Diaphania nitidalis Stoll Lepidoptera Pyralidae

Natural History

Distribution. Pickleworm is a tropical insect that occurs widely in Central and South America and the Caribbean. In the United States, it routinely survives the winter only in south Florida and perhaps south Texas. Pena et al. (1987a) documented the overwintering biology in south Florida, but overwintering has been observed as far north as Sanford, in central Florida, during mild winters. Pickleworm is highly dispersive, and invades much of the southeast each summer. North Carolina and South Carolina regularly experience crop damage by pickleworm, but often this does not occur until August or September. In contrast, northern Florida and southern Georgia are flooded with moths each year in early June as warm, humid tropical summer weather conditions become firmly established. Although it regularly takes one or two months for the dispersing pickleworms to move north from Florida to the Carolinas, in some years they reach locations as far north as Michigan and Connecticut. Presumably they are assisted in their northward dispersal by favorable wind patterns. However, there is some evidence that picklworms also can overwinter in particularly mild coastal areas of southeastern states (M. Jackson, pers. comm.). In Canada, pickleworm has occasionally been found in southern Ontario. In Puerto Rico, it is more common in the mountains than at low elevations, and is not found at all in dry areas of the island (Wolcott, 1948).

Host Plants. Pickleworm feeds only on cucurbits, but both wild and cultivated species are suitable hosts. Creeping cucumber, Melothria pendula, is considered to be an important wild host. Wild balsam apple, Mor-mordica chorantia, which has also been reported to be a host, is of questionable significance (Elsey et al., 1985). Summer and the winter squash species are good hosts. Pumpkin is considered of variable quality as a host, probably because pumpkins are bred from several Cucurbita species. The Cucumis species — cucumber, gerkin, and cantaloupe—are attacked but not preferred. Among all cucurbits, summer squash is most preferred, and most heavily damaged. Culti-vars vary widely in susceptibility to attack, but truly resistant cultivars are unknown (Dilbeck et al., 1974). Cucurbits are intolerant of cold weather. Although diapause is unknown in pickleworm, it is the lack of host plants during the winter months that functionally limits the distribution of pickleworm. (See color figures 2 and 26.)

Natural Enemies. Pickleworm has several natural enemies, but none reliably suppress damage. Generalist predators such as Calosoma spp. and Harpalus (both Coleoptera: Carabidae); the soldier beetle Chauliog-nathus pennsylvanicus De Geer (Coleoptera: Canthari-dae); and the red imported fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae); have been reported to be important mortality factors. Also, several parasitoids are known, including Apanteles sp., Hypomicrogaster diaphaniae (Muesebeck), Pristomerus spinator (Fabricius) (all Hymenoptera: Braconidae); Casinaria infesta (Cresson), Temelucha sp. (both Ichneu-monidae); and undetermined trichogrammatids (Pena et al., 1987b; Capinera, 1994). The braconid Cardiochiles diaphaniae Marsh (Hymenoptera: Braconidae) has been imported from Colombia and released into Florida and Puerto Rico in an attempt to obtain higher levels of parasitism (Smith et al., 1994).

Life Cycle and Description. The pickleworm can complete its life cycle in about 30 days. Over much of its range, multiple and overlapping generations may occur. The number of generations was estimated to be four in Georgia (Dupree et al., 1955) and two or three in North Carolina (Fulton, 1947).

  1. The eggs are minute, measuring only about 0.4-0.6 mm wide and 0.8 mm long. The shape varies from spherical to flattened. Their color is white initially, but changes to yellow after about 24 hours. The eggs are distributed in small clusters, usually 2-7 per cluster. They are deposited principally on the buds, flowers, and other actively growing portions of the plant. Hatching occurs in about four days (Smith, 1911). Elsey (1980) estimated egg production to be 300-400 per female.
  2. There are five instars. Total larval development time averages 14 days. Mean duration (range) of each instar is about 2.5 (2-3), 2 (1-3), 2 (1-3), 2.5 (2-3), and 5 (4-7) days, respectively. Head capsule widths for the five instars are about 0.25, 0.42, 0.75,1.12, and 1.65 mm, respectively (Smith et al., 1994). Body lengths average 1.6, 2.5,4.0,10, and 15 mm during instars 1-5, respectively. Young larvae are nearly white with numerous dark gray or black spots. The dark spots are

Pickleworm young larva.

Pickleworm young larva.

Pickleworm mature larva.

Pickleworm mature larva.

lost at the molt to the fifth instar. Larval color during the last instar is somewhat variable, depending largely on the insect's food source. For example, they tend to be orange when feeding on blossoms, green when feeding on stem tissue, and white when feeding on fruit. Before pupation larvae tend to turn a dark copper color. When mature, larvae often attain a length of 2.5 cm. Smith (1911) provided a good description of the larval instars. (See color figures 25, 28, 74, and 75.)

  1. Pupation usually occurs in a leaf fold; often dead, dry material is used. There is only poor evidence of a cocoon, usually just a few strands of silk. The pupa is elongate, measuring about 13 mm long and 4 mm wide. It is light brown to dark brown, and tapers to a point at both ends. Pupation usually lasts about 8-9 days.
  2. Emerging moths fly during much of the evening hours, but most flight occurs 3-5 h after sundown, with peak flight at approximately midnight (Valles et al., 1991). The female moth produces a pher-omone that attracts males, with peak production occurring at 5-7 h after sunset (Klun et al., 1986; Elsey et al., 1989; Valles et al., 1992). Moths are fairly distinctive in appearance. The central portion of both the front and hind wings is a semi-transparent yellow color, with an iridescent purplish reflection. The wings are bordered in dark brown. The wing expanse is about 3 cm. Both sexes often display brushy hairpen-cils at the tip of the abdomen. Moths are not found in the field during the daylight hours, and probably disperse to adjacent wooded or weedy areas during the heat of the day. Moths do not produce eggs until they are several days old. (See color figure 212.)

Good sources of information on pickleworm biology were supplied by Dupree et al. (1955), Fulton (1947), Quaintance (1901), and Smith (1911). Rearing techniques were provided by Elsey et al. (1984), Robinson et al. (1979), and Valles et al. (1991).

Damage

Pickleworm may damage summer and winter squash, cucumber, cantaloupe, and pumpkin. Water-

Cucumber Fruit Fly Parasitoids
Adult pickleworm.

melon generally is not a host. The blossom is a favored feeding site, especially for young larvae. In plants with large blossoms, such as summer squash, larvae may complete their development without entering fruit. They may also move from blossom to blossom, feeding and destroying the plant's capacity to produce fruit. Very often, however, the larva burrows into the fruit. The larva's entrance is marked by a small hole, through which frass is extruded. The presence of the insect makes fruit unmarketable, and fungal or bacterial diseases often develop once entry has occurred. If larvae burrow into fruit just before harvest, their presence is difficult to detect, yet a considerable amount of larval growth and feeding damage may occur. When all blossoms and fruit have been destroyed, larvae attack the vines, especially the apical meristem. Cantaloupe is not a preferred host, and larvae often seem reluctant to burrow into the fruit. Rather, they feed on the surface or "rind," causing scars. Thus, pickleworm is sometimes referred to as "rindworm."

Management

  1. It is very difficult to scout for this insect and predict its appearance. Moths are not attracted to light traps, and pheromone traps have had limited success (Elsey et al., 1991; Valles et al., 1991). Phero-mone lures are not currently available commercially. Brewer and Story (1987) developed sampling plans for pickleworm larvae in squash. They suggested that the most reliable sampling unit was the large green staminate flower bud. However, the small eggs, night-flying behavior, and inability to trap the insect reliably has led most growers to depend on preventa-tive applications of insecticides.
  2. Cucurbit producers in areas where pickleworm damage is likely to occur usually apply chemical insecticides from the onset of fruiting through harvest. The internal feeding behavior of larvae, which is so difficult to detect at harvest, causes particular emphasis on prevention of damage. In areas that are on the fringe of the normal range there are many seasons when damage do not occur, but producers apply insecticides as a preventative measure because prediction of occurrence is so difficult. Botanical insecticides can be effective (Arant, 1942).

Pollinators, particularly honeybees, are very important in cucurbit production, and insecticide application can interfere with pollination by killing honeybees. If insecticides are to be applied when blossoms are present, it is advisable to use insecticides with little residual activity, and to apply late in the day, when honeybee activity is minimal.

Biological Control. The entomopathogenic ne-matode Steinernema carpocapsae has been shown to effectively suppress pickleworm injury in squash (Shannag et al., 1994). Nematode survival is quite good in large-blossomed squash, where the nematodes can kill the young pickleworm before it burrows into the fruit. This approach is probably ineffective for species with small, open blossoms such as cucumber, however, because the nematodes die quickly when exposed to sunlight. Bacillus thuringiensis can kill pickleworm, but is usually not recommended because the internal feeding behavior puts the larvae beyond the reach of a stomach-active toxin.

Cultural Practices. It is possible to cover plants with screen or row covers to prevent moths from depositing eggs on the foliage (Webb and Linda, 1992). However, as the plants must be pollinated, usually by honey bees, some allowance must be made to leave the plants uncovered. Given the night-flying behavior of the moths and the daytime activities of honeybees, this is not a difficult task on a small planting but prohibitive on large acreage.

Some growers are able to prevent plant injury through careful timing of their cropping cycle. By planting early, it is often possible to harvest part of the crop before pickleworms appear. Usually the crop is eventually infested, so some yield is lost. Plowing under of the crop residue is recommended to destroy pupae in the leaf debris (Smith, 1911).

The presence of aluminum or polyethylene mulch of various colors was shown by Dupree (1973) not to influence pickleworm damage to squash. Wolfenbarger and Moore (1968) reported that white, black, and aluminum mulches did not reduce pickleworm infestation of squash as compared to unmulched crop, but that white mulch produced lower levels of fruit injury than aluminum mulch.

In a comparison of monocultural and polycultural crop production systems conducted by Letourneau (1986), no difference in abundance of pickleworm on squash was observed. In the same study, distribution of melonworm, Diaphania hyalinata (L.) was significantly lower in polycultures, a common response for an insect with a restricted host range. The differential response between the two species is likely due to the more active, dispersive nature of pickleworm.

Smith (1911) reported that squash could be used as a trap crop to keep pickleworm from attacking cantaloupe, a less preferred host. He recommended that destruction of squash blossoms, or even the entire plant, be done periodically to keep pickleworms from exhausting the food supply and then moving onto adjacent cantaloupes. In contrast, Dupree et al. (1955) reported unsatisfactory results with trap cropping.

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