UNIVERSITY OF CALIFORNIA
COLLEGE OF AGRICULTURE
AGRICULTURAL EXPERIMENT STATION
BERKELEY, CALIFORNIA
The Emergence of Pear Thrips in the
Healdsburg Area of California
in 1932
LESLIE M. SMITH
BULLETIN 562
NOVEMBER, 1933
UNIVERSITY OF CALIFORNIA
BERKELEY, CALIFORNIA
ACKNOWLEDGMENTS
Credit for the successful operation of the thrips traps is largely due
to H. A. Weinland, Farm Advisor, Sonoma County, and numerous
growers who cooperated with the University in this test. G. E. Stanley,
manual training teacher in the Healdsburg High School, and his stu-
dents constructed the traps.
THE EMERGENCE OF PEAR THRIPS IN THE
HEALDSBURG AREA OF CALIFORNIA IN 19321
LESLIE M. SMITH2
Unsatisfactory control of pear thrips, Taeniothrips inconsequens (Uzel) ,
has frequently been reported within the last few years by the prune
growers in the Healdsburg district of Sonoma County, California. Many
growers were of the opinion that the thrips emerged from the soil for a
much longer period in this area than is usually the case in other areas.
It was suggested that the emergence from the soil in the Santa Clara
Valley, where sprays and dusts have proved effective, might be much
shorter, so that one or two applications yielded an effective control. In
order to gather definite information on the emergence of thrips in the
Healdsburg area, forty-five traps were distributed through the district,
and the data given in the following pages were gathered.
METHOD
Each trap was shaped like a four-sided pyramid (figs. 1 and 2), with
the base 3 feet square and the apex 2 feet in height. The base was con-
structed of 1 x 2 inch redwood. From each corner of the base a piece of
lxl inch redwood extended at an angle of about 45° to a small block
% x 3 x 3 inches at the top. In the center of the top of the block a hole was
bored half way through of such size as to admit a small vial. This hole
was continued by a smaller hole which opened on the lower side of the
block (fig. 1C) . A vial could be inserted in the hole in the top of the block,
and the rim of the smaller hole formed a shoulder on which the mouth
of the inverted vial rested when installed.
The frame was covered with a cheap but close-mesh muslin. The cloth
was applied to the inside of the frame in order to reduce recesses which
would impede the progress of the thrips or serve as hiding places for
them. The cloth was tacked to the frame, the edges were then covered
with half-round screen molding, and three coats of Tree-White were
applied to the inside (fig. IB) . White was selected as the most desirable
i Eeceived for publication October 11, 1932.
2 Junior Entomologist in the Experiment Station.
[3]
University of California — Experiment Station
Fig. 1. — A, Exterior of trap. The hole in the top is for the insertion of the vial.
B, Interior of trap. C, Section of vial, cone, and block at the top of the pyramid,
showing arrangement and method of insertion.
Bul. 562] The Emergence of Pear Thrips 5
color, since it would contrast with any adult, or "black" thrips which
failed to pass from the pyramid into the vial, and by inverting the trap
and counting the thrips on the inside the efficiency of the trap could be
determined.
Although three successive coats of paint were applied to the inside of
the traps, there still remained minute openings through the cloth, which
in some cases were large enough to permit a thrips to pass through. As a
check on the efficiency of the cloth traps, a number of traps were covered
on the outside with a medium-weight roofing paper which was strapped
along the edges by screen molding.
Fig. 2. — Traps in position in the field. Left, cloth ; right, paper-covered trap.
The vials inserted in the tops of the cones were prepared to function
as traps, similar to the common fly trap. A small truncate cone was glued
in the mouth of each vial. In order not to impede the passage of light,
necessary to attract the thrips to the top of the trap, the cone inside of
the vial was constructed of transparent paper (Cellophane) and held in
place by water-proof cement.
The traps were placed in the hands of the growers, with mimeo-
graphed directions describing the procedure to be followed and with
materials to enable them to carry out the instructions. Growers were re-
quested (1) to remove the vial from the top of the trap every second day,
at the same hour, preferably in the morning, and replace with an empty
vial; (2) to place a cotton plug in the end of the vial and pour on it a
few drops of ether to kill the thrips; (3) to mark the vial with the trap
number and date removed; (4) to move the trap to a new location in the
same vicinity at the time of changing the vial, that is, every second day.
6
University of California — Experiment Station
If the trap had stood on bare soil (that is, on soil from which the cover-
crop had been cut immediately before the trap was placed) they were
requested to cut off the covercrop before placing the trap, otherwise to
place it over the covercrop. The traps were moved every second day in
order to avoid any change in soil temperature which might have resulted
had the trap stood for a long period on the same square yard. Each time
after the traps were moved, the lower edges of the frames were covered
with soil to exclude light. At regular intervals the vials were collected,
and the thrips removed and counted. The traps were distributed in the
Healdsburg area in such a way as to cover as wide a variety of cultural
practices as possible. An attempt was made to gather information on the
relation of soil types, irrigation, and covercrops to thrips emergence.
In order to facilitate a comparison of results, each catch was divided
equally between the days during which the vial was on the trap. With
very few exceptions, the cooperators followed directions and changed
the vials every second day. Traps were placed in representative orchards
which were known to have been infested with thrips the preceding
spring. Each trap was placed near the base of a tree, under as much of
the top as possible (fig. 2), since it was presumed that the majority of
thrips drop straight to the ground and transform and emerge without
much lateral movement.
RESULTS
Efficiency of the Traps. — During the first few days of the emergence
several questions bearing on the efficiency of the traps were settled. A
comparison was made between the catches in cloth-covered traps and
those in cages which, in addition to cloth, were covered with roofing
TABLE 1
Efficiency of Types of Traps
Total thrips caught
Type of trap
First set
Second set
Third set
Average
37
190
1,373
571
1,623
2,165
1,011
Cloth
975
paper. In several cases these two types of traps were placed within a
few feet of each other and kept in this relation throughout the period
of this test. The total catches of three such pairs of traps were as indi-
cated in table 1.
Bul. 562] The Emergence op Pear Thrips 7
From these counts it may be seen that the traps covered with roofing
paper in addition to cloth were no more efficient than traps which were
covered with cloth alone. The counts from these traps are included
among those given in table 2. During the course of the experiment sev-
eral factors appeared which indicated the superiority of the cloth alone.
It was found that the temperature and humidity rose much higher in the
paper-covered traps. Water condensed on the cloth and in the vial and
interfered with the counting of the thrips. The paper-covered cages
were more expensive to construct, heavier to handle, and more apt to
break or puncture.
On numerous occasions when the vial at the top of the trap was found
to contain a considerable catch, the trap was inverted and examined on
the inside for thrips. Thrips were rarely found on the sides of the trap,
and at most in very small numbers. From such observations it was con-
cluded that after leaving the soil, or the covercrop, the thrips moved
rapidly up the sides of the pyramid and entered the vial at the top.
Occasionally spiders caught in the traps constructed webs over the
mouths of the vials and preyed upon the thrips and thus reduced the
efficiency of the traps. Such spiders were removed as frequently as
found. They rarely interfered with the operation of the trap for more
than two days.
Total Emergence. — Twenty-one traps were placed in the field on Feb-
ruary 18. On February 21, eleven of these had caught some thrips. The
total accumulated catch on this day was 44. When this catch is divided
as equally as possible between the three days during which the thrips
were caught, and the odd thrips included with the catch on the latest
day, the average catch per trap was as follows :
Daily emergence, Cumulative total,
Date, 1932 thrips per sq. yard thrips per sq. yard
February 19 0.5 0.5
February 20 0.7 1.2
February 21 1.0 2.2
While a few thrips may have emerged prior to February 19, the number
must be very small and for the purposes of this paper, may be disre-
garded. The emergence on February 18 and preceding days is regarded
as zero in the following computations.
The total number of thrips caught in the Healdsburg section (table 2)
has been utilized in preparing the graphs of total emergence, figure 3A
and B.
8 University of California — Experiment Station
These data include emergences from irrigated and nonirrigated soils,
light, medium, and heavy soil types, and various covercrop conditions.
They may therefore be regarded as covering nearly all possible times of
emergence, and present an approximation of the average emergence of
all conditions. No records of soil temperatures were kept. The mean daily
TABLE 2
Total Emergence in the Healdsburg Area
Total
thrips
Num-
ber of
traps
Average thrips
per square yard*
Date, 1932
Total
thrips
Num-
ber of
traps
Average thrips
per square yard*
Date, 1932
Daily
Cumu-
lative
total
Daily
Cumu-
lative
total
/
2
3
4
5
1
2
3
4
5
Feb. 19
Feb. 20
Feb. 21
10
15
20
22
128
249
487
501
586
1,079
1,318
902
1,293
1,505
952
637
915
1,018
1,949
21
21
21
21
21
35
39
41
42
41
41
40
40
39
38
42
41
41
43
0.3
0 5
0.7
0.7
4.0
4.6
8.1
7.9
9.1
17.1
20.9
14.7
21.0
18.6
16.3
9.9
14.5
16.1
29.5
0 3
0.8
1.5
2.2
6.2
10.8
18 9
26.8
35.9
53.0
73.9
88.6
109.6
128.2
144.5
154 4
168.9
185.0
214.5
Mar. 9
Mar. 10
Mar. 11
2,208
1,487
1,148
1,103
975
765
791
614
335
231
210
149
175
76
39
43
26
14
3
43
43
43
43
43
43
41
41
39
38
38
36
35
26
26
24
20
18
18
33.4
22.5
17.6
16.7
14 8
11.6
12.6
9.8
5.6
4.0
3.6
2.7
3.3
1.9
1.0
0.9
0.8
0 4
0.1
247.9
270.4
288 0
Feb. 22
Mar. 12
304.7
Feb. 23
Mar. 13
319 5
Feb. 24 .
Mar. 14
331 1
Feb. 25
Mar. 15
343.7
Feb. 26
Mar. 16
353.5
Feb. 27
Mar. 17
359 1
Feb. 28
Mar. 18
363.1
Feb. 29
Mar. 19
366.7
Mar. 1
Mar. 2
Mar. 3
Mar. 20
Mar. 21
Mar. 22
369.4
372.7
374.6
Mar. 23
375 6
Mar. 5
Mar. 6
Mar. 24
Mar. 25
376.5
377 3
Mar. 7
Mar. 26
377 7
Mar. 8
Mar. 27 .
377.8
* Since some of the traps were on bare soil and some on the covercrop, the counts given in these
two columns have been corrected by the formula:
10TW10_84n
AT
H^j
43
in which 10 is the number of traps on bare soil. 33 is the number of traps on covercrop (see covercrop
data, table 3), 43 is the total number of traps, T is the total thrips caught on any day (given in col. 2 of
table 2), N is the number of traps on the same day (given in col. 3 of table 2), 10.84 is the average thrips
per square yard emerging throughout the season from clean soil, and 19.82 is the number of thrips per
square yard emerging throughout the season from soil under a covercrop. This formula is used to subtract
those thrips which were on the covercrop at the time the traps were placed over it.
temperatures recorded at Santa Rosa are indicated by the dashed line
in figure 3 A. The arithmetical mean of the curve of emergence falls on
the twentieth day of the test, or on March 9. The curve shown in figure
SA approximates the form of the normal curve except between the first
and seventh of March. At this time a sharp depression in the numbers
of thrips occurred. A study of the graph (fig. 3 A) will show that this
depression is correlated with a sharp depression of the mean tempera-
Bul. 562]
The Emergence of Pear Thrips
9
ture. During the first three days of lower temperatures the number of
thrips emerging increased slowly; during the second three days of cold
weather, the number emerging showed a marked decrease. It may be that
the soil retained sufficient heat from the warm period of February 27-29
to account for the slight increase in emergence of thrips on March 2.
The drop in mean temperature is therefore probably responsible for the
departure in emergence from the expectancy indicated by the cumula-
tive curve in figure 3B.
February March
Fig. 3. — Emergence in the Healdsburg area in relation to temperature. The con-
tinuous line shows the emergence; the dashed line shows the mean temperature,
based on three-day averages. (Data from table 2.) B, Accumulated total of average
thrips per square yard. (Data from table 2.)
10
University of California — Experiment Station
The total number of thrips emerging during the entire season aver-
aged 377.8 per trap. This means that in orchards that were infested in
the spring of 1931 every square yard that was overhung by branches
gave egress to 378 thrips, on the average, this spring. The total period of
emergence extended from February 19 until March 27, or a total of 38
days. The average daily emergence throughout the entire period from
February 19 to March 27 was 9.9 thrips per square yard. The largest
catch was 1,234 thrips in two days, or 617 thrips per square yard per
day. Significant emergence, arbitrarily selected at 5 or more thrips per
square yard, extended from February 23 to March 21, or a total of
28 days.
TABLE 3
Comparison of Significant Emergence Periods, Healdsburg and San Jose
Locality
Year
First
emergence
Mode
Last
emergence
Length of
emergence
period.
days
San Jose
1909
Feb. 15
Mar. 3
Mar. 20
33
San Jose
1910
Feb. 9
Mar. 4
Mar. 17
36
San Jose
1911
Feb. 7
Mar. 12
Mar. 18
39
San Jose
1912
Feb. 6
Mar. 1
April 8
62
Healdsburg
1932
Feb. 23
Mar. 9
Mar. 21
28
The traps were moved every second day, as stated above, to a new loca-
tion. In order to gather information regarding the total number of
thrips in a selected square yard, and to obtain data for comparison with
traps which were moved, three traps were left stationary throughout
the emergence period on the spot where they were first placed. These
traps caught a total of 311, 1,853, and 1,211, or an average of 10.0, 66.2,
and 60.6 thrips per da}-, respectively. These three traps computed to-
gether show an average of 42.6 thrips per square yard per day, which is
considerably in excess of the average emergence of all the traps, 9.9.
A comparison of the length of the emergence period in the Healds-
burg area with that in the San Jose area is possible by contrasting the
data herein presented with that obtained by Foster and Jones3 in San
Jose. In table 3 "first emergence" and "last emergence" refer to "signi-
ficant emergence" as described above. The dates for San Jose, 1909-1912,
are taken from Foster and Jones. Although the data from the two studies
were obtained by somewhat different techniques, they are sufficiently
comparable to indicate that the emergence period in the Healdsburg
s Foster, S. W., and P. E. Jones. The life history and habits of the pear thrips in
California. U. S. Dept. Agr. Bui. 173:33, table IV.* 1915.
Bul. 562] The Emergence of Pear Thrips 11
area is not appreciably longer than in the San Jose area. Foster and
Jones reared thrips from blocks of soil removed intact from infested
orchards, and carried to their laboratory. In this paper, their data has
been computed to thrips emerging per square yard of surface soil, and
again significant emergence is recognized as 5 or more thrips emerging
per square yard per day.
Covercrop.- — Covercrops are generally believed to retard thrips emer-
gence. This may be due to the fact that the covercrop shades the soil and
maintains a lower temperature therein. No study of such a hypothesis
was conducted during the test, but the relation of the total emergence to
the mean temperature, already discussed, lends some support to it.
A second method by which the covercrop may delay the appearance
of the thrips consists in retarding the flight of the emerged adults to the
trees. In other words the covercrop may serve as a temporary abode for
the adults. In order to obtain information on this question, ten of the
forty-three traps reported upon in table 2 were placed on bare soil, that
is, on undisturbed soil from which the covercrop was cut, just prior to
placing the traps. These traps were moved every second day, and each
time the covercrop was cut and removed before they were placed. In
table 4 the daily average per trap computed from ten traps on clean soil
is contrasted with the daily average per trap of thirty -three traps which
were moved every second day and placed over fresh covercrop. As will
be shown later, the emergence per square yard is greatly modified by soil
types. Although the traps used in this covercrop test were placed on all
types of soils, the soil factor does not render the results incomparable
since of the bare-soil traps 70.0 per cent were placed on medium and
heavy soils and of the covercrop traps 78.8 per cent were placed on soils
of this type.
It was assumed that the average number of thrips emerging from
freshly cleaned soil would approximate the number of thrips emerging
from soil under a covercrop. Any additional thrips caught in traps on
the covercrop were presumed to have emerged earlier and to have been
resting on the covercrop when the trap was placed over it. The number
thus present on the covercrop (column 4 in table 4) is computed as the
difference between catches from the covercrop and from bare soil. The
thrips on the covercrop probably enter the vial during the first twenty-
four hours after the trap is placed because of their pronounced positive
phototropism. During the second twenty-four hours the catch in cover-
crop traps is increased only by the further emergence from the soil.
Since the traps were moved only once in two days, these two values are
12
University of California — Experiment Station
averaged. If the traps had been moved every day, the number of thrips
caught in the covercrop traps would probably have been nearly doubled.
These data show that the number of thrips per square yard remaining
on the covercrop, after emerging, increased from none on February 19
TABLE 4
Number of Thrips Remaining on Covercrop
Date, 1932
Average thrips
caught in traps
on covercrop
Average thrips
caught in traps
on bare soil
Average thrips
per square y arc-
remaining on
covercrop
J
2
S
4
Feb. 19
Feb. 20
Feb. 21
0 5
0.7
0.9
1.1
6.1
9.8
12.9
12.2
14.9
28.7
34.3
25.2
39.6
0.5
0.5
1.0
0.0
4.3
7.0
12.3
11.0
11.1
17.3
17.9
11.0
6.4
0.0
0.2
—0.1
Feb. 22
1.1
Feb. 23
1 8
Feb. 24
2 0
Feb. 25
Feb. 26
0.6
1.2
Feb. 27
3.8
Feb. 28
Feb. 29
Mar. 1
Mar. 2
11.4
16.4
14.2
33.2
Mar. 3
42.6 17.1
25 5
Mar. 4
Mar. 5
24.2
17.1
26.3
30.5
49.7
55.7
36.6
27.8
31.6
30.0
21.3
20.5
15.8
10.4
7.3
6.1
3.8
5.4
4.4
654.0
19.8
15.2
6.6
8.1
7.4
31.0
36.9
31.7
25.9
14.8
10.9
6.3
15.8
11.9
3.6
2.2
3.5
4.5
3.1
0.9
357.7
10.8
9 0
10.5
Mar. 6
Mar. 7
Mar. 8
18.2
23.1
18.7
Mar. 9
18.8
Mar. 10
Mar. 11
4.9
1.9
Mar. 12
16.8
Mar. 13
Mar. 14
Mar. 15
Mar. 16
Mar. 17
Mar. 18
19.1
15.0
4.7
3.9
6.8
5.1
Mar. 19
Mar. 20
Mar. 21
Mar. 22
Total
Average
2.6
-0.7
2.3
3.5
297.1
9.0
to 33.2 on March 2. From March 2 on, the number of thrips remaining
on the covercrop decreased. From this standpoint alone, it would seem
that the covercrop acted as an important retarding factor up to March 2,
at which time its importance in this connection began to decrease, so
Bul. 562]
The Emergence of Pear Thrips
13
that the orchards could have been disked any time after March 2 with-
out appreciably affecting the numbers of thrips on the trees.
Irrigation. — Fall irrigation has been both recommended and con-
demned in the literature as a means of destroying pear thrips in the soil.
TABLE 5
Effect of iRRiGATioisr on Thrips Emergence
Date, 1932
Feb. 19
Feb. 20
Feb. 21
Feb. 22
Feb. 23
Feb. 24
Feb. 25
Feb. 26
Feb. 27
Feb. 28
Feb. 29
Mar. 1
Mar. 2
Mar. 3
Mar. 4
Mar. 5
Mar. 6
Mar. 7
Mar. 8
Mar. 9
Mar. 10
Mar. 11
Mar. 12
Mar. 13
Mar. 14
Mar. 15
Mar. 16
Mar. 17
Mar. 18
Mar. 19
Mar. 20
Mar. 21
Mar. 22
Total
Average
3 irrigations
2 irrigations
2.0
3.0
3.0
1.0
2.0
22.3
25.0
25.3
25.3
16.7
16.7
0.0
0.0
14.7
14.7
1.0
1.3
2.0
0.3
0.3
3.3
4.0
1.7
1.7
0.3
5.0
1.0
0.0
0.3
1.0
0.3
0.7
0.7
6.0
irrigation No irrigations
Average thrips per square yard
0.5
0.5
1.0
0.5
34.0
27.7
83.0
83.0
83.7
136.0
136.3
2.0
2.5
31.5
32.0
4.3
4.3
5.0
5.0
5.0
7.3
7.7
1.3
2.0
0.0
0.0
0.7
0.3
0.7
0.0
2.0
2.0
2.0
703.8
21.3
0.0
0.0
0.0
0.0
1.0
2.0
1.8
1.8
1.8
0.3
0.7
0.0
0.0
3.0
4.0
4.5
4.8
5.3
6.8
7.3
10.0
10.3
63.0
63.5
33.5
33.8
34.3
11.5
11.5
12.0
5.3
5.3
6.3
345.4
10.5
0.0
1.0
1.0
0.0
1.0
0.7
1.3
1.3
1.3
1.7
1.7
49.7
49.7
14.0
14.3
17.0
17.3
17.7
391.3
11.9
With the cooperation of Blaine McClish the following test of the effi-
ciency of irrigation was conducted. Portions of his orchard were irri-
gated once, twice, and three times, while a fourth portion was not
irrigated. The contour-check system of irrigation was used. At each
irrigation, the land received approximately 6 acre-inches of water. The
14
University of California — Experiment Station
three irrigations were applied on July 21, August 28, and September 29;
the two irrigations were applied on August 28 and September 29; and
the single irrigation was applied on September 29. Three emergence
traps were placed in each irrigated area and three in the nonirrigated
portion. The counts from these traps were included among those given
TABLE 6
Emergence of Thrips from Heavy and Light Soils
Date, 1932
Heavy and
medium soils,
average thrips
per square yard
Light soils,
average thrips
per square yard
Increase in
average thrips
per square yard
surviving in
heavy soils
Feb. 19
0.6
0 0
0.6
Feb. 20
0.9
0.0
0.9
Feb. 21
1.1
0 0
11
Feb. 22
1.3
0.0
1.3
Feb. 23
7.8
0.0
7.8
Feb. 24
11 3
1.4
9.9
Feb. 25
16.6
14.6
0 4
0.8
16.2
Feb. 26
13.8
Feb. 27
18.9
1.6
17 3
Feb. 28
34.7
1.8
32.9
Feb. 29
41.1
28.6
41.7
47.9
28.4
2.2
2.7
3.7
5.0
4.2
38 9
Mar. 1
25.9
Mar. 2
38.0
Mar. 3
42.9
Mar. 4
24 2
Mar. 5
18.0
3.8
14 2
Mar. 6.
27.8
3.8
24.0
Mar. 7
31.7
4 3
27.4
Mar. 8
58.8
4 1
54 7
Mar. 9
67.1
3.3
63.8
Mar. 10
25.8
4 0
21.8
Mar. 11
33.8
7.3
26.5
Mar. 12
34.6
7.0
27.6
Mar. 13
32 1
5.4
26.7
Mar. 14
20.8
10.4
10 4
Mar. 15
21.6
11 8
9.8
Mar. 16
16.3
10 .2
6.1
Mar. 17
9.6
7.0
2.6
Mar. 18
6.4
5 5
0 9
Mar. 19
5.4
5 1
5.9
13
-0 5
Mar. 20
3.8
Mar. 21
5.8
1.5
4 3
Mar. 22
3.2
0.7
i.i
Total
719 4
121.1
599 3
Average
218
3.7
18.2
in table 2. The data in table 5 were gathered. These data are so irregular
that it is difficult to base an opinion on them. They indicate, however,
that under some conditions as many as two irrigations applied late in
August and September fail to reduce materially the number of thrips
which emerge the following spring.
Bul. 562] The Emergence of Pear Thrips 15
Several orchards were flooded in December, 1931, when, as a result
of heavy rains, the Russian River overflowed its banks. Three traps were
placed on situations that had been submerged from 1% to 6 feet deep,
and for periods of one to several days. The data from these traps are in-
cluded in table 2. These traps caught a total of 277, 1,334, and 1,894
thrips, or an average of 40.3 thrips per square yard per day. This is con-
siderably greater than the average daily emergence, 9.9.
The data gathered on emergence from irrigated and flooded land in-
dicate that under certain conditions even large amounts of water are not
inimical to thrips survival.
Soil Types. — The soils involved in this test were classified before the
emergence into three groups: heavy, medium, and light. The data in
table 6 are based on ten traps on light soils and thirty traps on heavy
and medium soils. The data from these traps are included in table 2.
These two latter types are treated as one, since no appreciable differences
existed in numbers of thrips emerging from them. All of the traps were
placed in orchards where thrips were known to have been injurious the
preceding spring, so that the numbers of thrips which entered the
various soil types were probably similar.
These data show a much greater emergence of thrips from heavy than
from light soils. The emergence from heavy soils was more than twenty
times as great as that from light soils on March 9 and averaged about
six times as great throughout the period of emergence.
Control Measures to Reduce the Thrips Population the Following
Spring. — Considerable importance is attached to the possibility of kill-
ing the larvae while still on the trees, in order to reduce the thrips popu-
lation the following spring. During the spring of 1931 various growers
applied sprays and dusts in an attempt to control adults and larvae.
Many of the traps used in these emergence tests were placed in orchards
which had been sprayed and dusted the preceding spring. The types of
applications and emergence of thrips from them are given in table 7.
These data fail to show any correlation between spray practice and
numbers of thrips emerging the following spring. However, it should be
borne in mind that the amount of spraying which is done is generally in
proportion to the amount of injury or numbers of thrips present, so that
the orchards which received multiple applications may be regarded as
having the heaviest infestations. The counts indicate that as many as
three to six applications of sprays and dusts failed to reduce the thrips
population to a negligible number.
16
University of California — Experiment Station
TABLE 7
Results of Sprays and Dusts Applied the Preceding Year
Control measures
Material
Nicotine dust
Nicotine dust
Nicotine dust
Nicotine spray
Soap and nicotine
Nicotine dust
Nicotine dust
Nicotine dust
Nicotine spray
Nicotine dust
Nicotine spray
Nicotine dust
Nicotine spray
Nicotine dust
Nicotine spray
Nicotine dust
Nicotine dust
None
None
None
None
None
Number of
applications
Number
of
traps
Average daily
emergence
per square yard
1.5
11.1
15.8
4 1
44.5
12
5 5
11 1
3.7
43 2
0.0
1.0
27.7
3 5
8 1
SUMMARY
A satisfactory type of trap suitable for gauging thrips emergence was
devised.
The total emergence occurred over a period of 38 days, from February
19 until March 27. The peak of the emergence (empirical mode) and the
arithmetical mean occurred on March 9. The normal curve of emergence
was distorted by the influence of temperature.
A covercrop was found to delay the movement of thrips to the trees,
after they had emerged from the soil. The number of thrips remaining
on the covercrop reached a maximum of 33.2 thrips per square yard on
March 2.
Irrigation and natural flooding did not appreciably reduce the emer-
gence.
Heavy soils showed a much greater emergence than light soils. The
average daily emergence per square yard from heavy soils was 21.8,
from light soils 3.7.
Control measures applied in the spring of 1931 did not produce
demonstrable results in 1932.
7m-ll,'33
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