Distribution. Several Drosophila species affect vegetable crops in North America. Origin of the economically important pest species is uncertain, as they are nearly cosmopolitan in distribution. However, most are thought to be immigrants from the tropics. Among the important species are Drosophila affinis Sturtevant, Drosophila busckii Coquillett, Drosophila melanogaster Meigen, Drosophila repleta Wollaston, Drosophila simulans Sturtevant, and Drosophila tripunctata Loew. The importance of these species varies among crops, as well as geographically and temporally. However, they are quite similar in biology.
Host Plants. Although small fruit flies are normally found in association with "true" fruits, such as apple, banana, citrus, pineapple, and tropical species, the fruits and tubers of vegetable crops sometimes support Drosophila spp. Tomato, squash, cantaloupe, watermelon, and to a lesser extent cabbage and potato become infested, but almost any crop that bears fermenting tissue is attractive to ovipositing flies. Some species, such as D. repleta, also are attracted to animal excrement, though most fruit and vegetable-breeding Drosophila spp. do not display this behavior (Harrison et al., 1954). The larvae of the aforementioned species are primarily yeast feeders, and when decay progresses to the state that bacteria and fungi become dominant, the host is no longer suitable for larval development.
Natural Enemies. Parasitoids seem to be an important natural enemies of Drosophila spp., though fungi and mites sometimes are present. Among the parasitoids known to attack Drosophila spp. are Asobara tabida (Nees), Phaenocarpa drosophilae Fischer (both Hymenoptera: Braconidae), Spalangia drosophilae Ashmead, Cyrtogaster typherus (Walker), and Pachycre-poideus vindemiae (Rondani) (all Hymenoptera: Ptero-malidae). Probably the most important mortality agents, however, are beetles drawn to decaying fruit and which also ingest fly larvae, notably rove beetles (Coleoptera: Staphylinidae) and sap beetles (Coleoptera: Nitidulidae).
Life Cycle and Description. These flies breed very rapidly, so many generations can be completed annually if they are not interrupted by temperature extremes. A complete generation can be completed in about eight days when cultured at 29°C, and 1113 days at 24°C. These Drosophila spp. reproduce best at cool to warm rather than hot temperatures, often thriving under autumn conditions rather than summer. The important Drosophila spp. apparently do not overwinter successfully in cold areas, surviving indoors on produce or reintroduced to temperate areas in the spring. Thus, in temperate areas the population density is very low in the spring, only moderately abundant in the summer due to high and unsuitable temperatures, and then building to very high densities in the autumn.
A very good introduction to Drosophila, including keys and species descriptions, was given by Sturtevant (1921). Wheeler (1987) provided a recent key to the genera. Wheeler (1981a,b) presented interesting bio-geographic data on Drosophilidae. The publication by Demerec (1950) also contained a great deal of information, and though emphasizing anatomy and development, monitoring and rearing procedures also were provided. Ditman et al. (1936) provided a concise summary of Drosophila biology as it relates to crop damage.
The eggs of Drosophila spp. are deposited on fruit that is cracked or otherwise damaged. They do not attack undamaged produce. Moderately or completely ripe tomatoes are preferred over the green and pink stages (Collins, 1956). Attack usually occurs at harvest time because much produce is injured by the picking operation. The larvae develop very quickly, and if there is any delay in processing or consumption, larvae may attain a visible size and cause degradation or rejection of the crop. Drosophila melanogaster is normally the most abundant and damaging of the small fruit flies in vegetable crops. The injury is usually limi
ted to tomato, and perhaps melon crops, but Drosophila spp. are not generally considered as serious pests.
Cultural Practices. The presence of small fruit flies in a crop is favored by several factors, including the presence of weeds and grasses, because these provide shade for the flies, which do not prefer hot and sunny conditions; the proximity of a favorable breeding site, often a melon or tomato crop that has already matured, or piles of culled fruit; and injury to fruit, often from equipment or insect or hail damage, which provides oviposition sites. Small fruit fly problems are alleviated by early and frequent picking, which helps to eliminate some of the cracking associated with fruit maturity; the uniform watering of crops, which similarly reduces the frequency of cracking; and careful handling of fruit, which reduces breakage and limits access to oviposition sites by flies.
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