Dr. Geoff Zehnder, Professor, Department of Entomology and Coordinator, IPM & Sustainable Agriculture Programs, Clemson University
Scientific Names: Pickleworm (Diaphania nitidalis); Melonworm (Diaphania hyalinata); Crambidae (formerly Pyralidae); LEPIDOPTERA
This article presents an overview of the biology, behavior and damage associated with pickleworm and melonworm, two closely related insect pests of cucurbit crops. Discussed are management approaches that are appropriate for certified organic farming systems. Although the two species are similar in their biology and distribution, they differ in the damage that they cause to cucurbit crops, with pickleworm generally considered to be the more economically important pest.
Pickleworm and melonworm are tropical insects, and cannot tolerate cold temperatures. In the U.S., they are known to overwinter only in south/central Florida and southern Texas. The adult moths disperse northward each spring. During the summer, the pickleworm moth migrates into the Carolinas and can move into more northern states and even westward to Oklahoma and Nebraska. The melonworm, however, is rarely found north of the Gulf states.
Figure 1. Adult pickleworm moth. Photo credit: Natasha Wright, Florida Department of Agriculture and Consumer Services, Bugwood.org.
The pickleworm adult is a flashy moth with wide triangular wings and a wingspan of about one inch (Fig. 1). The wings are mostly iridescent brown, with a central band of semi-transparant yellow. The melonworm moth is slightly smaller with white, slightly iridescent wings centrally, and edged with a broad band of dark brown (Fig. 2). Both moths display brush-like appendages at the tip of the abdomen when at rest.
Pickleworm and melonworm moths are generally inactive during the day, but melonworm will fly short distances when disturbed (Smith, 1911). Peak flight activity of pickleworm moths occurs 3–5 hours after sundown (Capinera, 2005).
Moths lay tiny eggs in on growing areas of the plant, such as new leaf buds, flowers and shoots, and eggs generally hatch in 3–4 days. Pickleworm and melonworm can produce as many as four generations per season.
Figure 2. Adult melonworm moth. Photo credit: Alton N. Sparks, Jr., University of Georgia, Bugwood.org.
The younger larvae of pickleworms are thin white caterpillars with numerous small black spots but as they mature they become plump and darker in color, and lose their spots. Larval color in the last instar stage is variable depending on food source; off-white, green, yellow and orange colored larvae can be found. Pickleworm larvae prefer to feed on blossoms initially, and blossom damage can severaly reduce fruit set. When about half-grown, pickleworm larvae often bore into fruit and continue to feed there causing internal damage and producing soft excrement (Fig. 3). Both young and old fruit are attacked, but they prefer young fruit before the rind has hardened. Pickleworm usually pupates within a leaf fold, and dead, dry leaf material is often used.
Figure 3. Pickleworm larvae and excrement inside fruit. Photo credit: Clemson University-USDA Cooperative Extension Slide Series, Bugwood.org.
Newly hatched melonworm larvae lack color, but are pale green in the second instar stage and mature melonworm larvae are dark green with two lateral white stripes (Fig. 4). Melonworm larvae construct a silken structure on the undersides of leaves and continue to develop and feed on foliage. Pupation occurs inside a silken cocoon on the plant, often within a folded section of green leaf material.
Figure 4. Melonworm larvae. Photo credit: Alton N. Sparks, Jr., University of Georgia, Bugwood.org.
These two caterpillars infest only cucurbits; both wild and cultivated species may serve as hosts. Please refer to the Management section below for a listing of resistant varieties.
Though the pickleworm prefers squash (summer squash is highly preferred), it may also cause damage to cucumber and cantaloupe. Generally speaking, varieties of Cucurbita maxima and C. moshata are more resistant than those in the C. pepo species. Therefore, muskmelon, winter squash, and gourd are rarely damaged by pickleworm, and watermelon is also not a preferred host. Pumpkin is a variable host, probably because pumpkins have been bred from several Cucurbita species.
Melonworm commonly infests summer and winter squash. The Cucumis species—cucumber, gerkin, and cantaloupe—are attacked but not preferred, and watermelon is a rare host. Pumpkin is a variable host.
In their overwintering areas of southern Florida and Texas, crop damage can occur early in the growing season. Both species disperse northward as the season progresses and it regularly takes one or two months for the dispersing moths to move into the Carolinas. Therefore, in the southeastern states north of their overwintering habitats, infestation of cucurbits by pickleworm and melonworm is usually not observed until late June or early July. Early season crops usually escape injury. In some years they may travel further north where infestation of crops may not begin until August or September.
Melonworm damage is mainly on foliage, especially if foliage of a favored host plant is available (Fig. 5). Usually the leaf veins are left intact, resulting in lace-like plant remains. However, if the available foliage is exhausted, or the plant is a less preferred species, then the larva may feed on the surface of the fruit, or even burrow into the fruit (Fig. 6). Thus, growers may refer to these melonworm larvae as rindworms because they cause scars on the surface of melons.
Figure 5. Melonworm damage to foliage. Photo credit: Alton N. Sparks, Jr., University of Georgia, Bugwood.org.
Figure 6. Left: Illustration showing pickleworm life stages and damage. Credit: Art Cushman, USDA; Property of the Smithsonian Institution, Department of Entomology, Bugwood.org.
Right: Pickleworm entrance hole on squash. Photo credit: Alton N. Sparks, Jr., University of Georgia, Bugwood.org.
Unlike the melonworm, the primary concern with pickleworm is damage to the fruit. Young pickleworms usually feed for a time among small leaves at the growing tips of vines or within blossoms. Growing vines sometimes become riddled with holes and cease to grow. Larvae are often found in the squash flowers where they hide under the ring of stamens at the base of flowers. When about half grown, pickleworms bore into the fruit and continue to feed there causing internal damage. Entry holes are usually marked with a pile of white excrement or frass. Both young and old fruits are attacked, but they prefer young fruit before the rind has hardened. After the fruit rind has been punctured the fruit soon rots.
It is difficult to detect the presence of pickleworm and melonworm before damage occurs because of their small egg size, and nocturnal flight behavior of the adult moths. Although sex pheromones of both species have been discovered and synthesized, at present pheromone trapping systems are not commercially available. Checking plants for early stages of leaf damage and the presence of larvae are the most effective ways to monitor crops for melonworm.
For pickleworm, research has suggested that the most reliable sampling method is to begin sampling flower buds (Fig. 7) for small caterpillars before they bore into the fruit (Brewer and Story, 1987). Check with Extension specialists or other growers in your area to find out when pickleworm and melonworm generally appear. In conventional cucurbit production, growers often begin weekly insecticide sprays if larvae are detected in the buds. The same approach may be used with insecticides approved for organic production, although these materials are not generally as effective as conventional insecticides (see section under Management below).
Figure 7. Pickleworm entrance hole in squash blossom. Photo credit: John L. Capinera, University of Florida, Featured Creatures
Because these pests are difficult if not impossible to control once they infest cucurbit plants, and in keeping with the organic approach to pest management, implementation of preventative cultural practices will be the best way to avoid problems associated with pickleworm and melonworm. These include:
If preventive management strategies alone are unable to provide adequate contol, the application of insecticides approved under the National Organic Program (NOP) and/or entomopathogenic nematodes may help to reduce damage. A combination of several strategies will probably be the most effective.
Before development of effective insecticides, cucumbers and other susceptible cucurbit crops in the Carolinas were only grown in the early part of the summer, because crops grown after that time were usually destroyed by pickleworm (Anderson and Hofmaster, 1948). Thus organic growers may take advantage of early markets and plant susceptible crops like squash early to avoid serious damage.
Although these insect species do not tolerate cold temperatures, immature stages may survive warm winters in the southeastern states by overwintering in plant material. Therefore, removal and destruction of infested plants including vines and fruit following harvest is a good cultural practice to reduce populations (Smith, 1911). Also, some weeds in the cucurbit family, including creeping cucumber (Melothria pendula) and wild balsam apple (Momordica chorantia), may serve as alternate hosts (Elsey et al., 1985). Therefore it is a good practice to remove these plants in areas adjacent to cucurbit crop fields. The USDA–NRCS Plants Database provides additional information on weeds in the cucurbit family, including images and distribution.
Research has shown that cucurbit resistance to both pickleworm and melonworm is based on oviposition nonpreference (Elsey, 1985). It is likely that there are plant compounds in the less preferred cultivars that deter egg-laying by female moths. A search of the literature did not identify any commercially available cucurbit varieties with resistance to melonworm specified. However, research by Brett et al. (1961) demonstrated marked resistance to pickleworm in the following varieties: Butternut 23, Summer Crookneck, Early Prolific Straightneck, and Early Yellow Summer Crookneck. The varieteis more susceptible to pickleworm are Cozini Zucchini, Black Caserta Zucchini, and Benning's Green Tint Scallop squash.
The use of floating row covers has been shown to exclude pickleworm, melonworm, and other insect pests such as whiteflies and aphids from squash plants (Webb and Linda, 1992). Row covers prevent moths from laying eggs on the plants, and should be applied immediately after planting; however they must be removed to allow for pollination by bees and other pollinating insects. There are two approaches to this depending on the size of the planting and effort one wishes to devote to manipulating row covers:
Several researchers including Smith (1911) have shown that, because squash is a preferred host for pickleworm, it can be used as a trap crop to deter pickleworm from attacking other cucurbit crops like cantaloupe and cucumber. Smith recommended that squash blossoms be destroyed periodically to keep pickleworms from moving to adjacent cantaloupes. Research in Alabama demonstrated that 6-row cucumber plots (Vlas-Pic variety) bordered on both sides by two rows of squash (Dixie variety) sustained significantly lower pickleworm damage compared with cucumber grown without border rows of squash (Zehnder, unpublished data). However, in these experiments a synthetic insecticide was applied to the trap crop as soon as larvae were detected on plants, which would not be allowed in certified organic production. Organic growers may use approved materials such as spinosad (see below). Squash may also be used as a sentinel crop to detect the first appearance of pickleworms in flowers. This information can be used to help time applications of Bacillus thuringiensis and nematodes, as described below.
Figure 8. Two row perimeter trap crop of Buttercup squash around a main crop of Butternut squash (for cucumber beetle control). Photo credit: Andrew Cavanagh and Ruth Hazzard, University of Massachusetts Extension Vegetable program.
Capinera (2005) reported that, although pickleworm has many natural enemies including predatory beetles, fire ants, and several species of parasitic wasps, none can reliably suppress damage. Other authors suggest that natural enemies have a significant impact on pickleworm and melonworm and that hard insecticides should be avoided to preserve the effectiveness of beneficial organisms (McCleod, 2008). Elsey (1980) reported parasitism of pickleworm eggs by Trichogramma wasps as high as 69%, but this occurred only late in the season. He also reported that fire ants were an important predator of pickleworm pupae.
Important: Before using any pest control product in your organic farming system:
Note that, although OMRI and WSDA lists are good places to identify potentially useful products, all products that you use must be approved by your certifier. For more information on how to determine whether a pest control product can be used on your farm, see the related article, Can I Use This Input On My Organic Farm?
Nematodes that attack insects are called entomopathogenic nematodes and several species are available commercially for control of various insect pests, particularly those that live in the soil. This is because nematodes survive better in the soil than above ground where they will die when exposed to sunlight and to hot, dry conditions. Several researchers have reported significant control of pickleworm by application of entomopathogenic nematodes such as Steinernema carpocapsae. Shannag et al. (1994) reported that this nematode could effectively reduce pickleworm injury in squash because it has large, enclosed flowers. Because the nematodes are protected in that moist environment they can attack pickleworm larvae before they bore into the fruit. They suggest that nematodes may not be effective on cucurbits with small, open flowers such as cucumber because the nematodes would not be as protected and would desiccate and die from exposure to sunlight. However, Zehnder (unpublished data) found that spray application of Steinernema carpocapsae (strain 25; Biosys Corp.) to cucumber every 4 days beginning when first flower buds were formed significantly reduced pickleworm damage compared to an untreated control.
Bacillus thuringiensis products, commonly called Bt, contain a toxin produced by the Bt bacteria that will kill certain insects. The Bt variety called “kurstaki” is most effective on caterpillars like pickleworm and melonworm. The Bt toxin must be ingested by the insects for mortality to occur; therefore good spray coverage is essential. There are several OMRI-approved Bt kurstaki products including the following brand names: Agree (Certis), Dipel (Valent), Javelin (Certis), XenTari (Valent). As mentioned above, because the Bt product must be ingested, control of pickleworm may be problematic because the larvae are protected inside leaf and/or flower buds or in fruit. Therefore if pickleworm is present, in addition to good spray coverage, it is important to begin application of Bt when the first buds or open flowers are present, or at least when larvae are first detected in plants. Zehnder (unpublished data) found that spray application of Javelin WG (1.0 lb/acre rate) beginning either with first presence of buds or flowers, or when larvae were first detected, and applied every 4–7 days significantly reduced pickleworm damage to cucumber fruit compared to an untreated control.
Two additional insecticides approved for organic production and control of melonworm and pickleworm include neem and spinosad. Neem products (e.g., Neemix 4.5™) are botanical insecticides with the active ingredient azadirachtin that is extracted from the neem tree. Neem insecticides are effective against a variety of insect pests through contact toxicity, disruption of insect molting, and feeding deterrence. Labels direct users not to apply neem when honeybees are actively visiting flowers in the area.
The active ingredient in spinosad (e.g., Entrust™) insecticide is derived by microbial fermentation and is effective against a wide variety of insect pests. Spinosad is safe to most beneficial insects; however the wet spray can kill bees. The label indicates that once the spray dries there is no risk to bees foraging on plants.
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