The utilization of parasites for reducing oriental fruit moth injury to peaches

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The utilization of parasites for reducing oriental fruit moth injury to peaches
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Allen, H. W
United States -- Bureau of Entomology and Plant Quarantine
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U.S. Department of Agriculture, Bureau of Entomology and Plant Quarantine (Washington, D.C )
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June 1942


THE UTILIZATION OF PARASITES FOR REDUCING ORIENTAL
FRUIT MOTH INJURY TO PEACHES

By H. W. Allen, Division of Fruit Insect Investigations


*INTRODUCTION

Of all the insect pests that attack the peach in the Eastern and Cen-
tral States, the oriental fruit moth is the most difficult for the grower to
deal with. Extensive efforts on the part of both State and Federal ento-
mological agencies, since the discovery of this insect in the United States
in 1916, have not yet resulted in the development of practical measures for
*its control that can be generally utilized by growers. In this situation,
efforts to utilize more fully the natural enemies of the insect in its con-
trol have assumed unusual importance and are of special interest to commer-
cial growers. This circular summarizes briefly current information about
the parasites of the oriental fruit moth, what has been done about them,
how they are being used, and the trend of investigations under way. Al-
though the oriental fruit moth causes damage to several fruit crops, the
only crop in which parasites are generally recognized as being valuable in
checking injury is the peach. The subsequent discussion will be limited to
the use of parasites in reducing injury to peaches.

HOW REDUCTION IN FRUIT INJURY IS PRODUCED BY PARASITES

Over a large portion of the area infested by the oriental fruit moth
the principal damage to the main commercial varieties of peaches is caused
by larvae of the third generation. This generation is preceded by two
others which develop chiefly in the tender twigs. The fruit moth is able
to reproduce itself in such large numbers that if the increase which might
normally develop from the overwintering survivors in any locality is not
checked before the eggs of the third, or fruit-infesting, generation hatch,
an extremely heavy infestation of third-brood larvae develops in the ripening
fruit. Fortunately, certain species of parasites destroy enormous numbers
of twig-infesting larvae, and other parasites attack the eggs or the stages
of the fruit moth in the cocoons. Even when weather and other conditions


E-570






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are generally favorable to moth increase, the activities of parasites may
reduce the numbers of larvae at harvest time below the numbers that were
present at the beginning of the season. Under such conditions the fruit
infestation will be relatively low.

For three successive years, irom 1937 to 1939 inclusive, an effort
was made by the Bureau of Entomology and Plant Quarantine to measure more
exactly the effect of parasitism of the oriental fruit moth on the injury
to peach fruit. The studies have included a series of 51 orchards in 11
more or less distinct peach-growing districts east of the Alleghenies,
extending from Lovingston, Va., to Princeton, N. J., and supplement several
less extensive surveys made by State experiment stations. It has been
found that both the parasitism and the fruit infestation vary widely in dif-
ferent years, between different districts, and even between orchards in the
same district, but in general within any district the orchards having the
heaviest parasitism of twig-infesting larvae also have the lowest fruit
infestation. This was particularly true for 1933 and 1939, in which years
conditions other than parasitism in the districts surveyed were apparently
favorable for moth increase. In 1938, in 6 orchards in the Virginia, West
Virginia, and Maryland area which had parasitism in excess of 66.7 percent
there was an average of only 3.2 percent fruit injury, as compared with an
average of 25.5 percent injury in 8 orchards in the same area having less
than 33.3 percent parasitism. In 1939 a group of 6 orchards in the same
section with more than 66.7 percent parasitisr had only 1.4 percent fruit
injury, as compared with 19.2 percent injury in 11 orchards having less than
33.3 percent parasitism.

THE RELATIVE IMPORTANCE OF THE VARIOUS SPECIES OF PARASITES

Native parasites.--In the United States the fruit moth is attacked
by more than 100 species of native parasites. No species is present in
all orchards every season. Only a few species are generally abundant,
but more than a dozen species are known to occur in abundance in some
orchards every season. A few more occasionally become abundant enough to
destroy a considerable portion of the fruit moths in some restricted dis-
tricts. Of all these kinds of parasites, one species, named Macrocentrus
ancylivorus Roh., has proved to be outstanding in effectiveness and in adapt-
ability to a wide variety of conditions, and the greater part of the work
connected with the utilization of parasites for oriental fruit moth control
has involved this species.

Foreign parasites.--The Bureau of Entomology and Plant Quarantine
has searched infested peach districts in Australia, Europe, and Japan for
other promising species of fruit moth parasites. Twenty-nine such foreign
species have been imported, propagated at Ioorestown, N. J., and colonized
widely over the infested area. The results have not been encouraging.
Several of these introduced species produced high initial rates of parasiti-
zation, and at least two wore able to persist in the locality of liberation






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for 1 or 2 years following release, but, so Par as known now, none have be-
come estbalished. The work of importing, breeding, and liberating foreign
species of parasites was discontinued in 1938.

WORK ACCOMPLISHED WITH THE PARASITES 01 THE ORIENTAL. FRUIT MOTH

Parasites may be liberated for either of Lwo purposes. The first purpose
is the establishment of a parasite species in an area in which it does not
occur. In such cases a small number of parasites are placed in an already
infested orchard, and if they succeed in establishing themselves, they arc
expected utlimately to increase in numbers to the !aximum possible under
the conditions existing in the orchard, and also to spread into other or-
chards in the locality. Most of the work of this Bureau from 1930 to 1936
was on this basis. When the parasites are well adapted to conditions
existing in a given locality, they continue to function year after year
as valuable crop protectors, destroying enormous numbers of fruit moths
each season, without any effort being made on the part of the growers or
others inter ested inprotecting the peach crop.

The second purpose for which liberations may be made is to obtain
a reduction in injury to fruit the same season. Since 1936 an increasing
amount of attention has been given to the possibility of effecting immediate
control of the oriental fruit moth through the mass liberation of parasites.
Since the purpose of mass liberations is to increase immediately and greatly
the numbers of? the oriental fruit moth destroyed by parasites over the whole
area included in the liberations, the manner of making the liberations dif-
i'ers considerably from that of liberations made for the purpose of establish-
ment of the parasites.

Colonization of parasites of the oriental fruit m.oth.--In 1929, early
in the study of the parasites of the oriental fruit moth, the distribution
of Microcentrus ancvivorus was found to be limited to a few counties in
central Connecticut, southern New Jersey, Delaware, and eastern Maryland.
A program of rearing and releasing this parasite was, therefore, under-
taken. From 1930 to 1935, the period when most of this work was accom-
pli.h.Cd, the Bureau of Entomology and Plant Quarantine reared and, in cooper-
ation with several State agencies, liberated about 132,000 adults of this
parasite in 402 colonies well distributed throughout the infested area from
Massachusetts to Michigan and south to Georgia and Arkansas. This work is
being continued on a much reduced scale as additional counties in outlying
States become infested with this pest. In addition the Bureau has collected
and shipped nearly 700,000 strawberry leaf roller and fruit moth larvae
containing this parasite to cooperating State stations to assist them in
rearing and colonizing additional supplies of Marcocentrus.







-4-


As a result of this work Macrocentrus ancylivorus soon became established
in many of the principal peach growing districts from Connecticut west to
Hichigan and south to Georgia and Arkansas. Its establishment has been
followed in most instances by a high rate of parasitism and a decided re-
-uction in fruit infestation. In some districts in which this parasite
h's b-conie dominant there have been instances of the recurrence of mod-
erate fruit infestation, but none are known in which the pest has been
able to duplicate the heavy damage caused by it in the early days before
M. ancylivorus became established.

Experiments with liberation of parasites for immediate control.--
Experiments in the mass liberation of parasites conducted for 4 succes-
sive years in orchards of southern New Jersey have been moderately en-
couraging. Macrocentrus anc livorus has been used in these liberations at
the rate of approximately 500 per acre. In most cases these liberations
have been followed by an increase in parasitism, a decrease in the number
of moths emerging, and substantially lower fruit injury in the orchards
receiving The parasites. The average fruit injury in the orchards receiv-
ing parasites was about 50 percent less than that in those not receiving
parasites, and 22 of the 25 orchards receiving parasite liberations have
had less fruit injury than that occurring in the check orchards. In Burling-
.on County, in 1939, the five orchards receiving parasites had an average
of 34 injured fruits per tree, which was about one-fifth the injury in the
check orchards not receiving liberations. Mass liberations of M. ancylivo-
rus ha- ben tried in sections where the species released was already domi-
nant and also in districts where it was not abundant. The results were
similar in both cases.

Mass liberations of Trichograma minutum, a common native egg para-
site, were made during two successive years at the rate of about 50,000
parasites per acre. No substantial reduction in fruit injury was observed
following the liberation of Trichopramma.

Production of Macrecentrus for liberation.--During the last few years
m pro4,g$ess has been made in the large-scale production of M. ancylivorus.
There arc several ways in which this parasite can be bred or reared for use
n lieations. Tn southern New Jersey the best results have been obtained
reading the parasite in large field cages of aster cloth erected over
stra4berry. The caged strawberry plants are heavily stocked with strawberry
i~a rollers, and later on with parasite adults. A few days later the para-
sitized leaf rollers are removed to rearing cages, from which the parasites
that develop are assembled for liberation as they emerge. The principal
problems encountered in this procedure have been difficulty in carrying
eouh leaf rollers and parasites over winter to stock large parasite-breed-
ing cages, and trouble in holding strawberry pests such as the red spider
in check during the process of breeding in the field cages. These diffi-
culties have now largely been overcome. A propagation rate of 10-fold is
not unusual, and enough parasites can be produced from an acre of straw-
berry under cage for mass liberations over 1,200 acres of orchard. Theprop-






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agation of these parasites requires a certain amount of'equipment and en-
tails a considerable degree of experience and skill in the manipulation of
the material. The technique of producing them is not at the present tim
suitable for adoption by individual peach growers, unless the services of
individuals experienced in parasite production are available.

POINTS TO BE CONSIDERED IN PLANNING AND I\:AKING PARASITE
LIBERATIONS

Thus far Macrocentrus ancyjlivorus is the only known oriental fruit
moth parasite liberations of which have been frequently followed by bene-
ficial results. In orchards in which the species does not occur, libera-
tions for the purpose of establishment are advisable unless experience with
similar liberations during previous years in the sane orchard or in other
orchards of that district indicates that M. ancyjivorus is unsuited to that
environment. Fortunately there appear to be few localities in the infested
area unsuited to this parasite. When the purpose of' the liberation is estab-
lishment, a colony of 100 lively, mated females should be sufficient. (A
female Macrocentrus can be readily distinguished from the male by the
presence of the ovipositor, a slender structure, about as llng as the rest of
the insect, which protrudes from the tip of the abdo n.) They should be re-
leased when plenty of worms are available for tbem to attack. Usually the best
time for release is in June or July when the larvae of the second brood of the
fruit moth are present in freshly wilted twigs of the peach. In some sections
moderately heavy twig infestations on peach trees after harvest, or the
presence of, some other insect such as the strawberry leaf roller which this
parasite is known to attack during the periods when orinental fruit moth
larvae are not available in twigs, make it feasible to release later than
July. At present it is difficult to obtain this parasite in large numbers
to be available for colonization earlier than the middle of June.

The Bureau of Entomology and Plant Quarantine of the United States De-
partment of Agriculture does not have the facilities to produce parasites
of the oriental fruit moth for general distribution to individual growers
Several agricultural experiment stations, however, e producing such para-
sites for liberation within the borders of their respecLive States, and limit-
ed supplies of Macrocentrus ancylivorus can now obtained from commercial
sources.

When mass liberations for immediate control are being considered ,the
number of parasites that should be liberated is an important consideration.
The use of large numbers per unit of area increases the cost and the diffi-
culty of obtaining the necessary parasites. It is ohvious, however, that if
the number liberated is lowered beyond a certain point, the reduction in in-
festation which could be produced by the parasites during Lhe short period be-
tween their release and fruit harvest would be negligible. Approximately
500 parasites per acre have been used in the expe~rns conducted by the
Bureau of Entomology and Plant Quarantine. It would be perhaps be economically
feasible to use more than this, and, on the other hand, possibly a slightly

LIBRARY
STATE PLANT BOARD






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Ialier number would have given nearly as satisfactory results. From
aailable information, however, it seems rather unlikely that liberations
o ess than 100 parasites per acre would cause a substantial reduction in
fruit injury. If only one shipment is to be received, the parasites
used in mass liberations should be released at or just before the peaks
of first-brood or second-brood infestations of oriental fruit moth larvae
in peach twigs. If large numbers are being liberated and successive ship-
--,ts can be arranged, it is advisable to make several liberations in each
'lock, starting as soon as freshly wilted twigs with young larvae are ob-
setTed and continuing them for about 2 weeks. Macrocentrus ancylivorus
eeased after a majority of the second-brood larvae have completed their
feding in twigs could not possibly reduce fruit injury caused by the fol-
lowin- brood. Proper timing is therefore highly important. Since only fe-
male parasites can attack the oriental fruit moth worms, the parasites
released should contain a satisfactory proportion of that sex. Only vig-
orous, active adults should be used, and the degree of mortality noted
n the cages at the time of liberation is a fair indicator of vitality.
UTi s in which the mortality among females is in excess of 10 percent would
be considered relatively unsatisfactory for mass liberations. In most
.1 the material used in the experimental work described above, the mor-
tality of females at the time of liberation was less than 1 percent.

Parasites shipped for liberation must be handled with great care to
pre. nt high mortality or loss of vitality. Macrocentrus does most of
Ls work at night, in twilight, or on cloudy days. It is sensitive to
xtromos of heat or dryness, and the adult lives for only a few days in
*idsummer. In transporting it to the orchard, the containers should be
kept cool and damp, which can be done by wrapping them in a clean wet
;Ranket or by placing them in a wooden box with ice. The packing should
be done in such a way that water from the melted ice will not seep into
t7e parasite containers. "Dry ice," or solid carbon dioxide, should never
e used to keep parasites cool, since the gas generated would kill the
-nsects. If possible, the parasites should be released on the day of
arrival, though preferably not during the hours of excessive midday heat.
They can be released in moderate rain if the adults are active enough to
fly to protection on the under side of the leaves.


SUMMARY

In the absence of any other practical measure for the control of
the oriental fruit moth that can be utilized by growers, a determined
effort is being made to utilize more fully the natural enemies of this
insect in its control, Parasitic insects destroy great numbers of the
oriental fruit moth and are particularly valuable in reducing the infes-
tation at the time of harvesting the main commercial varieties. Many
foreign species of parasites have been imported, but their colonization







- 7 -


in the United States has not been successful. In the United States the
fruit moth is attacked by more than 100 species of native parasites. Of
these, one species named Macrocentrus ancylivorus is outstanding in effec-
tiveness and in adaptability to a wide range of conditions, and can be
manipulated to increase the total of parasitism in orchards infested with
the oriental fruit moth.

The colonization of Macrocentrus ancylivorus for the purpose of
establishment has been most successful. As a result of this work,
Macrocentrus ancylivorus has become established as the most valuable
parasite of the fruit moth in many of the principal peach-growing dis-
tricts from Connecticut west to Michigan and South to Georgia and Arkansas.
The work of colonization is being continued by several State agencies with-
in the borders of their respective States, and by the Bureau of Entomology
and Plant Quarantine in the more recently infested areas. Experiments in
the mass liberation of Macrocentrus ancylivorus during the last 4 years
have been moderately encouraging, the average fruit injury in the orchards
treated being only about half as heavy as in the orchards not treated.
Much progress has also been made in developing methods for large-scale
production of this parasite. It is now possible to produce enough para-
sites under one acre of field cage to make mass liberations over 1,200
acres of peach.

Growers who are expecting to make liberations of Macrocentrus in
their orchards should consider whether such liberations are to be made
for the purpose of establishment or for the immediate reduction of the in-
festation. They should also try to deliver parasites to the orchards in
vigorous condition, and to make their liberations ataperiod suitable for
maximum effectiveness of the colonization.





UNITED STATES DEPARTMENT OF AGRICULTURE
OFFICE OF THE SECRETARY
WASHINGTON, D. C.

OFFICIAL BUSINESS


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