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CIRCULAR No. 18 JANUARY, 1928

UNITED STATES DEPARTMENT OF AGRICULTURE
WASHINGTON, D. C.

EXPERIMENTAL DISSEMINATION OF iE
TABANID EGG PARASITE PHANURUS EMERSONI GIRAULT
AND BIOLOGICAL NOTES ON THE SPECIES

By D. C. ParmMan ~

Assistant Entomologist, Division of Insects Affecting Man and Animals,
Bureau of Entomology

CONTENTS
Page Page
VOSGES ELOY (EC CONT he ea es 1 | Tabanid egg collecting and parasite
Distribution and hosts of the para- dissemination e222 Usk eee ee 5
ST ea aa SS ee eee ae Sa 2 | Some factors that have probably con-
Physical and climatological conditions tributed in controlling the horse-
of the parasite breeding grounds UTC Seah Sos ai ele RE aN 6
in southwestern Texas —_ DT CONCIUSTO MS heey ace a Ee ii 6
Life cycle and seasonal activities of
themtparasite = 25 ose e ae os 2
INTRODUCTION

For a number of years prior to 1913 stockmen whose ranches lie
along the south escarpment of the Edwards Plateau, north of Uvalde,
Tex., experienced considerable trouble from the attacks of horseflies
on their cattle and horses. During the summer of 1913 a rather
severe outbreak of anthrax occurred in this region, and circumstan-
tial evidence, as well as some investigational work done by the State
board of health, indicated that these horseflies played a part in the
transmission of the disease. This combination of conditions made a
study of the horseflies and their control of distinct economic im-
portance.*

The study was begun in the summer of 1914, and- during the
first days of September of that season an ege parasite was found,
which was later described as a new species, Phanurus emersont, by
Girault.2, The horsefly eggs were so abundant at Laguna Crossing
on the Nueces River, 20 miles north of Uvalde, that a boy was able
to collect 9 pints of ege masses, or approximately 2,000,000 eggs, in
one day. In sample masses of these eggs, 97 per cent of the masses
were parasitized, and from 6 to 83 parasites emerged from each mass.
From these data it was inferred that the most feasible means of

1The work was initiated and carried on under the direction of F. C. Bishopp, and the
writer is indebted to W. E. Dove for assistance in summarizing the notes and for sugges-
tions as to preparation of manuscript and illustrations.

2GIRAULT, A. A. A NEW PHANURUS FROM THD UNITED STATES, WITH NOTES ON ALLIED
Species. Can. Ent. 48: 149-150, 1916.

67824—28

2 CIRCULAR 18, U. S. DEPARTMENT OF AGRICULTURE

control of the fly would be to collect the horsefly eggs and place them
along the streams in vessels which would allow the parasites to
escape but which would retain the fly larvae to be destroyed. This
circular reports some biological studies of the egg parasite and ex-
perimental work in horsefly control by egg collection and parasite

= = fealtoo)
d_ssemination. f

DISTRIBUTION AND HOSTS OF THE PARASITE

The parasite has been reared from the eggs of 7'abanus hyalinipen-
nis Hine from the counties of Sutton, Edwards, Kinney, Menard,
Kimble, Kerr, Bandera, Real, Uvalde, Zavalla, Medina, Bexar,
Comal, Kendall, Blanco, Gillesp e, Mason, Llano, San Saba, and
McCulloch in southwestern Texas. The parasite was found in most, if
not all, of these counties before general d‘stributions were made.
Webb and Wells® reported it as parasitizing practically every egg
mass of 7’. punctifer O. S. at Topaz, Calif. Cameron states that
eggs of Chrysops moerens Walker and C. mitis O. S. were fairly
commonly parasitized by TYrichogramma minutum (Riley) and
Phanurus emersoni in western Canada.

PHYSICAL AND CLIMATOLOGICAL CONDITIONS OF THE PARASITE
BREEDING GROUNDS IN SOUTHWESTERN TEXAS

The contour of the south escarpment of Edwards Plateau may
be- described as rather rugged. The elevation of the plateau is
about 2,300 feet above sea level, and the elevation of the stream
beds where they leave the valleys and enter the Gulf plains is about
1,000 feet. Some of the peaks and divides near the mouths of the
valleys are about 2,000 feet above sea level, and there are many high,
rocky cliffs. The parasite has been found to breed at elevations of
from 800°to approximately 2,000 feet. The escarpment is cut by
narrow valleys from 20 to 60 miles long, with many shorter tribu-
taries leading out. Nearly ail of these valleys have permanent
springs in the heads, and most of the main valleys have permanent
streams, some of considerable size. They all have heavy growths
of timber along the streams, and in places there are small swampy
areas with dense vegetative growths. The heads of the streams
are narrow and are more or less shaded, but in the lower parts of
the valleys they spread out over gravel beds 100 or more yards
wide and are exposed to the sun, since flood waters have stunted
the timber growth for some distance from the main channels.

The climate of this area is semiarid, with hot summers, usually
dry, and mild winters. During the summer the max?mum tempera-
tures are generally near 100° F., the nights are cool, and the mini-
mum temperatures are usually about 70°. The minimum tempera-
tures during the winters are rarely below 20°. The annual rainfall
at Uvalde has varied from 12 to 40 inches, with an average of
approximately 21 inches. The annual rairfall increases toward
the east and diminishes westward. Usually more rain falls during
the months of May and September than during other months.

8 WeBB, J. L., and WELLS, R. W. HORSEFLIES: BIOLOGIES AND RELATION TO WESTERN
AGRICULTURE. U.S. Dept. Agr. Bul. 1218, 35 pp.. illus., 1924.

4CamerRoN, A. E. BIONOMICS OF THE TABAN!DAE (DIPTERA) OF THE. CANADIAN PRAIRIE,
Bul. Ent. Research 17 (pt. 1) : 1-42, illus., 1926.

TABANID EGG PARASITE PHANURUS EMERSONI GIRAULT 3

The eggs of Zabanus hyalinipennis are deposited mostly on stones
from 3 to 15 inches in diameter, projecting above the water from
1 to 12 inches in rapids where the streams spread over the gravel
beds in sunny, open places. (Fig. 1.) Some eggs are deposited on
plants growing in the rapids and along the water’s edge. The eggs
have never been found in the swamps and rarely ever in places
shaded for most of the day. The parasite prefers the open, sunny
stretches of the streams, and the eggs on stones are usually most
heavily parasitized, but at times the egos on the plants are quite
as heavily parasitized. The adults are “most active during bright
hot weather and have never been observed active in misty or rainy
weather. The heaviest infestations of the horseflies have occurred
following seasons when there were heavy and continued summer
rains; this may be accounted for by the facts that during such

Fig. 1.—Gravel bar along the Frio River, where Phanurus emersoni parasitizes a
: large percentage of Tabanus eggs

seasons the fly-breeding grounds in the streams are greatly increased
and the activity and breeding of the parasite are retarded.

LIFE CYCLE AND SEASONAL ACTIVITIES OF THE PARASITE

The adults copulate almost as soon as they emerge and are dry.
Oviposition begins in from 4 to 24 hours after emergence. In the
act of ovipositing, the female inserts the ovipositor into the mem-
brane of the fly egg. She appears to be able to insert the ovipositor
between the eggs of the top layers into the eggs below. The females
prefer fresh eggs to older ones and have never been induced to
oviposit in eggs more than 5 or 6 hours old. The average rate
of oviposition in fresh eggs has been observed to be one every 25

seconds, but it takes longer to oviposit in older eggs. From 1 to 8

4 _ CIRCULAR 18, U. S. DEPARTMENT OF AGRICULTURE

females have been observed ovipositing on the same egg mass at
the same time. They appear to be able to determine to some extent
which eggs have beet parasitized, since females have been observed
to fly away without attempting to oviposit after examining an egg
mass upon which several females had previously oviposited. No
egg mass has ever been found to be completely parasitized, since
some tabanid larvae have hatched from each of the several hundred
masses collected. In collected egg masses, 93 per cent is the highest
percentage of parasitization of any single mass; the average in
any considerable number of masses has never been above 60 per cent,
as determined by the number of adult parasites emerging and the
number of tabanid larvae hatching. (Fig.2.) —

Fic. 2.—Mass of eggs of Tabanus hyalinipennis (viewed
from above), showing emergence holes of the parasite.
These egg massses are usually about one-half of an
inch across

There are not enough data at hand to determine the number of
young a single female Phanurus is capable of producing. From
three masses of fly eggs which had been attacked by a collected
female parasite, 114 adult parasites have been reared.

The males usually die within a few hours after copulation and
the females die soon after oviposition has been completed. Adults
have rarely ever lived longer than two days in the breeding jars and
none for more than five days. The adults have been supplied with
water, sweets, and flowers in the breeding jars, but have never been
cbserved to feed. They have bred quite freely in 4-ounce tumblers,
pint Mason jars, and smaller shell vials. It has been found advan-
tageous to introduce moisture into the jars by placing clean moist

€

TABANID EGG PARASITE PHANURUS EMERSONI GIRAULT 5

sand in the bottom. Females have been transferred from one egg
mass to another without appreciably interfering with the process of
oviposition.

At Uvalde the total developmental period for the immature stages
has ranged from 12 to 19 days, the average being 17.3 days, from
oviposition to emergence of the adults. The incubation period of
Tabanus hyalinipennis eggs has been from 5 to 9 days, usually 7 to 8
days. The immature stages appear to require a certain amount of
moisture, since many did not develop in egg masses collected and
kept in pill boxes without any moisture during hot dry weather.

Adult parasites begin to appear in the early summer as soon as
the weather has continued hot for some days. The earliest recorded
appearance was from eggs collected April 23, 1920, from which
adults emerged on May 10. The latest emergence was October
L190 14.

TABANID EGG COLLECTING AND PARASITE DISSEMINATION

After the tabanid eggs had been located in great numbers and the
ege@ parasite discovered in the late summer of 1914, some ranchmen
about Laguna Crossing, following the suggestion of the writer, col-
lected the eggs along the stream for a distance of 14 miles and placed
them in tin cans and buckets along the river. It was not possible to
determine accurately the quantity of eggs collected by the ranchmen,
but it was estimated at approximately 10 to 12 gallons of masses, or
20,000.000 to 25,000,000 eggs. This section was the most heavily
infested with horseflies of any during 1914.

During the early part of the season of 1915 it was estimated that
there had been a 50 per cent greater decrease in the number of flies
in the area where the eggs had been collected than in other similar
areas not so treated. Eggs collected at the time of the examinations
along this part of the river were more heavily parasitized than at
other places, samples averaging 50 per cent or above.

During July, August, and September, 1915, parasitized eggs were
collected along about 40 miles of the Nueces River and distributed
in streams over the other parts of the escarpment. One hundred
and five distributions of from one-fourth to 1 pint at each place
were made, and a total of 52 pints of eggs were used. At the time
the distributions were made, samples of egg masses were collected
from each place to determine whether the parasite was already
present. Parasites were reared from 51 of the 105 collections. They
were found over the entire escarpment but were found more gen-
erally distributed in the western half. From 1916 to 1926, inclusive,
egg masses were collected each year over portions of the area to
determine if the parasite was still present. A total of 139 collec-
tions have been made and parasites have been reared from 136.
They have remained rather well distributed, but the percentage of
eggs parasitized has usually been less in the eastern half than in the
western half of the area. This indicates that the parasite would
be more effective in tabanid control in areas of light rainfall.

6 CIRCULAR 18, U. S.. DEPARTMENT OF AGRICULTURE

SOME FACTORS THAT HAVE PROBABLY CONTRIBUTED IN
CONTROLLING THE HORSEFLIES

There has been a general decrease in the number of horseflies on
the escarpment since 1914. Several factors have entered that might
have augmented this decrease; however, it is believed that the
general distribution of the parasite, supplemented by egg collections,
has been one of the potent factors. Of the several thousand speci-
mens reared from collected egg masses, Phanurus emersoni has been
the only egg parasite recognized. Several predators of the fly eggs,
larvae, pupae, and adults have been observed, but none of these
has been recognized as having any considerable value in keeping
the tabanids in check.

Undoubtedly climatological factors have had a considerable effect
on the breeding of the flies and their enemies. A very severe drought
during the years of 1917 and 1918 decreased the fiy-breeding area by
drying up the streams except in the heads of the valleys. This con-
dition has interfered with obtaining more accurate data as to the
benefits that might be expected from ‘the work of the parasite. There
was a general outbr eak of tabanids during the summer of 1920, about
75 per “cent as many asin 1914. The summers of 1913 and 1919 were
characterized by heavy summer ra.nfall. The streams in the area
have flowed as freely during most years as during 1913 and 1919, but
when the summers have been characterized by light rainfall and a
high percentage of sunshine the tabanids have decreased. It is esti-
mated that the parasite has destroyed 50 per cent or more of the
tabanid eggs during the most favorable seasons for its work, and 10
per cent or less in the most unfavorable seasons.

CONCLUSIONS

Phanurus emersoni has been a potent factor in the control of
tabanids along the south escarpment of the Edwards Plateau, north
of Uvalde, Tex.

The parasite does its most effective work during seasons with a
high percentage of sunshine.

The artificial dissemination of the parasite has apparently estab-
lished it more generally over the escarpment on which its host breeds.

Egg- -parasite rearing and dissemination, augmented by tabanid egg
collecting (under conditions to be determined by the percentage of
tabanid eggs parasitized, the number of tabanid eggs present, and the
accessibility of the eggs for collecting), is a feasible method of taba-
nid control under certain climatic and physical conditions.

ORGANIZATION OF THE
UNITED STATES DEPARTMENT OF AGRICULTURE

December 22, 1927

pecretaryio; Agriculture - ——-. = 28s. 2. W. M. JARDINE.
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Director of Personnel and Business Adminis- W. W. STOCKBERGER.
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ICO UILCT REN ULG CG Is mie nie ns 2a 5! 1 ie CHARLES F. MArRviINn, Chief.
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LGA EOI EA) Fan NSS SEES Oa ane CLARIBEL R. BARNET?, Librarian.

This circular is a contribution from

BUreConl Of- ENLOMOlOGY_ = 2 = Se C. L. Maruatr, Chief.
Division of Insects Affecting Man and F. C. BisHopp, Senior Entomolo-
Animals. gist, in Charge.

of

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