very
injurious species, taken in plum blossoms at Evesham, England, had
been sent to him by Mr. Walter Collinge. So far as we know, this
and the previously mentioned account by Theobald are the only
published reports of the occurrence of this species outside of the
United States.

Two other species of Thysanoptera (Thrips physapus L. and T. flava
Schrank) are mentioned by Carpenter as the ‘“‘pear-blossom thrips”’

AN FRANCISCO-f)

SAN worl

SAN BERNARDINO

1 Parrott, P. J. Occurrence of Euthrips pyri Daniel in New York State Jn Science,n s., v. 34, no. 864,
p. 94, July 21; 1911.

2 Bagnall, Richard S. A contribution to our knowledge of the British Thysanoptera (Terebrantia), with
notes on injurious species. Jn Jour. Econ. Biol., v. 4, no. 2, p. 33-41, July 7, 1909.

6 BULLETIN 173, U. S. DEPARTMENT OF AGRICULTURE.

me his report before the Royal Dublin Society for 1900,1 and as the

“year thrips” in his report to the same society for 1901.2. In the
report for 1900 he states that these two species were found feeding
m unopened pear blossoms near Dublin, and he attributes the failure
of the fruit that season to the work of these insects. The report. for
1901 states that a Dr. Barton tried a dressing of kainit around the
trees, with very satisfactory results.

In December, 1914, Mr. W. M. Scott* reported the occurrence of
the pear thrips in a Kieffer pear orchard near Baltimore, Md. The
insect was so abundant as completely to destroy the crop of fruit.

THEORIES AS TO ORIGINAL HOME.

Various ideas have been advanced as to the original home of the
pear thrips. Dr. Pietro Buffa, a well-known student of Thysanoptera,
in private correspondence under date of April 17, 1909, suggested that
while it is a good species it should be put only in the genus Physopus,
and expressed the belief that it was not a European species. Prof.
Silvestri suggested that it was introduced from China or was of other
oriental habitat. Several leading fruit growers have expressed the
belief that the insect was introduced into this country from France
or England, giving as the reason its apparent partiality to prunes,
which are varieties of European plums.

The occurrence of the pear thrips in England lends some weight to
the theory that it is of European origin. It may be that natural
conditions hold it in check in England and that its advent into Cali-
fornia under conditions more suitable for its rapid increase explains
its presence there in such enormous numbers. Now, however, that
its presence is definitely established in the eastern United States, it
is probable that the insect had been in this country for years before
it was discovered.

It may be possible that the pear thrips is native to the Santa Cruz
Mountains, with some wild rosaceous plant as its original food plant.
Upon this supposition it is probable that it has been present in the
Santa Clara Valley for many years, and that it first became notori-
ously destructive with the advent of favorable conditions. While
this species has been taken upon a great variety of plants and has
been found to be able to subsist on many of them, it is distinctly an
enemy of deciduous fruits, to which it shows a decided preference.

COMMON NAMES.

Many common names have been assigned to this insect, as ‘pear

thrips,’ ‘prune thrips,” ‘“‘cherry thrips,” etc. The first mentioned,

1 Carpenter,G.H. Report on economic entomology for the year 1900, p. 96-97. Reprinted from the
Report of the Council of the Royal Dublin Society for 1900.

2 Carpenter, G.H. Injurious insects observed in Ireland during the year 1901, p. 153-154. In The Econ-
omic Proceedings of the Royal Dublin Society, v. 1, pt. 3, no. 5, July, 1902.

3 Jour. Econ. Ent., v. 7, No. 6, p. 478-479, Dec., 1914.

THE PEAR THRIPS IN CALIFORNIA. 7

namely, ‘pear thrips,’ has been more extensively used, following
the original designation of the insect, because the species was first
described from specimens taken upon pear trees. The word “thrips”’
is a general term for the species of the order Thysanoptera and is
sometimes erroneously applied to certain other insects, as the grape »
leafhopper (T'yphlocyba comes Say). The word “thrips” is both
singular and plural.
ECONOMIC IMPORTANCE.

DESTRUCTIVENESS.

This minute insect, which until 1904 was unknown to science, is at
present one of the most important insect pests with which the growers
of deciduous fruits in the San Francisco Bay region and adjoiming
counties have to contend. The rapidity with which the insect
spreads, its suddenness of attack and complete blasting in a few days
of all prospects for a crop of fruit, and the difficulty experienced in its
control, combine to make its subjugation a matter of considerable
difficulty. Moreover, as the insect is each year developing an ability
to subsist on other and new food plants, its capabilities for dissemi-
nation become correspondingly increased. There is no reason to
believe that the thrips will disappear in a few years, and it should be at
once realized that only the most careful attention each year to neces-
sary control measures will make it possible to continue the profitable
culture of fruit in regions where this insect is present in any con-
siderable numbers.

In the Santa Clara Valley this insect has been worse some years
than others, notably in 1905, 1907, 1908, 1909, and 1910, but it is
safe to say that from now on the maximum prune crop possible for
this valley will never again be reached unless every orchardist does
the utmost in his power to control the thrips. While it may be pos-
sible for unfavorable weather conditions to reduce the possibility of
a good crop of 100,000,000 pounds of dried prunes for this valley to
something like 40,000,000 or 50,000,000 pounds, the thrips, in a
great measure, has been responsible for the small crops since 1907,
and will continue to be so, first, by killing the fruit buds before they
bloom; secondly, by depositing the eggs in the fruit stems, and,
thirdly, by the feeding of the larve on the fruit, causing it either to
drop prematurely or to develop misshapen and scarred on the trees.
While the thrips is doing much serious work in the Santa Clara
Valley to cherries and pears and the damage done to different varie-
ties of peaches is increasing, yet on account of the small acreage of
these fruits the chief loss from a commercial standpoint is to the
prune industry. Some idea of the destruction caused by the pear
thrips during the previously mentioned bad years may be gained
from the following figures, giving the approximate yield of prunes in
pounds each year for the years 1900 to 1912, inclusive.

8 BULLETIN 173, U. S:. DEPARTMENT OF AGRICULTURE.

TABLE I.— Yield of prunes for the Santa Clara. Valley, 1900-1912, inclusive.

l
| =< - — a
=% Yield of dried . Yield of dried
Season. | fruit. | Season. ait

| | Pounds. Pounds. S

| 31900 ences 120, 000, 000 GQ (reer ee 50, 000, 000

[i 1QOMES Sst 5 es: 35/000; 0002 ||:19082-4 5.595 hee ae / 40,000, 000
1902S eT ak te cee 12050005000 1)" 219C9E. 2 ee eee | 85, 000, 000
L9Q03 So here ES 90, 000, 000 1910 35, 000, 000
190425 ces A eese|(. OOS O00 OOD Il my ge? gor as oege N 40, 000, 000
TODD are See 90500050000 Wie TOU ee eee | 100, 000, 000

PL OOGS “hae Aes 120,000; 0005) |pp19122 =e ee | 65, 000, 000

| | | |

1 Severe frost.

In 1911 the pear thrips probably caused a heavy loss in spite of
the fact that there were not more than one-half as many thrips
present in this valley as in 1910. The good prune crop in the Santa
Clara Valley in 1911 was due to hght thrips injury and the very
heavy rainfall. The amount of rainfall, which was about 8 inches
more than the normal, not only placed the trees in excellent shape
to bear a heavy crop, but, coupled with other climatic conditions
during the early part of 1911 and latter part of 1910, lessened the
work of the thrips very materiaily. Notwithstanding a favorable
fruit year from a weather point of view, thrips in some places caused
a great amount of damage. The thrips damage in the Santa Clara
Valley for 1910 was caused principally by the adults, with very
little larval work, while for 1911 it was just the reverse, the adults
doing comparatively little injury because of less numbers and strong
fruit buds as a result of the heavy winter rains. The scarcity of
adult thrips in 1911 may have been due to several causes. Two
heavy rains during the early part of April of the previous year
knocked off many young larve before they were sufficiently mature
for transformation. In addition the season for pupating, June to
December, 1910, was abnormally dry, showing a deficiency in rain-
fall of 5.28 inches, while the emergence period in the spring of 1911
was unusually wet and cold. All of these conditions caused a higher
mortality than would be the case under normal conditions. However,
im orchards which showed comparatively few adults the larve were
sufficiently abundant to riddle the foliage and cause much of the
young fruit to drop. The heavy rains during the emergence period
also checked to some extent the work of the adults.

In estimating the economic loss to the fruit industry of California _

eaused by the pear thrips it is necessary to begin with the year 1904,
when it was first known that the insect was doing commercial damage,
and continue down to the present time. An attempt will be made
fo give a fair estimate of the amount of damage done yearly to the
prune industry alone in the Santa Clara Valley for the years 1904
to 1911, inclusive.

€,)

THE PEAR THRIPS IN CALIFORNIA. 9

The average size of prunes grown in the Santa Clara Valley is
60-70; that is, dried prunes requiring from 60 to 70 to make a pound.
The price paid for prunes during the years from 1904 to 1911, inelu-
sive, Was variable, but would average close to a 3-cent basis;
that is, 3 cents per pound for dried prunes running 80 prunes to the
pound. In order to be conservative, the average size, 60-70, is
disregarded, and the loss is figured on the regular 80-to-the-pound
basis. In 1904 the loss was estimated at 500 tons, or 1,000,000
pounds (dried prunes), which, at 3 cents per pound, amounts to
$30,000. For the year 1905 it was placed at 10,000,000 pounds and
the damage at $300,000; in 1906 at 5,000,000 pounds, worth $150,000;
in 1907, 15,000,000 pounds, worth $450,000; in 1908, 20,000,000
pounds, worth $600,000; in 1909, 30,000,000 pounds, worth $900,000;
in 1910, 40,000,000 pounds, worth $1,200,000; and in 1911, 20,000,000
pounds, worth $600,000. The total of all of these years would be
141,000,000 pounds, valued at $4,230,000... The estimates for some
years probably have been close to the actual damage done, but more
frequently the loss has undoubtedly been underestimated. In 1904
all the fruit of one orchard, comprising 100 acres of Imperial prunes,
was totally destroyed, and this alone at an average crop of 5 tons of
green prunes per acre, on a 3-cent basis for dried prunes, would have
been valued at close to $30,000, because of the large size of this
variety of prune, only from 30 to 40 of which make a pound.

In estimating this loss no account is taken of the great deprecia-
tion in value of the crop caused by scabbing. The entire yield each
year has been counted as merchantable fruit, and estimates of damage
made solely from orchards showing total loss or a marked reduction
in tonnage produced.

To explain more fully the commercial! quotation of a 3-cent basis,
it is meant that 3 cents per pound will be paid for dried prunes
averaging 80 prunes to the pound. For prunes which are larger
and free from scab or defects the price is usually $1 per ton more for
each point in size, and for smaller prunes the price decreases corres-
pondingly. |

As to the extent of the damage the pear thrips will cause in this
county if left unchecked, it is difficult to estimate, but the fact that
thrips were twice as numerous in 1910 as in 1909 shows their ability
to double the damage performed in any preceding year. The cause
for the notably light prune crop in 1910 is not attributed altogether
to the work of the pear thrips, but partly to unfavorable weather
conditions, which pervented many of the blossoms from setting fruit.
However, all the large producing prune districts of the Santa Clara
Valley were very seriously injured by the pear thrips, and hundreds

. 1 These estimates are based on fuller and more complete reports than could be obtained in time for
Circular 131 of the Bureau of Entomology, and these figures more nearly represent the actual loss.

73390°—Bull. 173—15——2

10 BULLETIN 173, U. S. DEPARTMENT OF AGRICULTURE.

of acres in these districts were prevented from blooming—a fact not
attributable to unfavorable weather conditions but solely to ravages
of the thrips. Other orchards, under same weather conditions but
with little or no thrips injury, produced a full crop of blossoms.

During the year 1911 another type of mjury that was different —

from previous years, which may be called cumulative jury, was
noticeable in many orchards. Barring the three heavy frosts in
April, the blooming and fruiting season in 1911 was exceedingly
favorable in so far as climatic conditions were concerned. Never-
theless about the Ist of May the trees in many orchards turned a
sickly yellow owing to the work of the thrips m 1911 and from devas-
tations by this insect in previous years. Some orchards which were
out of the frost belt and which were not severely injured by thrips in
1911 showed this condition noticeably. It is possib!e that much of
this was due to neglect of the orchards by fruit growers who did not
obtain crops of fruit during the preceding four years because of the
injury of the thrips to the buds, blossoms, and young fruit.

As mentioned before, practically all of the Santa Clara Valley
came into full bloom in 1911 and gave promise of a record crop, but
larval injury was very heavy over the entire valley. This, with the
result of injury in previous years, apparently greatly weakened the
trees and caused much of the fruit to fall at the first unfavorable
weather. —

Injury to pears in the Santa Clara Valley has never risen to great
proportions from a financial pomt of view, for the reason that most
of the acreage of this kind of fruit is set out near Santa Clara and
Alviso, sections of this valley where the thrips has not yet become
dangerously numerous. However, during the season of 1911 a num-
ber of orchards in these localities became badly infested. The amount
of damage done to cherries in this valley has not been determined
on account of the scattered acreage planted to cherries in the infested
area.

The distinctly severe years for thrips injury in Contra Costa County

in pear orchards were 1908 and 1910, when the crops were practically

annihilated. Also there was great loss the two previous years,
1906 and 1907. The prune orchards suffered in these years and in
the year 1909, producing less than one-third of a normal crop any
one year. The fruit crop has been seriously menaced each year

since 1905, the area increasing yearly, and in 1911 it aggregated a -

total loss to the county of between $1,000,000 and $1,250,000.
Solano County has in some ways been more fortunate, as the thrips

has been known to cause serious injury only since 1907, but even -

in that time the thrips has spread rapidly and caused great damage
on large areas; the damage in 1911 was very extensive and the total

‘

THE PEAR THRIPS IN CALIFORNIA. au

loss to the county attributable to the work of the pear thrips amounted
to at least $750,000.

The damage in Sacramento County was noticeable only in a com-
paratively limited area in 1909, mcreasing considerably both in area
and destructiveness during 1910 and 1911, and the total loss to that
county probably amounted to at least $250,000.

No accurate figures are available for the dames caused in Alameda
County, but a considerable area has been intested for several years
and many conservative estimates put the total loss to, but not melud-
ing, 1912, as more than $150,000.

The pear thrips has more recently been found in slightly injurious
numbers in Yolo and Napa Counties, in the eastern part of Sonoma
County, in the northwestern part of San Joaquin County, and in
some parts of San Benito County.

Including the infested areas in Santa Clara, Contra Costa, Solano,
Sacramento, Alameda, Yolo, Napa, and Sonoma Counties, it is safe
to say that the thrips, in absence of treatment, would cause an average
yearly loss of over $2,000,000. With each additional year an addi-
tional loss of several hundred thousand dollars, due to the increase of
the area infested and the increased losses in the areas previously
infested, is to be expected. The total damage to the fruit industry
of the State of California since the first appearance of the insect
ageregates, it is believed, at least $6,630,000 up to but not including
1912.

FOOD PLANTS.

While the pear thrips is distinctly a deciduous-fruit insect and
practically all of its damage is confined to this class of plants, it has
been found upon a great variety of plants the list of which is increas-
ing each year. The fact of its wide range of food plants makes
extermination practically impossible, whereas control can be readily
practiced. It has been taken upon the following plants and could
probably subsist upon a number of them long enough to make it a
constant menance to the fruit mdustry of California: Apricots,
apples, almonds, cherries, figs, grapes, pears, plums, prunes, walnuts,
madrofia (Arbutus menziesi), wild California lilac (Ceanothus thyrsi-
florus), poison oak (Rhus diversiloba), dogwood (Cornus sp.), acacia,
willow (Salix sp.), laurel ( Umbellularia califormea), mustard peas
sica nigra), live oak (Quercus wislizent), miner’s lettuce (Montia

perfoliata), and various grasses and weeds.

_ CHARACTER OF INJURY.
MANNER OF FEEDING AND TYPE OF MOUTHPARTS.

Injury to plants by the pear thrips is caused directly by the feeding
of the adults and larve upon the various portions of the fruit, buds,
flowers, and leaves, and also by the deposition of eggs in the leaf
Bert aces: fruit ee and newly formed fruit.

12 BULLETIN 173, U. S. DEPARTMENT OF AGRICULTURE.

The mouthparts of the Thysanoptera present many difficulties for
study and are not thoroughly understood. They are so modified
that various writers have disagreed regarding their homologies.
They appear, however, to belong chiefly to the suctorial type, and
they show many traces of a transition from the mandibulate type to
the suctorial. (See Pl. I, fig. 7.) Viewed as a whole, the mouthparts
appear as a broad and jointed cone attached to the posterior edge of
the underside of the head and resting for a large part under the
pronotum. The apex of the cone is quite sharp, but not so slender
and drawn out as in the Hemiptera. The mouthparts as a whole
are strikingly unsymmetrical. The most evident marks of this are
the forms of the labrum and the left mandible. The first, which
makes the front wall of the cone, is unsymmetrical in the whole
order, but especially so in the Terebrantia. It is irregularly triangular
in form and is attached by its broad base to the clypeus. It becomes
narrower as 1t approaches the tip and is usually rounded in the
Terebrantia but more variable in the Tubulifera, where it is pointed
in some species and broadly rounded in others. The maxille are
broad and flat and constitute the side walls of the mouth cone. They
also taper toward their tips. The labium forms a hind wall of the
mouth cone and is usually considerably broader at the tip than at
the other parts. Within this hollow cone he the piercing organs,
which are three nm number. First, there is a single large mandible
lying on the left side of the mouth cavity, whereas the right side has
no corresponding member.’ The other two organs are the maxillary
lobes. These are more slender and longer than the mavdibles and are
developed alike on each side. All of the mouthparts are strongly
chitinized at the tip, bemg more so in the adults than in the .a 7
although the mouthparts of the latter are otherwise closely similar
to the former.

The members of this order are thought to use the mandibles for
piercing the exterior portion of the plants, while the maxillary lobes,
which are longer, are used to penetrate deeper into the tissues, and
are moved with a rasping motion, causing the juices of the plant to
flow, so that they may be sucked up into the alimentary canal. In
feeding, as observed by aid of a hand lens, both adults and larve
exhibit an up-and-down motion of the head combined with a forward
motion which might be properly termed rooting. Most of the species

under the writers’ observation prefer to enlarge a wound into the -

plant tissues where the juices flow more readily rather than to select
new areas for feeding. This continual macerating of the fruit by
the pear thrips for a period of several days causes on deciduous
fruits what is known as the characteristic pear-thrips scab, which

1The mandible in the Tubulifera is shorter and more bent than in the Terebrantia.

Bul. 173, U. S. Dept. of Agriculture.

<A
RRR

as > LLL seta se moe
ox KC WN
Vea ~~ A MAY OX QO
Mr AY AMS

QQy MQ uy \\

P> sa

THE PEAR THRIPS (TAENIOTHRIPS PYRI DANIEL).

Fic. 1.—Adult. Fie.2.—Eggs. Fic. 3.—First-stage larva. Fie, 4.—Full-grown larva. Fic. 5.—
Pupa, first stage. Fic. 6.—Pupa, last stage. Fic.7.—Side view of head showing mouth parts,
All greatly enlarged. (Original.)

Bul. 173, U. S. Dept. of Agriculture. PLATE Il.

Fig. 1.—MATURE PEAR SHOWING INJURY RESULTING FROM FEEDING OF LARVA OF THE
PEAR THRIPS. (ORIGINAL.)

FiG. 2.—TOMATO-SHAPED PEARS RESULTING FROM FEEDING BY ADULT PEAR THRIPS IN
THE FRUIT BUDS BEFORE BLOOMING. (ORIGINAL.)

INJURY TO: (PEARS. By. THE, PEARGEEIRIPS:

THE PEAR THRIPS IN CALIFORNIA. te

is very noticeable when the fruit is picked in the fall. Although at
this time the insects in question have been in the ground three or
four months, the injury becomes more apparent with the maturity
of the fruit, and the seabbing or scarring shows as the result of the
early spring feeding by this species.

The most serious injury to deciduous fruits by the pear thrips is
caused, first, by the feeding of the adults; secondly, by the feeding
of the larve, and thirdly, by the deposition of eggs in the plant tissue
by the adults. The effect of this last injury is more apparent upon
the fruits of prunes and cherries than upon the other deciduous
fruits. Numerous cases have been observed by the writers in both
prune and cherry orchards where the trees blossomed heavily and
there was promise of the setting of a good crop of fruit, but where
practically all the fruit dropped, solely from the effect of having too
many eggs deposited in the fruit stems, thus weakening the tissues,
and because the larve, feeding directly on the fruit and foliage, so
weakened the tree that it would not support a heavy crop of fruit.
Perhaps the chief injury to cherries is caused by the deposition of
eggs in the fruit stems. The long and tender stem of the cherry
presents a most favorable place for the deposition of a great number
of eggs.

Injury to the various fruits by adults and larvee is different, but,
classed in regard to bud structure, those fruits in which only a single
blossom is produced in a fruit bud, such as the almond, apricot, and
peach, seem to be less liable to severe injury than are the fruits which
which form a cluster of blossoms amd later produce a cluster of
fruits, such as pear, prune, cherry, and apple. If the thrips had
their choice of food plants, pears would probably be attacked first
in the sprmg and destroyed; also, other things being equal, a given
number of thrips would do more injury no doubt in a pear orchard
than in a cherry or prune orchard.

INJURY TO PEARS.

The greater injury to pears is caused by the feeding of the adults
in the bud clusters before blooming. Coming out of the ground in
great numbers in the spring as the fruit buds are swelling, the thrips
soon work their way underneath the bud scales and there attack the
individual buds. The feeding is not a biting and chewing process,
but the thrips, by rasping the tender surfaces in the developing buds
with their hardened or chitinous mouthparts, rupture the skin, and
the exudation of sap begins. If only a few thrips are present this
injury may be slight and the buds may develop and bloom, producing
fruit of normal size, although sometimes short-stemmed, or scarred
and misshapen. (See Pl. II, fig. 1.) Plate II, fig re 2, shows two
Bartlett pears which grew from a cluster that was badly injured but

14 BULLETIN 173, U. S. DEPARTMENT OF AGRICULTURE.

not entirely destroyed. Plate III, figure 1, shows a mature Bartlett
pear the one-sided appearance of which was caused partly by adults
and partly by larve. When thrips are more numerous a greater
amount of the bud surface is injured, consequently there is a greater
loss of sap. If this loss is sufficient to cause the cluster buds to
“bleed’’ (sap to drop from the end), fermentation quickly sets in
and the entire cluster is soon destroyed. (See fig. 3, in comparison
with fig. 2, which shows the cluster buds developing normally.) In
many cases blue molds gain a foothold in this fermenting sap and
greatly accelerate the injury,
causing complete destruction
of all fruit buds. The dead
clusters later dry up without
opening. (See Pl. III, fig. 1,
and compare it with Pl. III,
fig. 2, which is from a photo-
oraph of the sprayed portion
of the same orchard, taken on
the same day.) These dead
buds may remain on the trees
for months unless washed off
by rain or blown by winds.
The writers have seen many
orchards so severely injured
that it was difficult to find a
single healthy blossom, and
the entire orchard from a dis-
tance presented at blossoming
time a brownish color and
dead appearance, due to these
blasted buds.

Weather conditions influ-
ence to a great extent the de-
struction following the injury
Fic, 2.—Cluster buds of Bartlett pears developing caused by the thrips. For

ee eee ee instance, the weather of 1909
in the interior valleys during late February and the first 20 days
of March was open and comparatively dry, with more or less
wind blowing, giving quick evaporation throughout the day. Many -
clusters of buds that were kept under observation throughout the
season, with from 10 to 20 thrips in the cluster, developed many of
their buds and produced fruit, a large percentage of which was first
class. During this period for 1910 there was considerable rain and the
atmosphere was warm and humid with very light evaporation. From
many observations in Contra Costa and Solano Counties it was shown
conclusively that in every case where as many as 10 to 15 thrips

THE PEAR THRIPS IN CALIFORNIA. 15

gained entrance into the bud cluster early in the season, and were left
unmolested, the entire cluster was sufficiently injured to prevent the
appearance of a single blossom. In 1909 there was greater evapora-
tion, comparatively little of the characteristic bleeding showed at the
tips of the buds, and far less of the blue molds appeared in any place.
Also the thrips came out of the ground more slowly than in 1910. The
latter year thrips were held back to a slight extent by cold wet weather,
but once the emergence from the ground commenced, thrips came very
rapidly. Then, too, they were more
numerous throughout the entire
section in 1910 than they were the
previous year.

The serious nature of this insect
can be understood when it is re-
alized that in a badly infested pear
orchard it is far more usual to find
from 75 to 150 and often as high as
200 thrips to the cluster than only
10to 15. Any spraying to be effec-
tive must be done before these thrips
have remained long, in numbers,
inside the bud clusters. A delay
_of four or five days in spraying the
badly infested orchards in the spring
of 1910 meant the loss of the entire
crop, and in many cases a delay
of two to three days for the first
application meant a loss of more
than half the crop.

In the ability completely to de-
stroy the crop the adult is of more
importance than the larva, and in
many large orchards the destruction
of the developing fruit buds by the Fic. 3.—Work of the pear thrips on pear at San
adults has been so complete that eis ma Cs
by the time the trees would normally come into bloom there was left
no possibility for a crop of fruit. The larva, together with the
injury which has been caused by the deposition of the eggs by the
adult, can lessen the prospects of a good crop of fruit after it has appar-
ently set. To secure the best results it is always desirable first to
apply efficient treatment against the adult in order to reduce the
early injury to a minimum so that the trees may bloom, and later,
to make additional treatment against the larve. This will usually
result in increasing the value of the crop from 10 to 25 per cent for

16 BULLETIN 173, U. S. DEPARTMENT OF AGRICULTURE.

pears and 40 to 50 per cent for prunes. If remedial measures are not
successfully used against the adult but only against the larve, it is
not to be expected that 50 per cent of a crop will be saved; but the
additional treatment against the larve after the adult treatments
have been applied will cause from 10 per cent to 50 per cent more
of the crop to remain on the trees. Without taking into account
the after effects of migration, good results can be had in pear
orchards by spraying against adults alone, if thorough work is done
at the proper time.
INJURY TO PRUNES.

Next to the pear, thrips injure prunes most severely; and, as the
larger fruit area in the Santa Clara Valley is devoted to this kind of
fruit, and smce the pear thrips has caused the failure over large areas
of the prune crop for several years, growers in the Santa Clara Valley
have commonly called this particular species the prune thrips. The
large acreage of prunes and the general distribution of the pear thrips
over the valley, together with the fact that the majority of the thrips
are out before many of the buds of the French prunes have started
to spread, make it very evident that these little insects, which are
waiting on the outside of the twigs in enormous numbers, will at the
first sign of life of the prune buds bury themselves into the very heart
of the tenderest parts, and rapidly carry on their work of destruction.
The numbers that will get inside of a prune cluster is really aston-
ishing. Many times the writers have, from a single cluster, taken
more than a hundred of these little insects feeding upon the tender
blossom stems, the tips of the petals, and the stigma and style of the
blossoms when they have opened. These parts mentioned seem to
be the choice bits for the adults when feeding upon the prunes. The
rapidity with which the thrips can destroy the whole year’s crop is
astonishing. Many a time orchardists have gone into their prune
orchards at the time the buds were about ready to spread, and, with
only casual observation, have failed to see these minute, dark-colored
insects crawling around or at rest upon the twigs and buds. Upon
inspecting the orchard four or five days later, expecting it to be in
full bloom, they have been astounded to find practically all the buds
destroyed, leaving no hope for a crop that year, the entire orchard
presenting a brown, burnt appearance, with only a stray blossom
now and then, a sight which is well known now to the majority of

the prune growers of the Santa Clara Valley. Anyone who hasever ~ &

seen one of these prune orchards with the burned, browned, and
blasted appearance beside another of snowy whiteness will never
forget the contrast. (See Pl. IV, comparing fig. 1 with fig. 2.) Again
there may be a very severe larval injury on prunes, such as was the
case In 1911. Very few adult thrips occurred i comparison with

PLATE III.

Bul. 173, U.S Dept. of Agriculture.

—UNTREATED PORTION OF PEAR ORCHARD, SHOWING LOSS OF PEAR BLOSSOMS

1

Fic.

(ORIGINAL. )

RESULTING FROM ATTACK OF THE PEAR THRIPS.

(ORIGINAL.)

FiG. 2.—SPRAYED PORTION OF SAME ORCHARD, SHOWING TREES IN BLOSSOM.

INJURY TO PEAR ORCHARDS BY THE PEAR THRIPS.

Bul. 173, U. S. Dept. of Agriculture. -; - PLATE IV.

Fic. 1.—UNSPRAYED PORTION OF PRUNE ORCHARD IN WHICH BLOSSOMS ARE COM-
PLETELY DESTROYED BY THE PEAR THRIPS. (ORIGINAL.)

Fic. 2.—SPRAYED PORTION OF THE SAME ORCHARD, SHOWING TREES IN FULL
BLossom. (ORIGINAL.)

INJURY TO PRUNE ORCHARD BY THE PEAR THRIPS.

Bul

. 173, U. S. Dept. of Agriculture. PLATE V.

Fig. 2.—NORMAL FRUIT, UNINJURED BY THE PEAR THRIPS. (ORIGINAL.)

PRUNES

INJURED AND UNINJURED BY PEAR THRIPS LARVA.

THE PEAR THRIPS IN CALIFORNIA. ey

1910, and they did not accomplish much injury in the Santa Clara
Valley, but larve were present in large numbers everywhere and
riddled the foliage (fig. 4) and weakened the fruit stems, making the
financial loss amount to about half as much as in 1910.

In regard to varieties, Imperial prunes seem to be attacked first
and injured, on the whole, more severely than French prunes in the
Santa Clara Valley. This may be explained in several ways: For
one thing, the acreage of this variety in the Santa Clara Valley is
much less than that of the French prunes and the blossoming period
is usually about a week or more earlier; then, too, the small develop-

Fic. 4.—Prune foliage riddled by pear thrips larve. (Original.)

ing fruit stems of the Imperial prunes seem to be more tender and
not so able to withstand the attacks of the thrips as are those of the
French prunes. Sugar prunes, which blossom at a period interme-
diate between the blossoming periods of Imperial and French prunes,
are, from a financial standpoint, not injured so greatly as are either
of the other varieties. This is partly due to the fact that this variety
sets an unusually large amount of fruit and is therefore able to with-
stand the loss of a considerable portion of it and still produce’ a fair
crop. The scabbing of the prunes on this variety, however, is often
so deep as to cause a large exudation of gum and to render a large
73390°—Bull. 173-153

18 BULLETIN 173, U. S. DEPARTMENT OF AGRICULTURE.

portion of the fruit unmarketable. Plate V, figures 1 and 2, shows
photographs of sprayed and unsprayed prunes, the prunes having
been picked from trees when full grown. Robe de Sargent prunes
blossom about the same time as French prunes, and are injured to
the same extent as that variety.

a

INJURY TO CHERRIES.

Cherries, as a whole, are not injured so severely by the feeding of a
given number of adults as would be the case for the same number of
thrips upon pears and prunes, but certain varieties, especially the
black cherries, suffer comparatively as much from a monetary stand-
point as either pears or prunes. Probably the worst damage accom-
plished on cherries is by the deposition of eggs in the long fruit stems
and in the leaves, and by the feeding of the larve upon the foliage.
The deposition of eggs in the fruit stems has at times caused a large
percentage of the cherry crop to drop, and it is a common sight to
see the foliage entirely riddled by the larve, thus greatly weakening
the trees. Many other mstances are on record where the adults
have mjured the fruit buds to such an extent that only a few blos-
soms appeared. Late varieties of cherries, such as the Royal Anne,
escape serious injury more than the earlier blooming black varieties.
Fortunately the manner of bud growth and blossoming of cherries
permits effective penetration of different spray solutions more ad-
vantageously than is the case with either pears or prunes.

INJURY TO APPLES.

While there are not many instances of great commercial injury to
apples, yet individual cases have been known where the adult thrips
have killed all of the buds in the cluster except the central one. This
was especially noticeable in an orchard of the Newtown Pippin variety
in the vicinity of San Jose in 1910. Some small orchards in Sacra-
mento County were ratber seriously injured during the same year.

INJURY TO PEACHES.

Following the apple, peaches come next in importance as regards
possibility of dangerous injury, the early varieties suffering the greater
loss. The more seriously injured varieties are the Muir, Nicol-cling,
Crawford, Foster, and Lovel, in order of damage done, injury being
more severe on the first two varieties mentioned. On account of the
hairy pubescence on the young peach fruits, the thrips prefer to feed
upon the nectary glands and the inside of the calyx cups; this pre- ~
vents proper pollination, and the young fruits drop to the ground a
few weeks after the blossoming period. Where the injury has been
severe, peaches are sometimes prevented from blooming, and the larve
feeding upon the tender leaves cause them to curl and become dis-

THE PEAR THRIPS IN CALIFORNIA. 19

torted somewhat in the same manner as does peach leaf-curl. Some-
times the larve feed on the young fruit, but rarely to the extent of
causing any great loss.

INJURY TO APRICOTS.

Apricots have not, as a rule, been injured commercially except
in cases where there are a few young trees around home grounds or
near an infested pear or prune orchard. They are sometimes injured
to about the same degree as peaches, and in some ¢ases isolated trees
have been observed which failed to bloom as a result of the work of
the thrips. Larval injury to the young fruit is usually more exten-
sive than is the case with peaches and may at times be serious. How-
ever, apricots are apparently not favorite breeding places for thrips.

INJURY TO ALMONDS.

Almonds are injured less by the thrips than any ot the foregoing
fruits. On account of the early blossoming of the trees and the rela-
tively greater amount of exposed leaf surface at the time the thrips
are out in numbers, together with the character of the blossom, which
is similar to that of the peach, feeding by the thrips very rarely causes
much commercial loss in almond orchards.

DESCRIPTION.

EGG.

The egg when first deposited is bean-shaped, translucent white, measuring on the
average about 0.416 mm. in length and about 0.166 mm. at its widest part in the
middle. (PI. I, fig. 2.)

Just before hatching it decreases in length, appears swollen, has a slight brownish
tint, and is faintly striated longitudinally where the antenne and legs are folded to-
gether. The dark brown spots, the eyes of the young larva, are apparent at one end.

LARVA.
FIRST STAGE (LARVA | DAY OLD).

Length 0.646 mm.; width of head 0.166 mm.; width of mesothorax 0.183 mm.;
width of abdomen 0.15 mm.; length of antenne 0.2 mm.; length of antennal segments:
I 20p, 11 40n, III 454, 1V 100. General color translucent white. General shape fusi-
form. Antenne, head, and legs large in proportion to the rest of the body, and unwieldy.
Antenne distinctly four-segmented, first segment short, cylindrical; second segment
about twice as long as first, oval cylindrical; third segment slightly longer than second,
urn-shaped; fourth about as long as rest of joints together, acutely conical. A few very
fine inconspicuous hairs present on all joints, more prominent on segment 4; Head
subquadrate; eyes reddish brown. Thorax about as long as abdomen, slightly wider.
Abdomen gradually tapering, 10-segmented, first eight segments subequal, 1X and X
longer and more abruptly tapering, with a fringe of long, white, nearly inconspicuous
hairs. Legs stout; femora and tibie nearly equal in length; tarsi one-jointed, ending
in a single black claw. (PI. I, fig. 3.)

20 BULLETIN 173, U. S. DEPARTMENT OF AGRICULTURE.

SECOND STAGE (FULL-GROWN LARVA).

Total length 1.833 mm.; length of head 0.15 mm., width 0.1083 mm.; length of
prothorax 0.1833 mm., width 0.2166 mm.; length of mesothorax 0.1833 mm.,- width
0.466 mm. Length of antennz 0.2833 mm.; segment I 26u, II 50”, III 76, IV 66y,
V i4u, VI 16, VII 334. Antenne: Segment I short cylindrical; II obtuse spindle-
shaped; III spindle-shaped, about as long as I and II together; IV nearly ag long as
III, broader than the rest, subconical; V short, narrow cylindrical; VI slightly nar-
rower and longer than V; VII twice as long as VI, narrower and cylindrical. All
joints transversely striated and with a few inconspicuous white hairs. General color
faintly yellowish white, obtusely fusiform in shape. Body longitudinally and laterally
faintly striated. Head quadrate; eyes prominent, dark reddish brown, situated a
little in advance of the middle; mouth cone broadly rounded, nearly as long as
the head, extending to the middle of the prosternum. Prothorax large, slightly
wider than long, diverging posteriorly. Mesothorax and metathorax short and broad,
twice as wide as long, subequal, in length about as long as prothorax. Abdomen
broad, gently rounded, 10-segmented, broadest at segments V and VI; first eight
segments subequal; segment IX distinctly longer, tapering to apex, the posterior
edge armed with a circle of strong, short, thick wedge-shaped spines, the two medio-
dorsal and medioventral ones shorter and smaller; segment X slightly tapering, not
quite as long as segment IX. Lateral edges of abdomen finely serrated, also with
a few long inconspicuous white hairs which are more prominent onsegment X. Legs
strong; femora and tibiz about equal; tarsi one-jointed, ending in a single black claw.
CES hie.)

NUMBER OF MOLTS, DEVELOPMENT.

When first hatched the larve are active and start feeding imme-
diately and soon become more robust. At the end of about seven to
eight days they molt into second-stage larve, where (see description)
they are still more robust and show also other differences. The total
time required for the development of the larve is about three weeks,
although this period is shorter during warm weather.

PUPA.
PREPUPA (FIRST STAGE).

Total length 1.333 mm.; length of head 0.1 mm., width 0.116 mm.; length of pro-
thorax 0.183 mm., width 0.266 mm.; width of mesothorax 0.35 mm.; length of abdomen
0.666 mm., width 0.383 mm. Shape similar to adult; color translucent white, deeply
tinted with brown. Head subquadrate, about as broad as long, eyes dark reddish
brown. Mouth-cone broadly rounded, extending to about one-half length of the pro-
sternum. Antenne extending backward on each side of head, apparently four-jointed;
first three segments nearly subequal in length, about as broad as long, thick and
unwieldy; segment IV about as long as remaining joints, clublike, and tapering to
an obtuse point. Antennz with a few inconspicuous white hairs. Prothorax nearly
twice as long as the head, broadly rounded posteriorly. Mesothorax broader; wing
pads short, those of first pair of wings extending to distal edge of third abdominal -.
segment. Abdomen 10-segmented, widest at III and IV, segments gradually tapering
from there posteriorly. First eight segments subequal, IX and X longer, distal end
of IX with broad spines somewhat similar to those of second-stage larvee but shorter
and smaller. Legs stout, similar to those of full-grown larva, whole body with sparse,
light-colored, inconspicuous hairs. (PI. I, fig. 5.)

THE PEAR THRIPS-IN CALIFORNIA. 21

PUPA (SECOND STAGE).

Total length 1.416 mm.; length of head 0.183 mm., width, 0.166 mm.; length of
prothorax 0.166 mm., width 0.25 mm.; width of mesothorax 0.35 mm.; length of
abdomen 0.783 mm., width 0.416 mm, Shape similar. to adult, which is visible
beneath the thin transparent shell. Apparently brownish in color, caused by adult
within. Head broader than long; eyes large, dark brown; mouth-cone of adult
within extending to posterior edge of prothorax. Antenne large, cumbersome, laid
back on the head and extending past middle of prothorax, four-jointed; I short; IT
elbowed, about twice as long as I; III short, cylindrical; IV longer than IIT, sides
uneven as knotted club gently tapering to obtuse apex. Joint I of adult is in joint I
of pupa, joint IIT of adult in joint II of pupa, and III of adult within III of pupa;
remaining joints of adult within IV of pupa; 3 or 4 white, inconspicuous hairs pro-
jecting cephalad from elbow on joint II. Prothorax broader than long. Mesothorax
about one and one-half times as broad as prothorax. Wing-pads extending to distal
margin of eighth abdominal segment, fore pair not quite so far. Abdomen widest at
third and fourth segments, tapering from there to obtuse apex. Posterior edge on
ventral side of segment IX with four strong spines resembling a meat fork. This is
apparently the cremaster. Legs stout. Entire body with numerous inconspicuous
white hairs. (PI. I, fig. 6.)

ADULT.

Length of head 0.13 mm., width 0.15 mm.; length of prothorax 0.13 mm., width
0.2 mm.; width of mesothorax 0.28 mm.; width of abdomen 0.31 mm.; total length
1.26 mm. Length of antennal segments: I 33y, II 45y, III 63y, IV 54, V 33h,
VI 66, VII 9u, VIII 12, total 0.31 mm. Color dark brown; tarsi light brown to
yellow. Head slightly wider than long, cheeks arched, anterior margin angular, back
of head transversely striate and bearing a few minute spines and a pair of very long
prominent spines between posterior ocelli. Eyes prominent, oval in outline, black
with light borders, coarsely faceted and pilose. Ocelli approximate, yellow, margined
inwardly with orange-brown crescents, the posterior ones approximate to, but not
contiguous with, light inner borders of eyes. Mouth-cone pointed, tipped with
black; maxillary palpi three-segmented; labial palpi two-segmented, basal segment
very short. Antenne eight-segmented, about two and one-half times as long as head,
uniform brown except segment III, which is light brown; spines pale; a forked sense-
cone on dorsal side of segment III, with a similar one on ventral side of segment IV.
Prothorax about as long but wider than head; a weak spine at each anterior and two
large, strong ones on each posterior angle; other spines not conspicuous. Mesothorax
with sides evenly convex, angles rounded; metanotal plate with four spines near front
edge, inner pair largest. The mesonotal and metanotal plates are faintly striate.
Legs moderately long, uniform brown except tibizw and tarsi, which are yellow,
Spines on tip of fore and middle tibize weak; several strong spines on hind tibiez.
Wings present, extending beyond tip of abdomen, about twelve times as long as
wide, pointed at tips; costa of forewings thickly set with from 29 to 33 quite long
spines; fore vein with 12 to 15 arranged in two groups ot 3 and 6, respectively, on
basal half of wing and a few scattering ones on distal part; hind vein with 15 or 16
regularly placed spines; costal fringe on fore wing about twice as long as costal spines.
Abdomen subovate, tapering abruptly toward the tip from the eighth segment;
longest spines on segments 9 and 10; abdomen uniform brown, connective tissue
wellow.<- (Pl, ¥; ‘fig. 1.)

22 BULLETIN 173, U. S. DEPARTMENT OF AGRICULTURE.
SYSTEMATIC POSITION.

The pear thrips belongs to that suborder of the Thysanoptera
called Terebrantia, which differs from the other suborder, the
Tubulifera, in the possession by the female of a sawlike ovipositor;
also, the terminal segments of the abdomen are conical and the wings
are not equal in structure, the fore pair being the stronger. The mem-
brane of the wings, also, has microscopic hairs. This species is placed
in the family Thripide and is separated from the Molothripide
in that the antenne usually have from 6 to 8 segments, the wings
usually are narrow and pointed at the tips, and the ovipositor is
downeurved. It is placed in the genus Teniothrips of this family
because the body is free from reticulation and the abdomen not closely
pubescent; the head nearly or quite as long as wide, with a pair of
long bristles between the anterior and posterior ocelli; the cheeks
swollen, curving abruptly to the strongly protruding eyes; the
antenne eight-segmented, with the last two segments (the style)
shorter than the sixth; the maxillary palpi three-segmented, the
prothorax very slightly, if at all, shorter than the head, with two
long bristles at each posterior angle; the fore tibize unarmed; the
bristles on the veins of the forewings not equidistant, and the last
abdominal segment of the female conical and without a pair of short,
stout bristles on the dorsal surface.

Until recently this species was placed in the genus Euthrips Tar-
gioni-Tozzetti, which most American authors had used in the sense
of Physothrips and Odontothrips, Tzniothrips and Frankliniella.
Hood! has recently shown that the name Euthrips Targioni-Tozzetti
(1881) was first used in a subgeneric sense as a substitute for the name
Thrips, which had been used for a subgenus of Thrips Linné (1758), .
and that it is consequently a synonym of that genus. The pear
thrips he places in the genus Teeniothrips Amyot and Serville, the
orange thrips in Scirtothrips Shull, and, partly following Karny,?
the various other species formerly assigned to Euthrips in the genera
Physothrips, Odontothrips, and Frankliniella.

ANATOMY.?*

OVIPOSITOR.

The ovipositor is attached to the ventral side of the eighth and
ninth abdominal segments and is composed of four distinct plates,
the under pair attached to the eighth segment and the upper or —
posterior pair to the ninth abdominal segment. The ovipositor in

1 Hood, J. Dougias. On the proper generic names for certain Thysanoptera of economic importance.
In Proc. Ent. Soc. Wash., v. 14, no. 1, p. 34-44, 1914.

2 Karny, H. Revision der von Serville aufgestellten Thysanoptera Genera. Jn Zoologische Annalen,
Bd. 4, Heft 4, p. 322-344, 1912.

3 For a description of the mouthparts see discussion under ‘‘ Manner of feeding and type of mouth-
parts,”’ p. 11-13. ,

THE PEAR THRIPS IN CALIFORNIA. 23

the pear thrips is curved downward. The passageway between the
plates is grooved so that the eggs can pass through readily. The
upper edge (of the upper plates) is fitted with sharp sawlike teeth,
while the lower plates have similar teeth for most of the way but
also bear a number of broad cutting teeth. The end of the ovi-
positor is sharp and pointed. When this is inserted into the plant
tissues, the slit or opening is enlarged by the action of the hard ser-
rate edges of the ovipositor as it is worked up and down by the
rather powerful muscles of the abdomen. The ovipositor when not
in use is protected in a sheath along the ventral side of the last two
seoements of the abdomen.
WINGS.

The wings are long and slender, membranous, with a fringe of
fine hair upon both the anterior and posterior margins, and are never
folded. Both pairs of wings are quite similar and when at rest are
laid back flat upon the abdomen, the pairs lying parallel in the Tere-
brantia. The wings of the family Thripide, to which the pear
thrips belongs, are slender, and taper from the base to the tip, which
is pointed; they bear a general resemblance to sabers. The veins
in the family Thripide are not so prominent as in the family A¢olo-
thripidee, and only one or two longitudinal veins are present, the
cross-velns being very obscure.

FEET.

The legs and feet of thrips form one of the chief characteristics
which separate this order from the various other orders of insects.
They are composed of the usual parts of an insect leg, namely, coxa,
trochanter, femur, tibia, and tarsus. There is nothing unusual in
the formation of the first four parts, the femur and tibia usually
being quite long and somewhat cylindrical. The tarsus is the most
peculiar structure on the leg, and may be either simple or of two
segments, and usually ends in one or two claws. In the family
Thripide, they belong to the former type. The remarkable bladder-
like structure, which for many years gave the name Physopoda to
this order, is protrusile from the end of the last tarsal segment. It
is present in both adults and larve. The end of the tarsus is cup-
shaped, and into this cup the delicate membranous bladder is
attached. When the foot is at rest the bladder is invisible and is
withdrawn into the end segment. The bladder is protruded and
brought into action when the adult is resting on some surface or
walking around. The mechanism of the bladder has been partially
worked out by Jordon and Uzel, but as it is somewhat intricate it
will not: be described here. If a swollen bladder is pricked or rup-
tured, the blood pours out and the bladder collapses quickly. The

24 BULLETIN 173, U. S. DEPARTMENT OF AGRICULTURE.

blood is probably what causes the protrusion of the bladder. Vari-
ous agencies have been used in experiments to hinder the thrips
in walking about on the surfaces of the plants they are attacking,
with the view that if nm some way the mechanism of the bladder was
affected, either by causticity or by absorption, the bladder would
not be able to perform its function, and the insects would fall from
any surfaces that were so treated. This has not been successful
from the writers’ experience, as they have observed on numerous
instances thrips crawling around on sticky surfaces, even on tangle-
foot, which was to all appearances and to the touch very sticky.
This bladderlike formation is probably so delicate that surfaces
which appear smooth or sticky or caustic to the naked eye and human
touch are rough and uneven to the thrips and are neither adhesive
nor caustic. The writers have never seen thrips stuck to any sur-
face by the ends of their tarsi, but only by their bodies, legs, or
wings. It is apparent that they are able to walk on practically
every kind of surface, especially after this treated surface has been
exposed to the atmosphere for a few hours.

LIFE HISTORY AND HABITS.

ADULTS IN SPRING.

EMERGENCE FROM GROUND.

The first form of the pear thrips to be seen by the orchardists during
the growing season is the adult (PL. I, fig. 1), which emerges from the
ground during the last winter months and the early spring. The
period in which they first appear upon the trees in Santa Clara,
Contra Costa, Solano, and Sacramento Counties is variable. Certain
sections in each territory are earlier than others and some orchards
are in advance of others in regard to blossoming conditions.

In the Santa Clara Valley during the year 1909 the first adult
thrips were collected February 15. (See Table IV.) By February
18 they were quite numerous in one of the orchards under observa-
tion and were common in all orchards by February 25. Maximum
emergence began about February 19 and lasted until March 18.
They continued to emerge until the first three days in April. In
Contra Costa County first thrips were out at the laboratory February
12 and in the field February 16, emerging in numbers by February
20. Maximum emergence was over by March 15 and all were out by
March 27. During the season of 1910 the first thrips taken in the |
field in Santa Clara County were observed on February 7, while the
first in emergence cages appeared on February 9. They were common
in the field from February 15 on. Thrips appeared in maximum
numbers from the cages (see fig. 5) beginning February 22 and
ending March 10, with the last stragglers coming out as late as
March 20. The emergence season for 1911 at first gave promise

THE PEAR THRIPS IN CALIFORNIA. 95

of being very early, as the first thrips were found in the field on
January 29 and in the emergence cages February 1; but the heavy
rains following in February and March caused it to be very back-
ward, so that thrips were not common in the field until March 14,
which was about the time of the true maximum emergence.

In Contra Costa County during the season of 1909 the maximum
number of thrips emerged in cages, which were put in the ground
in the yard at the laboratory, from February 23 to March 4. (See

Fic. 5.—Type of soil cage used for soil samples in obtaining emergence records
of the pear thrips at San Jose, Cal. (Original.)

Table VI and fig. 7.) In cages placed under trees (see fig. 6) in the
field the thrips emerged in maximum numbers from February 26
| to March 12 (see Table V and fig. 8). During the spring of 1910 the
Z first thrips found to emerge in the cages at the laboratory were out
2 on February 18 (see Table VI and fig. 9) and in the field cages on
February 21, reaching a greater daily emergence by March 1, and
continuing to emerge in considerable numbers until March 15, the
maximum emergence being March 7 (see Table V and fig. 10). By
comparing figures 7 and 8, which show the emergence records for
73390°—Bull. 173—15 —4

26 BULLETIN 173, U. S. DEPARTMENT OF AGRICULTURE.

1909, with figures 9 and 10, showing the record for 1910, it will be
seen that the time of emergence in any considerable numbers was
much shorter in 1910 than was the case in 1909. No actual daily
emergence records were kept in 1911, but no thrips were found in
the field until February 18 and then only very few in one early
almond orchard. On February 24 a few scattering specimens were
found in two pear orchards. Not until March 12 were they appearing
in any noticeable numbers, but the emergence was very rapid after
this, reaching the maximum between March 15 and 20. The emer-
gence of adults was mostly over by March 30.

i WH

Hut {| iif |
il MHRA BARHUA ERMA Mk

Pe Re

| |
[pies ws TAOS yr ee aa

Fic. 6.—Type of wooden cage used for field emergence records of the pear thrips in orchards at
Walnut Creek, Suisun, and Courtland, Cal., 1909-10. (Original.)

Emergence records and field observations in the Suisun Valley of
Solano County (see Table VII and fig. 11) show that for the season
of 1910 thrips came out of the ground in numbers on about the same
dates as for Contra Costa County. They were out in numbers in the
Courtland district of Sacramento County from two to.four days
earlier. Further observations in 1911 showed the emergence in
these two sections to be about the same time as for Contra Costa
County.

Records of the emergence for the years 1909, 1910, and 1911 are
summarized in Table IV. From this table it rill be seen that in
Santa Clara County in 1909 most thrips appeared on March 3 while
in 1910 March 4 yielded the highest number, with March 3 and 2

THE PEAR THRIPS IN CALIFORNIA. A

following close behind. The increase in emergence during the season
1909 (fig. 12) and the tapering off in the same year was more gradual

28 24 6 & 10 2 /4 es 22 24 26
FEERUAP MAP?CH

Fic. 7.—Curve illustrating emergence of adult pear thrips at laboratory, Walnut Creek, Cal., 1909.
(Original. )

than during the season 1910 (fig. 13). This difference was most
probably influenced during the latter season by the temperature.

eee eee eee
eo tt tT | A
|: SSE ee ee ieee
eee ee
coos a
| | ae Se ieee eee
SSR ee eee eee
oJ A ee ee
|| See Baas Sees
SS ies ae
» a Je eee eee
Pm el ET | TN TT TT a
>. J 2SSS sae
. | Aa Abe Seah Cee Rees
) 3 ee Ree, See

44 16 18 20 22 a ens Ve EES Ne COR COR
FEBRUARY Cr

Fic. 8.—Curve showing emergence of pear thrips in cages under trees in field at Walnut Creek, Cal.,
1909. (Original.)

RELATION OF EMERGENCE TO TEMPERATURE AND RAINFALL.

The average mean temperature for February and March, 1911, or
the two months when practically all of the thrips emerged, was

28 BULLETIN 173, U. S. DEPARTMENT OF AGRICULTURE.

50.7° F., or about the same as in 1909, and the emergence probably

would have been very similar to the emergence for that year but
for the abnormal precipitation in February and March, especially
in the latter month. ;

TTI ea
Hee et

/000
950

al aaa
SRR eee
| ete pe Sa ee
tt Ht

loa ee ee

fn? See AR eee
/4 /6 18 20 22242628 2 46 @& 10 l2 144 16/8
FEBRUARY MARCH

Fic. 9.—Curve showing emergence of adult thrips at laboratory, Walnut Creek, Cal., 1909.
(Original. )

TasBLe I1.— Mean temperatures for the months of February and March, 1909, 1910, and.

OTT e

S28.
Mean maximum temperature for month of February, 1909......-..------------ 59. 2
Mean minimum temperature for month of February, 1909-....-.----- ee en VAR
Average mean temperature for month of February, 1909.-..-.-.----------------- 51.0
Mean maximum temperature for month of March, 1909.--.-..-.---------------- 60. 0
Mean minimum temperature for month of March, 1909. -.-.-..------------------ 40. 0
Average mean temperature for month of March, 1909...-...--.---------------- 50. 0
Mean maximum temperature for month of February, 1910..-.-.--------------- 58. 8
Mean minimum temperature for month of February, 1910-..--.----------------- 38. 5

THE PEAR THRIPS IN. CALIFORNIA. 29

Average mean temperature for month of February, 1910...-....--......-......- 49.0
Mean maximum temperature for month of March, 1910................-...-.... 66.2
Mean minimum temperature for month of March, 1910........-..-.-.-...--.-- 44.5
Average mean temperature for month of March, 1910....:.....-....-..--..--.--- 55. 0
Mean maximum temperature for month of February, 1911...........--..-..--- 56.5
Mean minimum temperature for month of February, 1911................-..... 37.3
Average mean temperature for month of February, 1911-........-..-5-..-..--- 46. 9
Mean maximum temperature for month of March, 1911........-..-..-.------- 63:3
Mean minimum temperature for month of March, 1911..............-.--.-+---: 46. 0
Averase mean temperature for month of March, 1911]....................------ 54. 6

Deere Te
mae |

SEE]

BREWS
Semmes NT | TT)
wild Ae eens
Pema Te bgt | Ty
Dee ee ee \eeeeenee
eo OTe ee) eee
emma eS
ich eae le ee eae eee
a JAS ae i eo eee
> JO Ee ee eee
oo OS ee ee ae el ee
eww SER ee ie eae
cave Ree ee ieee
ono Sasa ae Bees ieee eee
pee Ee ee ei
oo ie ee Bee eee
Pomme era a re Py TP ee
op wl Ee ene Cees
perme hay | PA
Pome maw 7 iy LIN | ee
pine ee | LP) NN
ee aes GN EVES, 70 Gea 18 20 22 24 276 28

Fig. 10.—Curve showing emergence of adult pear thrips in cages under trees in field, at Walnut Creek,
Cal., 1910. (Original.)

It will be seen from the temperature records (Table II) that while
February, 1909, had 2 degrees higher average mean temperature

4 than February of 1910, March of 1909 had 5 degrees less average
mean temperature than March of 1910, making the average mean
temperature for the months in which most of the adults emerged
50.5° F’. in the year 1909 and 52° F. in the year 1910. Another factor
which held back the emergence greatly the former year was the

30 BULLETIN 173, U. S. DEPARTMENT OF AGRICULTURE.

greater rainfall, the month of February, 1909, having 4.87 inches
precipitation while February of 1910 had only 0.83 of an inch.

A comparison of the amount of precipitation for the three years
1909, 1910, and 1911 (see Table III) shows a large amount for 1909,

eee ie
BEREAN
PAGS Se ee
TNS] a
\4- i

14/6 /8 20 2224227 24 @ 10 /2 1416 18 20 22 CALE CEP
FEBRUARY MAFPCH

Fig. 11.—Curve showing emergence of pear thrips at Suisun, Cal., 1910. (Original.)

which with the low average mean temperature for the two emergence
months caused the emergence to be drawn out. The season 1911
was very abnormal in the large amount of precipitation, especially

Jo
AL
EHH AE

RRS Res Ree ees
TERE ae ae eee
St NA PL

/2 14 16 18 20 22 JO [2 14/6 18 3
FEEL MAFPCH APF.

ee Pee
Raa Seas

N
%
S
8
\

Fig. 12. Curve showing emergence of pear thrips at San Jose, Cal., 1909. (Original.)

during the latter part of February and early March, causing a late
blossoming season, and holding the thrips back to such an extent
that comparatively little injury was caused by the adults.

THE PEAR THRIPS IN CALIFORNIA. 81

Taste III.—Total precipitation for the years 1909, 1910, and 1911 at San Jose, Cal.,
laboratory.

Precipitation in inches.
Month.

1909 1910 1911

HODTUaG Ye. asl iol 4. 87 0. 83 2.03
Manchieucrcc ssc: 2.77 2. 84 6. 26

One curious fact about the emergence for 1911 was the double
maximum, one the latter part of February, from the 18th to the 26th,
and another from the 8th to the 15th of March. (See Table IV and

EERE EEEEEEESoE
See Bees Pease
EEE

3600

cee nae | Eee
3400
DUES eae eae | nea

2 eee ee bh eee
WL Eee el eee
[Tac i
Je eee el eee

JOBE 2 Ee
eeeae7 ir) i |) SAL

Fic. 13.—Curve showing emergence of pear thrips at San Jose, Cal., 1910. (Original.)

fig. 14.) From February 26 to March 11, inclusive, it ramed every
day from 0.02 of an inch to as much as 2.45 inches. Probably a
number of the thrips which emerged in February were killed by the
heavy rains in early March, or at least were not permitted to cause
much injury. The pear thrips emerges from the ground during rainy
weather, but not in such great numbers as durmg warm, sunshiny
days, which was the case during the latter part of February and the
early part of March of the year 1910. Whether the soil is clean or
covered with weeds and grass at this time of year influences the time

32 BULLETIN 173, U. S. DEPARTMENT OF AGRICULTURE.

of emergence by some two or three days. This was particularly
noticeable in pear orchards used in cultivation experiments in Contra
Costa and Solano Counties. In the plowed portions which were free
from weeds, the surface dried out and warmed up more rapidly and
thrips came out in numbers and into the trees three days earlier than
on the unplowed part of the orchard, which was covered with’a rank
growth of vegetation. The shading of the soil by the vegetation
seems to result in holding the thrips within the ground several days
later, or else they spend some time on this succulent erowth before
going into the trees.
The following tables give the emergence records for the years 1909
1910, 1911, and 1912 for Santa Clara County (San Jose, Table TV);

ACTON | a
SRR RRR eee Neha eee eee
ae a VS | AS ISAT) sepa al oe

pe eee le PP a EEN Tei as [|S
i BEE BREA BREE Bee
ER AS ky SRewad

e ee 74 22 26,2 178 22 = =
a
FEBRUARY MARCH

Fig. 14.—Curve showing emergence of pear thrips at San Jose, Cal., 1911. (Original.)

for 1909 and 1910 in Contra Costa County (Walnut Creek, Tables V
and VI), and for 1910 in Solano County (Suisun, Table VII). These

tables show the total number of thrips emerging on the given dates.

from soil in the cages. For the San Jose records, all the cages con-
taming soil samples from infested prune orchards were placed in the
ground at the laboratory. For the records in Contra Costa and
Solano Counties, part of the cages were brought to the laboratory
and buried in the ground and part were left in the ground under the
trees in infested orchards. (See fig. 6 for type of cage used for the
field emergence records in the northern counties.) It was not pos-
sible to take the emergence every day, but,so far as possible, counts
were made at regular intervals.

: 2a

THE PEAR THRIPS IN CALIFORNIA.

33

TABLE 1V.—Total emergence of pear thrips from all the cages kept at the laboratory at

Date

CmMISokw WIR

COMI? Or CO LD

San Jose, Santa Clara County, Cal., during 1909, 1910, 1911, and 1912.

Number | Number | Number
of thrips | of thrips | of thrips
emerging | emerging | emerging
in 1909 in 1910 in 1911
from 18 | from 18 | from 4
cages: cages. cages.
0 | 0 2
0 | 0 7
0 0 0
0 0 0
0 0 0
0 0 1
0 0 28
0 0 5
0 25 1
0 18 ay
0 16 1
0 16 22
0 4 0
0 88 0
18 22 11
0 27 5
DDL 34 2
192 33 17
192 14 62
169 23 41
75 62 32
119 129 33
135 375 25
552 272 26
459 297 18
444 455 8
414 574 0
781 657 0
781 1,975 0
535 3, 592 0
1, 299 3,011 2
714 4,217 4
508 1, 402 0
362 1, 595 0
438 539 1
219 275 21

Number

of thrips

emerging
in 1912
from 4
cages.

ROOD OWNRK OCF

bo

Date.

Apr.

Total. .

11,998

Number

Number

of thrips | of thrips
emerging | emerging

in 1909
from 18
cages.

776
497
498
338

28

—

SOO COCOOC CAF OCOWONONAWNWWOLO

in 1910
from 18
cages.

oooocoocoocooococooooocooocoooonnNnatorsns

20, 350

Number | Number
of thrips | of thrips

emerging | emerging

in 1911
from 4
cages.

SOO COCOCCOOCCCOCOOCOCOFF OFF AWOORNS

660

in 1912
from 4
cages.

i)

Or HW OV 1 BH ATO

17, 968°

}

prune orchards, Walnut Creek, Contra Costa County, Cal.

Number of
Date. thrips
emerging.
1909.
Feb. 13 0
16 20
19 By
22 30
26 110
Mar. 32 615
& 679
10 752
12 273
16 65
20 33
22 4
27 11

Date.

Mar. 1

Number of

thrips
emerging.

TaBLE V.—Emergence of pear thrips from cages placed in ground under trees in pear and

34 BULLETIN 173, U. S. DEPARTMENT OF AGRICULTURE.

Taste VI.—Emergence of pear thrips from soil samples taken from orchards in December
and January and kept in cages at laboratory, Walnut Creek, Contra Costa County, Cal.

Number of
Date. thrips
emerging.
1909.
Feb. 12 3
15 42
16 56
ly 38
18 56
20 89
23 125
wy 185
27 246
Mar. 1 196 |)
4 237
7 51
10 52
14 13
19 0
22 0

Number of |-
Date. thrips
emerging.
1910. > Z
Feb. 18 11
20 16 |
29 0 £'
24 12
26 30
28 75
Mar. 2 377
4 918
6 937
8 165
| 10 114
iy, 47
14 0
16 4

TaBLeE VII.—Lmergence

records of pear thrips for Suisun, Solano County, Cal., 1910.

|

Emergence of thrips

from cages placed
in ground under
trees in orchards,
Suisun, Cal.

| Number of

Date. thrips
emerging.
Feb. 17 | 3
19 0
21 0
23 0
25 1
27 20
Mar. 1 47
3 121
10 484

| Emergence of thrips
from soil samples
taken from _ or-
chards in Decem-
ber and January
and kept in cages
at laboratory, Sui-

sun, Cal.
Number of |

Date. thrips
emerging.

Feb. 16 1
ily 3

18 2

19 6

20 1

21 1

22 | 4

23 2

24 5

25 11

26 Ad

27 14

28 | 41
Mar. 1 | 105
2 247

3 243

‘L 612

12 357

16 82

19 8

The latest dates on which adult thrips were collected in the field
were about the same for the years 1909 and 1910, the last ones being
found from April 15 to April 25.

as late as the middle of May. They were very scarce, however, after

In 1911 living adults were found

May. The number of living adults as a rule decreases rapidly after

April 1.

The time adults will feed before they begin ovipositing varies.
Those individuals which emerge early and which do not have a suit-

THE PEAR THRIPS IN CALIFORNIA. 35

able place for ovipositing will feed from 15 to 20 days before placing
any eggs, while individuals which emerge at a later date, as, for in-
stance, from March 5 to 20, do not as a rule feed more than one or
two days before depositing eggs. Individuals which were taken from
emergence cages and placed in mica chimneys were observed ovi-
positing the day following their emergence. It is possible that in
the field thrips begin depositing eggs more quickly on certain varie-
ties of fruits than on others. This would be governed very largely
by the presence or absence of available tissue suitable for oviposition.
For this reason on the early blooming varieties of cherries thrips prob-
ably feed for a shorter time before oviposition commences than is the
case with other fruits.

PERIOD OF EGG LAYING FOR INDIVIDUALS.

The egg-laying period for individuals does not usually last for
more than three weeks. Individual thrips confined m mica chimneys
on March 5, 1910, did not deposit any eggs after the latter part of
March. The full period of egg laying for the entire brood throughout
all the infested areas extends from about February 20 until near
April 10, or a period of six to seven weeks.

LENGTH OF LIFE OF ADULTS.

Adult thrips confined in vials without food lived on an average
three days, while those confined in vials with food lived about two
weeks. Adult thrips confined on the trees with mica chimneys
lived from three weeks toonemonth. The length of life of individuals
in the field has not. been observed accurately, but probably ranges in
duration from three weeks to one month and a half.

RELATION OF EMERGENCE TO BLOSSOMING OF TREES.

The emergence period extends from early February to early April
and is closely associated with the blossoming periods for the different
varieties of fruits. Budding and blossoming of the different fruits
is as follows: Almond buds begin to swell during the latter part of
January and early February, and this variety of fruit is in full bloom
between February 8 and 24. Apricots show first blossoms from Feb-
ruary 12 to 23, and most varieties are in full bloom by from March 3
to 10. Peaches show first blossoms about February 23 and many
varieties are in full bloom from March 8 to March 17. Black Tar-
tarian cherries reach full bloom by March 15 to 20, while the Royal
Anne variety has not at that time opened its buds. French prune
buds are beginning to swell between March 8 and 11 and first blos-
soms appear by March 20. They are usually in full bloom between

36 BULLETIN 173, U. 5. DEPARTMENT OF AGRICULTURE.

March 26 and April 8. The Sugar and Imperial varieties precede the
French by about one week. Bartlett pear buds begin to swell the
last of February or the first of March, the first clusters usually spread-
ing from March 10 to 15 and are in full bloom for quite an indefinite
period between March 20 and April 10. Pears, prunes, and cherries,
which are spreading their bud clusters just after the maximum numbers
of thrips are coming from the ground, are the fruits most seriously
injured by the pear thrips.

MIGRATORY HABITS.

Evidences of the migratory habits of the pear thrips have been
noticed at times during the last three or four years. However, no
definite observations concerning their migration had been made until
the year 1910. Hitherto it had been noted that in some orchards
the adults were very numerous early in the season and doing extensive
damage. Later observations at an imterval of four or five days
showed very few adults present, and the entire orchard had the
characteristic browned and burnt appearance. It was quite evident
that after destroying all the fruit buds the thrips had migrated to
other orchards in search of food.

It was possible to obtain more definite knowledge regarding
migration in the year 1910 than had heretofore been known, for the
reason that the thrips were unusually numerous throughout all the
infested areas that year and weather conditions were such that
practically the entire brood emerged from the ground in a few days.
Also, following their emergence in great numbers, the weather was
sufficieatly warm that the destruction of the fruit buds in the various
orchards was accomplished in much shorter time than is usually the
case. Observations so far indicate that thrips migrate in swarms
only on bright, warmdays. Numerousinstances of supposed migration
were mentioned to the writers at various times during the season, the
reports stating that the pear thrips were flying in swarms, but most
of the cases reported lacked authentic evidence to bear them out,
such as the saving of specimens. However, in the afternoon of March
28, 1910, the junior author drove out from San Jose toward Saratoga
and had great difficulty in keeping both hands on the reins on account
of the great numbers of thrips which, flying through the air, filled his

eyes and covered his clothes. The prevailing direction of the wind

on this day was not observed; no distinct migration or swarm was
noted, however, although individuals were numerous flying across the
road and could be readily seen when the observer looked toward the
sun. They were more numerous on roads running north and south,
and extended over a territory of 4 or 5 miles; they were the most numer-

é

THE PEAR THRIPS IN CALIFORNIA. 37

ous at the west end of Hamilton Avenue and along the San Tomas and
Santa Clara and Los Gatos Roads.

On March 30, 1910, still more definite information was gained, and
this is probably the most unique record of thrips migration which has
yet been taken. The day was bright and rather warm and ended
with the evening warm and a gentle breeze blowing from the south.
Mr. E. L. Fellows, who was in Santa Clara on this day, started home
about 5 o’clock in the afternoon. About 5.15 p. m., out on the
Saratoga Road, he noticed a number of small, black imsects which
covered his face and hands, his hat and clothes, and got into his eyes.
When he was one-fourth of a mile north of Meridian Corners he met
the thickest part of the swarm, which appeared literally lke a black,
glistening, seething mass moving up and down like heat waves.
From this place the sects became less numerous as he went toward
home, which he reached about 6 p.m. He thought the swarm to be
about 8 miles long and 4 miles wide, from 4 to 15 feet high, moving
at the rate of about 10 miles per hour northward toward San Fran-
cisco Bay. As he was not sure concerning the identity of this insect,
he gathered several hundred specimens in a paper bag and submitted
them to the junior author for identification. They were found to
be the pear thrips, Txniothrips pyrv. This same swarm was noticed
by the junior author and by several fruit growers, but they did not
have the opportunity to view the whole swarm as did Mr. Fellows.

Continued observations during the season of 1910 showed that the
usual time for migration was from 3 to 6 p.m. on bright, warm days
during the latter part of the period of maximum oviposition, which
was also about the time many orchards have been so badly injured
that the trees will not bloom.

This migratory habit is undoubtedly influenced chiefly by a desire
for a new supply of food, better places for deposition of eggs, and
suitable weather conditions, especially the temperature. The
direction in which thrips will migrate depends upon the direction the
wind is blowing, ard the distance at which suitable feeding places
are found.

No distinct migration of the whole brood has ever been observed,
such as is the case with some species of Orthoptera. The migra-
tion from certain badly infested orchard localities has been in-
fluenced, without doubt, by the early destruction of the fruit buds
in these orchards. Many instances are known where thrips are
numerous and their injury severe in an orchard one year and not
very numerous the succeeding year, but they are usually highly
injurious again the third year. This phenomenon is more noticeable
in pear than in prune orchards, due probably to the fact that a pear

38 BULLETIN 173, U. S. DEPARTMENT OF AGRICULTURE.

orchard in which all fruit buds have been destroyed is poor feed-
ing ground for both adults and larve and reproduction is at the
minimum under such conditions. This reappearance in damaging
numbers the third year makes it evident that the orchardists should
not allow their orchards to go untreated. It should be noted that the
years 1907 and 1910 were the only seasons in which the pear thrips
migrated to any great extent. No migration was known in the
season of 1911, although it was watched for.

MANNER OF REACHING TREE TOPS FROM GROUND.

Most of the adults when emerging probably crawl around for a
while on the ground until their wings get sufficiently dry and then
fly up into the tree. Some, however, must undoubtedly crawl up the
trunk, as a few have been caught by tanglefoot bands. This, however,
can not be used as a method of control, since very few go up this way;
moreover, the thrips would not be caught unless the bands were
renewed every day or so, because the bands do not remain sufficiently
sticky after a short exposure to the atmosphere.

REPRODUCTION.

According to Bagnall 1 an example of the male pear thrips was found
by him among some specimens of this species taken from plum
blossoms at Evesham, England, and submitted to him by Mr. Col-
linge, director of the Cooper Research Laboratory at Berkhamstead.
His only description is that ‘‘It is much smaller than the female and
the wings considerably overreach the tip of the abdomen.” This is
the first report of the existence of the male of this species, and
in California very extensive observations by the writers and other
workers have failed to show a single male, and the only type of
reproduction known is by parthenogenesis. In all of the life-history
experiments to secure data upon the length of the egg stage indi-
vidual females were taken directly from the emergence cages and
isolated. It is highly probable that practically all of the eggs
which are deposited hatch, as no sterile eggs have ever been found.

OVIPOSITION.

Moulton ? states that he has observed the adult in ovipositing
to make first a hole in the epidermis of the plant tissue with the

mouth before depositing the egg. Repeated observations by the |

writers of a large series of adults during oviposition have failed to

1 Bagnall, Richard S. A contribution to our knowledge of the British Thysanoptera (Terebrantia), with
notes on injurious species. Jn Jour. Econ. Biol., v. 4, no. 2, p. 33-41, July 7,1909. Seep. 39.

2 Moulton, Dudley. The Pear Thrips (Euthrips pyri Daniel). U.S. Dept. Agr., Bur. Ent., Bul. 68,
pt. 1, rev., p. 7, Sept. 20, 1909.

THE PEAR THRIPS IN CALIFORNIA. 39

show a single one going through this procedure. The usual method
as shown by observations during the season of 1910 is as follows:
The female starts the ovipositor into the tissue by working the
abdomen up and down, gradually forcing the ovipositor its full length
into the tissue. After this is done the thrips remains quiet for a
short interval while the egg is passing out between the plates of
the ovipositor. When finished, the female vibrates her antennae and
jerks out the ovipositor. The prevailing posture during the whole
period of oviposition is with the abdomen arched and the legs spread
apart wider than when in walking. The average time required for the
operation by a number of individuals observed during the season of
1910 ranged from three to five minutes. After depositing an egg the
female usually resumes feeding for a short interval, but some indi-
viduals have been observed to deposit two and three eggs in suc-
cession without any feeding between times. The number of eggs
that a female can deposit in a day is probably not over seven or eight,
as the abdominal cavity is not large enough to hold more at one time.

EGGS.

PLACE OF DEPOSITION.

The eggs are always placed in the tenderest portions of the plant
tissue, such as exposed blossoms, fruit stems, leaf stems, ribs of the
leaves (preferably the midribs), and the leaf edges. Still others are
placed in the young fruits. The pear thrips apparently prefers to
oviposit upon cherries if a cherry tree is at hand, as the fruit and leaf
stems, on account of their length and tenderness, offer excellent places
for oviposition without making it necessary for the thrips to move
over a large area. However, the small prunes and the stems, as also
the stems and midribs of the young leaves of both prunes and pears,
are well suited for oviposition by this species. The counts in Table
VIII were taken upon leaf stems and fruit stems of French prunes and
show the comparative percentage of eggs deposited in each; they also
show the inability of the different spray mixtures to kill the eggs
within the plant tissues, as these stems in question had been sprayed
two days previously with a combination of tobacco extract and dis-
tillate emulsion.

40 BULLETIN 173, U. S. DEPARTMENT OF AGRICULTURE.

TaBLE VIII.—Comparative percentage of eggs deposited in fruit stems and leaf stems of -

French prunes, San Jose, Cal., season of 1910.

_ | Number _| Number
No. of ob- umber of eggsin || No. of ob- pees ofeggsin
servation. Teatctenia fruit servation. eerie fruit
cee “| “stems: ‘ stems:
1 2 i 44 1 12
2 1 10 45 0 11
3 5 5 46 7 8
4 0 2 47 3 8
5 2 13 48 7 9
6 1 6 49 5 11
7 3 8 50 2 9
8 0) 8 51 12 9
9 0 8 52 10 ial
10 il 9 53 2 9
11 1 8 54 3 10
12 2 10 55 7 12
13 2 4 56 0 6
14 5 6 57 9 10
15 3 10 58 5 10
16 2 11 59 12 4
17 0 5 60 5 11
18 3 12 61 0 17
19 3 10 62 6 9
20 1 8 63 2 13
21 0 6 64 4 9
22 0 3 65 5 12
23 3 10 66 8 6
24 1 9 67 0 7
25 1 5 68 11 8
26 2 13 69 8 9
27 1 10 70 5 16
28 1 5 71 9 7
29 1 9 2 3 8
30 2 8 73 2 9
31 0 16 74 2 7
32 2 8 75 9 11
3i3! 2 4 76 17 8
| 34 1 15 77 6 10
SB 0 9 78 11 4
36 0 11 79 12 8
37 1 7 80 9 14
38 4 19 81 8 9
39 7 16 82 2 §
40 2 13 83 1 11
41 5 7 84 0 il
42 3 12 ||—— = ae
43 3 9 Total 299 786

It will be seen from this table that the average number of eggs
placed within fruit stems of prunes is more than twice the number
placed in the leaf stems. In pears a very large proportion of eggs is
placed in ribs and veins of leaves and a comparatively smaller per-
centage in the fruit stems.

FIRST EGGS.
The first eggs that were noticed in the vicinity of San Jose and in

Contra Costa County were placed about March 10 for the season of
1909, while most eggs were being placed about March 15 to 25, and

the last eggs in early April. The first eggs were deposited in 1910 in »

the field about March 9, while maximum oviposition was from March
18 until about April 2. The last eggs were observed to be placed in
the field toward the middle of April. In the interior counties, espe-
cially Sacramento and Solano Counties, eggs were being deposited in
large numbers by March 15, and continued to be deposited in num-
bers until the latter part of March, a few being found in early April.

= ag Biersord

AY
2 i

THE PEAR THRIPS IN CALIFORNIA. 41
LENGTH OF EGG STAGE.

Moulton! records the length of the egg stage to be approximately
four days, but detailed observations during the season of 1910 at San
Jose show it to be considerably longer. The length of the egg stage
was first ascertained by inclosing twigs with paper bags before thrips
emerged so as to get no outside infestation. Later, when thrips were
ovipositing in the field, a considerable number of adults were placed
in mica chimneys which had been specially constructed to fit over the
twigs in such a manner as to give them as nearly natural conditions
as possible, and to permit the eggs to remain in living plant tissue
because they usually dried out when the twigs were removed from the
tree. These chimneys were made by sewing pieces of strong white
cloth in the shape of tubes about 5 or 6 inches long and gluing one
end of a cloth tube thus made to each end of the mica chimney.
When placed upon the tree, ends of the cloth were tied securely
around the twig so that no insects could get in from the outside.
The thrips kept for oviposition remained in the cages over night and
were removed the next day. To make sure that none would remain
in to continue ovipositing, new cages were placed on the twigs in each
case. Table IX shows the length of the egg stage.

TaBLe 1X.—Length of egg stage of the pear thrips, San Jose, Cal., 1910.

Average
Cage | Datede-| Date eae ote mean | Prevailing
T j ¢ 5 r
No. posited. | hatched. hatched. | stage. seer: weather.
, Days. Sait.
I Mar. 10 Mar. 16 25 6 56 Cloudy
ile 6 U 57 Do.
18 9 8 58 Do.
19 8 9 57 Do.
20 3 10 57 Do.
22 10 12 52 Do.
23 3 13 52 Do.
24 1 14 52 Do.
| It | Mar.10 | Mar.16 13 6 56 | Cloudy
| | 17 27 7 57 Do.
18 30 8 58 Do.
19 35 9 57 Do.
20 8 10 SV Do
Lit Mar. 10 Mar. 16 27 6 56 Cloudy.
7, 4 7 57 Do.
18 9 8 58 Do
19 14 9 57 Do
20 10 10 57 Do
22 f 12 52 Do
23 1 13 52 Do
24 1 14 52 Do
Iv | Apr. 7 | Apr.14 3 7 55 | Clear
Vv | Mar. 29 Apr. 5 1 i 56 Clear
) 8 1 10 56 Do
10 1 12 56 Do
VI ~| Mar.29 | Apr. 7 2 9 56 | Clear
| 9 4 11 56 (0)
10 1 12 56 Do
12 1 14 55 Do
1 Op. cit., p. 8.

42 BULLETIN 173, U. S. DEPARTMENT OF AGRICULTURE.

TaBLE IX.—Length of egg stage of the pear thrips, San Jose, Cal., 1910—Continued.

| Average
Cage Date de- Date pred meee mean | Prevailing
No posited. | hatched. hatched. | stage. peBere weather.
Days. | oF %
Vil Mar. 29 Apr. 3 1 5 56 Clear
8 1 10 56 Do
Vill Mar. 29 Apr. 7 1 9 56 Clear
8 7 10 56 )
9 3 il 56 Do
10 4 12 56 Do
TEX Mar. 29 Apr. 2 1 4 57 Clear
6 1 8 56 Do
if 1 9 56 Do
8 4 10 56 Do
10 4 12 56 Do
14 je 16 55 Do
x Apr. 6 Apr. 12 1 6 54 Cloudy.
XI Apr. 6 Apr. 13° | 1 7 54 Cloudy.
XII Apr. 6 Apr. 13 2 i 54 Cloudy.
SUMMARY.
| : l
Time | Time
Number eggs deposited. required for Number eggs deposited. required for
| incubation. incubation.
| Days. | Days.

PC Se eR ee ee ae eee 7 | ees: ee Renee ee ee ae Sek Seo 10
ae eee se eie, ores Soe se 5 | EER SE ee ye oe he SAS 11
OGM ee er re eee te ee 6 7 A AR re ea a) teres Sc 12
Ue ee ee ae el eee eer ee 7 Bisse Serciet fae eee ee eee eee 13

| TV, ath 4s Ee ce ele es ee Sy [icdins Pe ER ee ee 14
Loe a Se ote eee 9 Dix ets! 224 oecaoed epee eee eee 16

1

For the 296 eggs under observation, the maximum length of the
ego stage was 16 days, and the minimum 4 days, making 8.3 Chae
the average time required for incubation.

The eggs of the pear thrips are undoubtedly affected by tempera-
ture conditions, but rainy weather as compared with clear weather
seems to make no difference when the mean temperature is the same,
as all eggs are embedded in the moist plant tissue and do not require
additional moisture from the atmosphere.

It is evident that all of the eggs are not in the same stage of develop-
ment at the time they leave the abdomen of the female, since eggs
deposited upon the same day ranged from 4 to 16 days in the length
of the egg stage. An examination of the average mean temperature
for the various cages shows usually several degrees less mean tem-_
perature for a long egg stage in comparison with a short egg stage.

The maximum and minimum temperatures influencing the different
lots of eggs are given in Table X.

THE PEAR THRIPS IN CALIFORNIA. 43

TaBLeE X.—Mazximum and minimum temperatures during period of incubation Jor eggs
of the pear thrips, San Jose, Cal., 1910.

Maxi- Mini- Maxi- Mini-
mum mum E mum. mum
Date. temper- | temper- Date. temper- | temper-
| ature. ature. ature. ature.
CH ball oe eed
NSDL On are ees oe 73 44 Mary 28h os cee eee 64 40
MN Ee. whioe chen 72 48 DOSE Cia tiie ee nie e 69 40
Pie aC Sep Poe ae 57 48 SO eee weno eee 76 4]
Pomre sets ootoe sce eee 71 44 See 2 Coreen 78 43
ARSE Re. its renctcic so: 68 49 UN 0} nie ae Mis Os eC ine 70 45
Gy 92 83.8 Se eee 70 44 Dene RS et ati Oe a oe 63 43
(ae OS eh ae Ae a 70 53 2 Sen Pe AR eh sey 66 46
U7 hiss Ske eee ee 69 54 Nae epee UN ee a eek 7 41
U3) 21 oe ecto ee 62 50 DE mere eee eee eer 67 46
Omer ees ee ene ahd 61 48 Oar es eee ae ea 65 46
Pi Dis eae A See ae 60 dl EE co 8 ee Sap AR ode peg 64 40
| oe aa) Bok oC ete 57 47 SE A re te ne, ie aes a 66 45
De See 4 Ae a See 61 46 OTe. Sere ee settee 66 47
UR jc NOL i aor eee ae ener 57 39 INO et eee ee Seen teem ere 61 51
Oh Sth ee eee Bee 60 37 1 Ue Pene As Bees rae a 56 47
Dea oa tS ws 59 44 IAW ee eke re ete a 66 46
PAG ot al SS ee ae es 57 44 Miro ad ee ek eta a 72 41
RE et a os 51 42 aly Neg ions Se eal aig faa bs rene 74 41

NUMBER OF EGGS DEPOSITED BY A SINGLE FEMALE.

Up to the season of 1910 only conjectures had been made as to the
number of eggs a single female would deposit, but by taking indi-
viduals as soon as they emerged and placing them separately upon
twigs in the mica cages described under the heading “‘Length of egg
stage,” the total progeny of a single female was ascertained—approxi-
mately, therefore, the total number of eggs possible for one individual
to deposit. Each individual was allowed to remain undisturbed on
the twigs inside the cage. After the eggs hatched the larve were
removed and counted, yielding the following total number: Cage 1,
155 larvee; cage 2, 146 larve; cage 3, 142 larve; cage 4, 99 larvae;
cage 5, 117 larve. The maximum number of eggs laid is 155,
the minimum 99, and the average 131.8. This is probably close to
the average number of eggs that would be deposited by a single female
out in the field, although some few long-lived individuals would
perhaps exceed 200 eggs.

DEPTH EGGS ARE DEPOSITED IN TISSUE.

The eggs are deposited within the plant tissue immediately under-
neath the outer epidermis and are inclosed by the tissue. The places
where they have been deposited can readily be found with the aid of
a hand lens because of the little swellings on the stems and by the scars
left where the ovipositor had been inserted into the plant.

LARV.
FIRST APPEARANCE.
The very first larvee appear on almonds, apricots, and the early

plums, usually about the 1st of March. Larve begin to hatch on
prunes and pears the middle of March and usually are in maximum

44 BULLETIN 173, U. S. DEPARTMENT OF AGRICULTURE.

numbers in the interior valleys of Contra Costa, Sacramento, and
Solano Counties the last of March and the first 10 days of April, while
the maximum number in Santa Clara County appear the first 15 days
of April and the last ones in all the infested regions are found some
time in early May. |

TIME SPENT IN FEEDING.

The time spent in feeding, or the period required for the larve to
obtain their growth, is from two to three weeks, for individuals. For
the whole brood—that is, from the time the first larve are found on
any variety of fruit to the time the last ones are found in the trees—
a period of about two months and a half is spent, from the latter part
of February to the early part of May.

MOLTS.

After the larve have hatched and fed for some seven or eight days
they shed their skins, becoming more robust, and ovoid in shape, and
in this form they continue until they molt again into the prepupal
stage while in the ground. After the larve have molted the first
time they remain upon the tree from ten days to two weeks before
becoming full grown and dropping to the ground. The total time
spent upon the tree is from two to three weeks.

LEAVING TREES AND ENTERING GROUND.

On leaving the trees the larve do not crawl down but either fall or
are knocked off by rains or shaken off by winds. A large number
fall with the dropping calyces. Numerous instances were recorded
in the year 1910 in which heavy rains knocked off large numbers of
larvee, some of which reached their full growth by feeding upon
miner’s lettuce, which was at the time the only vegetation growing in
this orchard; but many of these immature larvee were quite small and
failed to reach full growth, which is partly responsible for the smaller
number of adults in some sections the following year, 1911. The
young and only partially grown larve that fall off the trees and do
not come in contact with any weed or grass in the orchard mostly
perish. Only the full-grown larve that fall to the ground in culti-
vated orchards work their way into soil Larve that fall off normally
do not ascend the trees again, but in some cases in cherry orchards
where foliage was near the ground on the trunks of the trees many of —
the larvee were noted to crawl back to lower foliage. This would not —
be likely to occur on pears or prunes, where there is little or no
foliage near the ground.

THE PEAR THRIPS IN CALIFORNIA. 45
HABITS OF LARVAL IN THE GROUND.

After the larvee have pentrated the soil to a sufficient depth they
hollow out for themselves a small oblong cell, the inner surface of
which is a hard, smooth wall, the cell proper being about one-half
inch long. These cells are made for safe places in which the larve
may pupate or transform to adults. It is here they spend most of
the year.

DEPTH TO WHICH LARVZ GO IN THE GROUND.

The depth that larve will penetrate the ground depends largely
upon the type of soil. Practically all of the larve go below the 3
or 4 inches of a loose topsoil mulch and establish themselves at
various depths in the harder soil below. The depths at which larve
are found in soils vary from 1 to 26 inches. Both of these are extremes
and very rarely contain many thrips. In Contra Costa, Solano, and
Santa Clara Counties from 50 to 95 per cent of the thrips do not go
below 9 to 10 inches, the gravelly soil having the highest percentage
of the larve nearest the surface. Some of the sedimentary, soils
along the Sacramento River are very open and porous—a recent
alluvial containing a great deal of decaying vegetable matter. The
larve in such soil may go much deeper, and in many cases they were
found in numbers 24 to 26 inches below the surface when none could
be found above this depth. In other cases where these light soils
have a good heavy sod, thrips have been found in large numbers from
1 to 3 inches below the surface in the cells constructed among the
erass roots.

DEPTH TO WHICH LARVZ GO IN DIFFERENT SOILS.

An absolutely definite statement as to how deep larvee will go in
the various soils, such as gravelly, sandy, sandy loam, sediment loam,
and adobe, can not be made, and only comparisons can be given from
samples taken from these various soils. On account of the local
character of thrips infestation it 1s important, when one is trying
to ascertain the depth of most of the larvee in an orchard, that several
samples be taken, to insure accuracy. The samples should come not
only from different parts of the orchard but also from various distances
and locations in the vicinity of the same trees. Soil samples for
determining the number of thrips per square foot and the depth to
which the larve go in the soil should be taken at about 2 to 4 feet
from the base of the tree.

The samples from which the records given in Table XI were made were
taken by sinking galvanized-iron cages into the soil and removing them
to the laboratory. The cages had a sliding fourth side which could be
be removed so that each /ayer could be examined by cutting off the
desired thickness and sifting the dirt upon a piece of black paper. The

46 BULLETIN 173, U. S. DEPARTMENT OF AGRICULTURE.

average depth to which larvee will penetrate in gravelly and sandy
loam soils is usually less than in heavy sedimentary loam. In those
soils which incline toward the adobe type and in the distinctly adobe
soil the larvee usually go deeper. On account of the cracking of this |
latter type of soil as it dries out in the spring, and the texture, which
is such as to prevent the making of a perfect soil mulch, suitable
places for making the cell are not found so near the surface. In soils
which can be worked readily except in cases of silt deposits or an
abnormal amount of vegetable matter below the surface, very few
larve, as a rule, penetrate to an unusual depth below the surface;
for this reason practically all the soils in the Santa Clara Valley that
are badly infested by thrips are such as render possible the obtaining of
practicable results from early fall plowing. Table XI shows the com-
parative depth of larve in a number of samples of soil taken from 10
orchards in Santa Clara County. While no sandy soil is present, these
samples represent fairly well the different types of soil of the Santa
Clara Valley.

f

47

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48 BULLETIN 173, U. S. DEPARTMENT OF AGRICULTURE.

In Contra Costa County the greater portion of the orchard area
is on the distinctively adobe soil. It is a noticeable fact that the
larve penetrate this soil to a greater depth than they do the hard
gravelly soils, probably owing to the greater prevalence of cracks.
An examination of Table XII, which is the record of the results of
soil examinations from five pear orchards and one prune orchard
during the winter of 1908—09, shows that all of the larvee in the hard
gravelly soils were within 8 inches of the surface, while in the adobe
soil only 79 per cent were found at this depth, the other 21 per cent
being between 8 and 13 inches below the surface.

TaBLeE XII.—Comparative depth of larve of the pear thrips in various soils near Walnut
Creek, Contra Costa County, Cal.

Pear and prune orchards.

Wescott and H. H.
Anderson, F. A. Bancroft, and Whit- Bancroft (pear)

man (pear), and Jones (prune) orch- orchards. Hard,
ards. Heavy loam to adobe. sandy, gravelly
soil.
24 samples. 12 samples.
Number
mn Depth.
of layer. Number | Per cent | Number | Per cent
of thrips.| above. | of thrips.| above.
| Inches.
7 ae aire! 1-2 Oo |e ae | 3 3. 33
5 eee 2-3 | OF See one 3 6. 66
eae 3- 4 Of Woese8 ae 9 14, 44
[tose eer 4-5 76 10. 33 18 36. 66
aed eee 5- 6 276 47. 83 33 73. 33
ie oy eae 6- 7 152 68. 7 18 93. 33
hy ake 7-8 82 79. 98 6 100. 00
| ee 8- 9 48 86. 23 Ose se See
QR ee: 9-10 32 90. 57 Onn |eo sata eeee
lies 10-11 42 96. 28 Oe ese ees
1 eee 11-12 24 99. 55 Ome eee cee
13 ‘| 12-13 4 100. 00 OUR eee
Total number of | |
larycer. 22s (pig | Reema eae | 90} | See eeace
Average number | |
of larve per | |
square foot..... 13a) | Son oaes ae SONA Sees

AREA AROUND DIFFERENT TREES IN WHICH THRIPS ARE MOST NUMEROUS.

The area around trees in which thrips are most numerous would
usually be within a radius of 6 to 8 feet of the base in prune orchards
where the trees are from 22 to 24 feet apart. Under prune trees
which are from 18 to 20 feet apart, and where the branches overlap,
the area infested will be more uniform, and more thrips will be-
present midway between the rows than nearer the base, as such trees,
erowing close together, usually do not have so many smaller limbs
in the center of the tree as nearer the end of the branches. Pear
trees are more upright and compact in growth; hence the greater
percentage of the larve are near the trunk of the tree, and in the

THE PEAR:..THRIPS IN CALIFORNIA. 49

average Bartlett pear orchard most of the larve in the ground are
within a radius of 2 to 3 feet of the base of the tree.

TIME SPENT AS LARVAE IN GROUND.

The time spent by larve in the ground before pupating varies.
The minimum time is about 2 months, with a maximum of about 8
months, while most of the larve will spend about 5 to 6 months
within the soil before pupating. Of many examinations of soil
samples in Contra Costa and Solano Counties no larve were found
after November 29; all had pupated prior to this time.

PUPA.

STAGES.

As soon as the white larva gets ready for transformation it sheds
its skin and develops into what is called the prepupa, which is also
white and resembles somewhat the full-grown larva, although also
having some features of the adult. In this stage the legs resemble
slightly the legs of the adult and the short wing pads extend to
about the end of the third or fourth abdominal segment. The
antenne in this stage do not project over the back, as in the case of
the pupa or second stage, but project latero-caudad. The exact
length of time spent in this prepupal stage has not been ascertained,
but from observations made upon other Thysanoptera by the writers
this stage is usually very short and in the pear thrips probably does
not last more than a week or 10 days before the prepupal skin is
shed and the insect passes into the second pupal stage or real pupa.

TIME OF FIRST, MAXIMUM, AND LAST PUPATION.

The earliest pupz are found during the month of May, and these
are very rare. It is possible that these will form late-emerging
adults, but more than likely they are premature larve that are
sickly or infected with some fungous organism which causes them
to develop prematurely. All of these early pupz probably die and
fail to reach the adult form. A few pupez can be found the latter
part of July, and there is a gradual increase in numbers through
August and September. During the month of October, however,
pupation reaches its maximum and may continue through Novem-
ber and into December, by which time it has practically ceased.

Samples taken from orchards in July and August show some
pup, while sometimes large numbers of samples taken from the
same orchards in September fail to show the presence of any. Table
XIII shows the relative number of early pupz and of larve found
in the Santa Clara Valley during the summer of 1909. Two samples
of soil were taken from each orchard for each examination.

50 BULLETIN 173, U. S. DEPARTMENT OF AGRICULTURE.

TaBLe XITI.—Comparative number of pupx and larvxe of the pear thrips found in the
soil during July and August, 1909, San Jose, Cal.

Landon and Cottle prune orchards.

Larve. Pupe.
Sample | Date ex-
Nos. amined.

Number. | Per cent. | Number.| Per cent.

1909. |
30-33...| July 15 556 99 66 1
34-37... 20 127 LOO Rie Soe oe | See ee
3841... 28 67 86 11 14
42-45...| Aug. 3 | 44 94 4 6
46-49... 17 22 LOOP bee ee ac | cece eee
50-53. - - 17 165 87 22 13
54-57... 23 65 80 13 20
58-61... 23 98 82 18 19

The time of pupation varies considerably with different orchards;
for instance, in orchards where irrigation is practiced in the early fall,
pupation probably starts at an earler date than in orchards where
this custom is not followed. Furthermore, from a number of exami-
nations made the past two years it seems evident that pupation
begins earlier in those orchards having a heavy sedimentary soil
than in orchards which have a light, gravelly soil. Fall plowing
would necessarily be more effective upon orchards which have a
gravelly soil on account of this habit of late pupation, which would
enable the owners to wait until the fall rains have started before
plowing, and also because a larger number of thrips are near the
surface.

EFFECT OF WEATHER CONDITIONS UPON PUPATION.

It is hardly probable that temperature conditions affect the length
of the pupal stage of the pear thrips very greatly, since the ground
does not freeze in the winter, except in the Eastern States, and the
mean temperature at 6 to 9 inches below the surface for the year
around is probably more even than it is above the ground. An early,
wet fall would probably cause the thrips to pupate earlier than would
be the case in a dry season.

The time spent in the pupal stage varies from one to four months,
while the normal time for most of the pupz is about two months.

ADULTS IN WINTER.

The first adults appear in the ground in late October, the number _
increasing gradually until December to early January, by which time
practically all pupe have transformed to adults. The time spent in
the ground as adults before emerging and appearing on the trees
varies from a minimum of one month to a possible maximum of five
months, averaging, however, about three months.

THE PEAR THRIPS IN CALIFORNIA. 5]
SEASONAL HISTORY.

Adult thrips first appear in early February upon the fruit buds
and continue to emerge until in the early part of April, appearing in
maximum numbers from February 22 to March 10, thus covering the
entire period of swelling of buds and blossoming of trees. By the
time the fruit buds have swollen sufficiently to separate slightly the
bud scales at the tip the adults force their way within, feeding upon
the tenderest parts of the buds. Egg laying usually begins when the
first leaf surface or fruit stems are exposed, depending somewhat upon
the variety of fruit attacked. First oviposition usually occurs the
latter part of February and the last toward the middle of April, while
maximum oviposition occurs from about March 10 to April 1. The
majority of eggs are deposited in the fruit stems, young fruit, and leaf
stems, and require from 4 to 16 days to hatch, averaging about 8 days.

By the time Bartlett pear and French prune trees are breaking
into full bloom the adult thrips have done practically all of the injury
they are able to accomplish. Injury by adult thrips is distinctly
associated with the fruit buds before blossoming.

Larve first appear in numbers toward the latter part of March and
ean be found upon the trees up to the middle of May. They appear
in maximum numbers from April 1 to April 15.

The larve feed upon the fohage and young fruit, causing on the
latter the well-known thrips scab, and individuals remain on the
trees for two to three weeks in attaining their growth, the entire
brood of larve requiring 8 to 10 weeks from the first-appearing to the
last-disappearing individuals.

All of the larvze have dropped from the trees by the middle of May
and penetrated the soil toa depth of from 1 to 26 inches, depending
upon the type and condition of same, in most cases the majority
being within 8 to 9 inches of the surface.

Sometimes in July a few larve transform into the tender pupe,
and by October the pupz are in maximum numbers, the last larve
pupating in November. ‘The pupal stage lasts from one to four
months, the usual time being about two months.

Early in February adults, which, in some instances, have remained
as such for several months in the ground, appear upon the trees and
wait for the first opening of buds, when they begin the work of

destruction.
NATURAL ENEMIES.

Probably no single order of insects of such great economic impor-
tance has so few effective natural enemies as the Thysanoptera.
This is partly due to the small size of the insects belonging to this
order, their manner of working, their great activity, their unique life
history, and the fact that not more than six or seven species in the
order have ever accomplished any great economic damage. Practi-

a2 BULLETIN 173, U. S. DEPARTMENT OF AGRICULTURE.

cally all the attempts to control the thrips by artificial means have
been within the United States. Of the few natural enemies of Thy-
sanoptera that do exist, the most important seems to be Triphleps
insidiosus Say, which feeds upon thrips by impaling them upon its
beak and sucking out the juices. Megilla maculata De G., chrysopid
larvee, and syrphid larve have also been found feeding upon thrips.
Uzel' has found Triphleps minutus L. preying on thrips and crecits
Heeger with the finding of Scymnus ater Kug., Gyrophaena manca Er.,
and some fly larve feeding m the same manner. Hinds? mentions
having found some small scarlet acarid attached to the membranous
area of the body of Anaphothrips striatus Osborn. Uzel*t and Quaint-
ance * have both found eggs of nematode worms within the bodies of
adult thrips. J.C. Crawford*in December, 1911, gives a short account
of Thripoctenus russelli Crawford, a new internal parasite of Thy-
sanoptera and later Russell® publishes a more complete account of the
hfe history and habits of this parasite. The first recorded host of T.
russelli was Heliothrips fasciatus Pergande, but it has been reared from
Thrips tabaci Lind. and Frankliniella tritici Fitch. Its oviposition
has been observed in Heliothrips femoralis Reuter and H. haemor-
rhoidalis Bouché. Great hopes were entertained by Mr. Russell for
its colonization among related injurious Thysanoptera.

Of plant parasites, Thaxter ® has taken an Empusa fungus destroy-
ing a species of thrips in the larval, adult, and pupal stages, and
Petit’ and Hinds® have found a fungus which they thought was
causing some of the species of thrips to die. :

No effective natural enemy has been found preying upon the
pear thrips. Moulton ® mentions some raphidians feeding upon the
younger forms of this species and has also found a species of ant
killing individuals. He mentions * a fungus which he regarded as
parasitic during the season of 1905 and 1906, but the last three or
four years have failed to show that any appreciable amount of
benefit has been derived from it. Very little of the fungus has been
observed during the years 1908, 1909, and 1910.

1 Uzel, Heinrich. Monographie der Ordnung Thysanoptera. KOniggratz, 1895, 472 p. 10 pl. See p. 362.

2 Hinds, W. E. Contribution to a Monograph of the Insects of the Order Thysanoptera Inhabiting
North America. Jn Proc. U.S. N. Mus., vol. 26, p. 119, 1902.

3 Quaintance, A. L. The Strawberry Thrips and the Onion Thrips. Fla. Agr. Exp. Sta., Bul. 46,
p. 79-114, 12 figs. July, 1898.

Crawford, J.C. Twonew Hymenoptera. Jn Proc. Ent. Soc. Wash., v. 13, no. 4, p. 233-234, 1911.

5 Russell, H. M. An Internal Parasite of Thysanoptera [ Thripoctenus russelli]. U.S. Dept. Agr.,
Bur. Ent., Tech. Ser. no. 23, pt. 2, p. 25-52, figs. 11, Apr. 27, 1912.

6 Thaxter, Roland. The Entomophthoreae of the United States. In Mem. Boston Soc. Nat. Hist.,.
v. 4, no. 6, p. 134-201, pls. 14-21, Apr., 1888. Seep. 151, 172, 174, pl. xvii, figs. 200-219.

7 Pettit, Rufus H. Some Insects of the Year 1898. Mich. State Agr. Coll. Exp. Sta., Bul. 175, p. 341-373,
20 figs, July, 1899. See p. 343-345, figs. 1, 2.

eHGOCs Cit:

3 Moulton, Dudley. The Pear Thrips (£uthrips pyri Daniel). U.S. Dept. Agr., Bur. Ent., Bul. 68,
pt. 1, rev., p. 14, Sept. 20, 1909. .

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