Recent Advances in Research on Control and Biology

 of Pickleworm and Melonworm

 Kent Elsey, Jorge Pefia, Joop Peterson, Todd Wehner


 USDA, ARS, in cooperation with other institutions, has
been conducting research on pickleworm and melonworm
(Diaphania spp.: Pyralidae) for several years. Recent advances
in this research are given. Topics discussed include: research
on plant resistance in cucumber and squash, role of oviposi-


tion stimulants, foreign exploration and importation of
parasites from South America and the Caribbean, and popula-
tion dynamics of Diaphania spp. in tropical Florida.
Keywords: Diaphania nitidalis, Diaphania hyalinata, Cucur-
bita, Cucumis.


 A mainstay of small farm operations is vegetable production,
with cucurbit vegetables, such as cucumber and squash, being an
important part of vegetable farming. For example: in North
Carolina almost 40,000 acres of cucumbers are grown, mostly on
small farms. In the southeastern U.S. the key insect pest of cucur-
bits is the pickleworm, Diaphania nitidalis Stoll. The pickleworm
causes primary injury by burrowing into fruit and rendering them
unfit for market. Since shippers and packers, particularly pickle
processors, cannot tolerate infested or damaged fruit reaching the
consumer, the economic threshold for the pickleworm is very low.
Thus, when pickleworm are present frequent applications of in-
secticide are necessary to control the pest. The pickleworm
undergoes an annual expansion and contraction of its range in
the eastern U.S. Populations move slowly north in spring and
summer and then retreat south during autumn and winter as
freezing temperatures kill pickleworms and available host plants.
Eventually midwinter populations are restricted to southern
Florida (Dupree et al., 1955). A closely related species, the
melonworm, Diaphania hyalinata L., has similar biology, except
that it is strictly a defoliator. It can build up to economic levels in
more southern areas in the U.S., such as Florida and South
Carolina. Both species are present in northern South America and
Caribbean islands.
 The United States Vegetable Laboratory in Charleston, S.C.,
has taken the lead in coordinating research on biology and alter-
native control measures for these pests. In this paper we will sum-
marize our progress in selected aspects of this work.

Plant Resistance Studies
 Most of our work on plant resistance of cucurbits to pickleworm
 and melonworm at Charleston and cooperating institutions has
 involved the roles and relative importance of antibiosis and
 oviposition nonpreference as resistance mechanisms.
 In order to facilitate testing for antibiosis to pickleworm among
 over 1,000 cucumber cultivars and breeding lines, we developed a
 simple laboratory method using detached leaves. In essence, the
 tests involved placing newly-hatched larvae on leaves and enclos-
 ing them in plastic petri plates for several days. The plates were
 then opened to count the larvae and score the leaves for damage
 using a scale of 1 (little damage) to 9 (very heavy damage). The
 exact methods differed slightly depending on whether the test
 was a preliminary screening of all lines or later tests of the most
 resistant and most susceptible. Of the 1,160 lines tested in the in-
 itial screening test, 8 were selected for further studies (5 resistant
 and 2 susceptible). These lines and their performance are shown
 in Table 1. The range of scores from most resistant to most
 susceptible was narrow, ranging from 3.0 to 6.5, or 3.1 to 4.5
 depending on the test. In a further study, a parent-progeny
 regression analysis run on a population developed from the resis-
 tant lines revealed a heritability of nearly 0 (Wehner et al., 1983).


100


Another series of antibiosis tests run on a number of cultivated
species of cucurbits found that only Lagenaria siceraria, an inedi-
ble species of gourd, demonstrated a significant degree of
resistance (Elsey, 1981). This and the cucumber work indicates
that antibiosis among cucurbit vegetables may not be a useful
factor in breeding for pickleworm resistance.
 Studies of oviposition nonpreference have focused on dif-
ferences between glabrous and pubescent cucumbers, com-
parisons of pickleworm resistant and susceptible squash species,
and a study of the chemical factors involved.
 Foliage of cucumber and muskmelon is normally pubescent
 but glabrous mutants have been described (Foster, 1963; Robin-
 son and Mischanec, 1964). Pulliam (1979) reported that glabrous
 cucumber plants seemed to be nonpreferred for pickleworm
 oviposition. Follow-up work by Elsey and Wann (1982), using ar-
 tificial infestations of moths and following natural populations in
 the field, found that both pickleworm and melonworm preferred
 to oviposit on the pubescent plant type (Tables 2 & 3). Unfortu-
 nately, the glabrous cucumber is less vigorous than the pubescent
 type, which may limit its use by plant breeders.
 Several studies on pickleworm resistance in squash have found
that some Cucurbita moschata Poir cultivars have lower infesta-
tions of pickleworm than more susceptible species (Brett et al,
1961; Dilbeck and Canerday, 1968; Dilbeck et al., 1974), but the
mechanism for this resistance was not known. In the summer of
1984 we have compared egg and larval infestations on two C.
moschata cultivars, Butternut and Calabaza with a susceptible
Cucurbita pepo L. cultivar, Tablequeen. During the peak
oviposition period ca. 10 times as many Diaphania spp. eggs were
found on Tablequeen and subsequently higher numbers of both
pickleworm and melonworm larvae (Table 4). Laboratory tests, to
date, have failed to detect any larval antibiosis or preference fac-
tors among the three cultivars, so we we feel that oviposition non-
preference is the most important resistant mechanism in the C.
moschata cultivars.
 It is probable that chemical differences among these three
cultivars are responsible for the observed oviposition non-
preference. Thus, we have been looking closely at the chemical
influences on pickleworm oviposition using a yellow squash varie-
ty in the species C. pepo. We have found that ethanol extracts of
foliage from which the volatile components have been removed
contain chemicals that stimulate oviposition when sprayed on a
suitable medium (pads of fiberglass insulation). We have further
found that pumping air containing squash volatiles into pads
sprayed with extract increases oviposition over pads with extract
only. From these and other experiments we can outline the
following hypothesized mode of action (Fig. 1).
We are now in the process of testing the model and trying to
identify the volatile and nonvolatile chemicals involved.


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