Diaphania hyalinata Linnaeus Lepidoptera Pyralidae

Natural History

Distribution. Melonworm is most abundant in tropical climates, where cucurbits can grow during the winter months. It occurs throughout most of Central and South America and the Caribbean. The United States is the northern limit of its permanent range, and its wintertime occurrence generally is limited to south Florida and perhaps south Texas. Melonworm disperses northward annually. Its distribution during the summer months is principally the southeastern states, extending west to Texas and, sometimes, north to New England and the Great Lakes region. Melon-worm has been reported in North Carolina as early as July (Smith, 1911).

Host Plants. Melonworm is restricted to feeding on cucurbits. The host preferences are nearly identical to that of pickleworm, Diaphania nitidalis (Stoll); both wild and cultivated cucurbits may be attacked. Summer and the winter squash species are good hosts. Pumpkin is 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. Watermelon is almost never eaten. (See color figure 23.)

Natural Enemies. Natural enemies of melon-worm are nearly the same as those of pickleworm. All of the parasitoids found by Pena et al. (1987b) to attack pickleworm also attacked melonworm: Apan-teles sp., Hypomicrogaster diaphaniae (Muesebeck), Pristomerus spinator (Fabricius) (all Hymenoptera: Bra-conidae), Casinaria infesta (Cresson), Temelucha sp. (both Hymenoptera: Ichneumonidae), and undetermined trichogrammatids (Hymenoptera: Trichogram-matidae) (Pena etal., 1987b; Capinera, 1994). However, additional species parasitized melonworm, including Gambrus ultimus (Cresson), Agathis texana (Cresson) (both Hymenoptera: Ichneumonidae) and an undetermined fly (Hymenoptera: Tachinidae). The tachinids known from melonworm are Nemorilla pyste (Walker) and Stomatodexia cothurnata (Wiedemann). Studies conducted in Puerto Rico (Medina-Gaud et al., 1989) reported levels of parasitism reaching 24%. 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 also been reported to be mortality factors.

Life Cycle and Description. The melonworm can complete its life cycle in about 30 days. It is present throughout the year in southern Florida, where it is limited mostly by availability of host plants. It disperses northward annually, usually arriving in northern Florida and other southeastern states in June or July, where not more than three generations normally occur before cold weather kills the host plants.

Egg. Melonworm moths deposit oval, flattened eggs in small clusters, averaging 2-6 eggs per mass. Apparently they are deposited at night on buds, stems, and the underside of leaves. Initially, they are white, but soon become yellow. They measure about 0.7 mm long and 0.6 mm wide. Hatching occurs after 3-4 days. (See color figure 257.)

Diaphania Nitidalis
Melonworm larva.
  1. There are five instars. Total larval development time is about 14 days, with mean (range) duration the instars about 2.2 (2-3), 2.2 (2-3), 2.0 (1-3), 2.0 (1-3), and 5.0 (3-8) days, respectively. Head capsule widths are about 0.22, 0.37, 0.62, 1.04, and 1.64 mm, respectively (Smith et al., 1994). Larvae attain lengths of about 1.5, 2.6, 4.5,10, and 16 mm in instars 1-5, respectively. Neonate larvae are colorless, but by the second instar larvae assume a pale yellow-green color. They construct a loose silken structure under leaves which serves to shelter them during the daylight hours. In the fifth instar, larvae have two subdorsal white stripes extending the length of the body. The stripes fade or disappear just before pupation, but they are the most distinctive characteristic of the larvae. Smith (1911) provided a complete description of larvae. (See color figure 76.)
  2. Before pupation, larvae spin a loose cocoon on the host plant, often folding a section of the leaf for added shelter. The melonworm cocoon is much better formed than the cocoon of pickleworm, and the melonworm's preference for green foliage as a pupation site also serves to differentiate the insects. The pupa is 12-15 mm long, about 3-4 mm wide, and fairly pointed at each end. It is light to dark brown. The pupal stage persists for 9-10 days.
  3. The moth's wingspan is about 2.5 cm. The wings are pearly white centrally, and slightly iridescent, but are edged with a broad band of dark brown. Moths frequently display brushy hairpencils at the tip of the abdomen when at rest. Melonworm moths differ from pickleworm as they remain in the crop during the daylight hours. While they are generally inactive during the day, they fly short distances when disturbed. Smith (1911) provided a detailed account of melonworm biology. Rearing techniques were given by Elsey et al. (1984) and Valles et al. (1991). (See color figure 211.)

Damage

Melonworm feeds principally on foliage, especially if foliage of a favored host plant such as summer or winter squash is available. Usually the leaf veins are left intact, resulting in lace-like plant remains. However, if the available foliage is exhausted, or the plant

Larvas Pyralidae
Adult melonworm.

is a less preferred species such as cantaloupe, the larva may feed on the surface of the fruit, or even burrow into the fruit. As happens with pickleworm, growers sometimes refer to these insects as "rindworms," because they cause scars on the surface of melons. In a study of melonworm damage potential to summer squash conducted in south Florida, melonworm caused a 23% yield loss due to foliage damage (indirect loss) and a 9-10% yield reduction owing to fruit damage (direct loss) (McSorley and Waddill, 1982). Kelsheimer (1949) considered this insect to be the most important pest of cucurbits in Florida.

Management

  1. Pheromone production by female moths peaks at about sunset (Valles and Capinera, 1992). The sex pheromone has been identified (Raina et al., 1986), but is not available commercially. Moths are not attracted to light traps.
  2. Historically, melonworm was considered to be a very damaging pest, but because it feeds preferentially on foliage it is easy to control with a variety of insecticides. In tropical areas it often is considered more damaging than pickleworm. In temperate areas, and especially in commercial vegetable production areas, it is treated as only a minor pest. In insecticide-free cucurbit production and in home gardens, melonworm can cause serious damage.

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 insecticides late in the day, when honeybee activity is minimal.

Biological Control. In addition to chemical insecticides, Bacillus thuringiensis is commonly recommended for suppression. The entomopathogenic nematode Steinernema carpocapsae provides only moderate suppression because the nematodes do not survive long on the foliage, where larvae are found resting and feeding (Shannag and Capinera, 1995).

Cultural Practices. Row covers can be used effectively to exclude melonworm adults (Webb and Linda, 1992). Intercropping of corn and beans with squash was shown to reduce damage by melonworm (Letourneau, 1986). Because melonworm prefers squash to most other cucurbits, trap cropping has been suggested, and of course destruction of crop residue which may contain melonworm pupae is recommended (Smith, 1911). Early plantings, except in tropical areas where melonworm overwinters, often escape serious damage.

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Responses

  • ROSAMUNDA CLAYHANGER
    What type of damage does the melonworm cause?
    8 years ago

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