Bactrocera cucurbitae Coquillett Diptera Tephritidae

Natural History

Distribution. Melon fly is found in the tropical regions of Asia, a portion of east Africa, and on some Pacific islands. In the United States, its distribution currently is limited to Hawaii. Melon fly has been recovered on several occasions in California, and though it has not become established on the mainland, California and Florida would likely be suitable environments.

Host Plants. Melon fly prefers such cucurbits as watermelon, cantaloupe, pumpkin, squash, and cucumber, but infests other vegetables including tomato, pepper, green bean, and cowpea. Wild hosts, particularly bitter melon, Mormordica charantia, can be important. Passion fruit, papaya, grape, and citrus also are suitable larval hosts, but it is the availability of vegetable crops that generally determines melon fly abundance (Vargas et al, 1989; 1990).

The adult often is found associated with plants that are not larval food sources, but rather provide sustenance for the adults. Honeydew produced by homo-pterous insects and secretions of extrafloral nectaries attract flies to corn; castor bean, Ricinus communis; spiny amaranth, Amaranthus spinosus; rattlepod, Crotolaria incana and C. mucronata; and other cultivated and wild plants (Nishida and Bess, 1957).

Natural Enemies. Extensive research has been conducted around the world to identify effective natural enemies of this serious pest. The wasp Psytallia fletcheri (Silvestri) (Hymenoptera: Braconidae), a larval parasitoid, is the most effective biological control agent located. It was introduced into Hawaii from India in 1916 to provide biological suppression of melon fly. Studies conducted in Hawaii found that though P. fletcheri was effective at parasitizing melon fly larvae infesting wild cucurbits, it was relatively ineffective in crops (Liquido, 1991). Therefore, its use is limited to reduction of background populations only. Predators and parasitoids attack all stages of melon fly in Hawaii. A good review was provided by Nishida (1955).

Life Cycle and Description. A life cycle can be completed in about five weeks in warm climates but may require 3-4 months in cool climates. The adults can survive for months, and will continue to reproduce if fruit is available. In tropical climates, such as Hawaii, they are present throughout the year. Their abundance is determined mostly by availability of suitable host plant material, but they tend to be most common in summer and autumn (Vargas et al., 1989; 1990).

  1. Females deposit eggs in small batches, usually 5-20 eggs each. Females may produce 800-900 eggs over their life span. The white eggs are about 1.3 mm long and 0.25 mm wide, and are deposited in the fruit or vegetative parts of plants. The lower- and upper-developmental thresholds for egg development are about 10° and 40°C, respectively (Meats, 1989). Eggs hatch, on average, in 1.3 days.
  2. The larvae are white, and when they hatch are about the length of the egg. The larvae immediately begin to grow, and attain a length of about 2.5, 5.5, and 11.0 mm long during instars 1-3, respectively. The mouth hooks are pale in the first instar, but thereafter they are darkened and easily observed. The larvae complete their development in 5-8 days, with instars 1-3 requiring about 1, 1-2, and 2-3 days, respectively, in soft hosts such as papaya. Back and Pemberton (1914) reported that larvae developing in thick-skinned fruit such as cantaloupe and watermelon may remain in the fruit as full-grown larvae for several days before emerging. The lower- and upper-developmental thresholds for larvae are about 12° and 34°C, respectively (Meats, 1989).
  3. When mature, larvae leave the host, burrow into the soil up to a depth of 10 cm, and pupate. The adults emerge from the tan or yellow-brown pupar-ium after about 10 days (range 7-14 days), and dig to the soil surface.
  4. Newly emerged adults crawl to a sheltered location where they rest for 2-3 h before taking flight. Adults are yellow or yellow-brown, with yellow wings marked with brown bands. They measure about 6-9 mm long. Dark markings resembling a "T" occur on the abdomen, and two lateral and a medial yellow stripe are found on the thorax. The presence of wing bands and the dark abdominal markings can be used to differentiate melon fly from oriental fruit fly and Mediterranean fruit fly, respectively. The pre-oviposi-tion period of adults is about 7.4 days, and they are capable of depositing eggs for about three months.

The adults spend most of their time associated with the adult host plants, seeking vegetable crops or other prospective larval hosts intermittently for oviposition. Fly activity in crop fields peaks in the afternoon hours. At dusk, males form aggregations called leks, and by making wing vibrations and releasing sex pheromone, they attract and copulate with females (Kuba and Koyama, 1985; Kuba and Sokei, 1988). Components of the sex pheromone were described by R. Baker et al. (1982).

Melon fly larva.

Melon fly larva.

Cucurbitae
Adult melon fly.

Life history information was given by Vargas et al. (1984) and Nishida and Bess (1957). Descriptions of all stages, and keys for the differentiation of melon fly from oriental fruit fly, Bactrocera dorsalis (Hendel), and Mediterranean fruit fly, Ceratitis capitata (Wiedemann), were provided by Hardy (1949), White and Elson-Harris (1992), and Foote et al. (1993).

Damage

In the absence of biological or chemical control, melon fly is extremely damaging. For a period of time after the introduction of melon fly to Hawaii in the late 1800s, farmers stopped growing cucumbers, melons, and tomatoes because of the great damage. Melon fly larvae develop in blossoms, fruits, and some vegetative portions of plants. Among vegetative plant material, newly emerged seedlings and terminal shoots are preferred. Similarly, among fruits, immature fruit is usually selected. Larval feeding also opens the plant tissue to secondary invaders, both microbial and insect. Damage can also occur from egg laying even when larvae cannot survive because oviposition allows entry of microorganisms or causes deformities in the growing fruit. Some differences in damage among cucurbits exists. Seedling and stem damage is more common in watermelon and cantaloupe than in squash, cucumbers, and pumpkin. Blossom damage is serious among all cucurbits except cucumber. Both male and female blossoms of squash and pumpkin are affected, but in watermelon and cantaloupe the male blossom generally escapes attack (Back and Pemberton, 1918a).

Management

Sampling. Fruit may be sampled for eggs and larvae, and soil for pupae, but this is not done routinely because of high labor requirements and expense. A

natural chemical constituent of some plants, raspberry ketone, is highly attractive to male melon flies, and a synthetic analog called cue-lure is equally or more attractive (Cunningham, 1989); the latter is useful with traps for sampling. Fluorescent yellow-sticky traps can also be used to sample adult densities (Katsoyannos, 1989; Cunningham, 1989), and odor and visual stimuli are sometimes combined in a single trap design (Economopoulos, 1989). However, the most widely used trap is the McPhail trap, a liquid-baited trap (McPhail, 1937).

Insecticides. Melon fly adults, unlike many insect pests, do not remain in contact with larval host plants for extended periods of time. Instead, they spend a great deal of time in weedy vegetation surrounding crop fields. A program of treating the surrounding vegetation, rather than the crops, was therefore developed. Treatment of border vegetation was much more effective than treatment of crops (Nishida, 1954). Bait sprays containing insecticide also can be used effectively for adult suppression. Sugar or protein hydroly-sate is mixed with various insecticides to produce bait sprays (Nishida and Bess, 1957; Roessler, 1989). Insecticide and bait treatments have been combined with mass release of sterile male insects to eradicate melon fly from some Pacific islands (Steiner et al., 1965a). Cue-lure can also be mixed with insecticide to attract and kill flies, but it is only effective against males (Cunningham et al., 1970; Cunningham and Steiner, 1972).

Cultural Practices. Field sanitation is the most important element of cultural management. High melon fly populations result from continuous availability of larval food, which may be due either to continuous cropping or failure to dispose of crop residues. Destruction of infested or unmarketable vegetables in a timely manner is essential.

Protective coverings have long been used to deter oviposition by melon fly. Paper bags and newspapers are used to wrap individual fruit of large produce such as cantaloupe and watermelon. This is effective but tedious, and not useful for small-fruited vegetables. Also, it does not protect vines and flower buds. Row covers provide complete protection, but pollination may be interrupted.

Trap crops are not effective for protecting vegetables, probably due to the relatively wide host range and vagility of the insects.

Biological Control. Melon fly larvae are moderately susceptible to steinernematid nematode infection (Lindegren and Vail, 1986). There may be some use for this approach in suburban rather than agricultural environments because financial cost would be less of a consideration.

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