Diptera Drosophilidae

Natural History

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.

  1. The female seeks cracks and wounds of fruit for oviposition. The eggs of Drosophila spp. are white, elliptical in form, and bear 2-4 filaments at one end, depending on the species. Similarly, filament length varies among species, but it is less than the length of the egg. The eggs measure 0.6-0.7 mm long and about 0.2 mm wide. The eggs are deposited at the surface of moist tissue; the filaments apparently keep the eggs from sinking. Fecundity is favored by cool temperatures. Females produce eggs at a rate of about 15 per day, with a mean total fecundity of about 430 eggs when reared at 25-30° C. However, when held at lower temperature of 19-25°C, egg production averages 26 per day and total fecundity increases to 940 eggs per female. Duration of the egg stage is 1.0-1.5 days.
  2. There are three instars. Larvae increase in size from about 0.9 mm at hatching to 6.5 mm at maturity. The larva is elongate and tubular in form, tapering at the anterior end to a point. Larvae are white, except for black mouth hooks at the anterior end. The number of teeth on the mouth hooks increases from 1, to 2-3, and 9-12 during succeeding instars. The mouthparts (cephalopharyngeal skeleton) measure about 0.22,0.45, and 1.0 mm long in instars 13, respectively. Large spiracles are evident on the posterior end of the larva. Duration of the larval stage is about four days at 29°C and 5.5 days at 24°C.
  3. The larva seeks a dry location for pupation, usually leaving the fruit and dropping to the soil. The puparium formed by the larva is whitish initially, but turns brown in 4-5 hours. It is basically oval, but bears two elongate-tubular structures which function as spiracles at the anterior end. The puparium measures 4.0-4.3 mm long and 1.4-1.5 mm wide. Duration of the pupal stage is about six days at 24°C and 5.5 days at 29°C.
  4. The adult is yellowish or grayish with dark bands on the abdomen. The abdomen sometimes appears entirely dark. Eyes are large, and usually red in color. Wings are colorless. The fly measures about 2.5 mm long, though D. melanogaster is smaller, averaging 2.0 mm. Oviposition commences when the adult is about two day old, and continues for 20-30 days. Adult longevity is inversely related to temperature, and flies are most active early and late in the day. Males survive for about 40 days and females for 70 days when reared at 18°C. However, when cultured at 28°C, males survive for only 22 days and females for 28 days.

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.

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

Drosophilidae

ted to tomato, and perhaps melon crops, but Drosophila spp. are not generally considered as serious pests.

Management

  1. Flies are attracted to light traps, yellow-sticky traps, and juice-baited jar or vial traps. Red containers are somewhat more attractive than other colors when jar or vial traps are used (Wave, 1964). However, fruit flies are detected easily by visual means because, though small in size, they allow close approach and are readily recognized.
  2. Insecticides can be applied to crops to reduce the number of flies present and ovipositing on fruits at harvest time. This is most effective before fruit matures, however, because the number of effective insecticides which can be applied up to the day of harvest is limited, and insecticide residues must be minimized.

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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Responses

  • gruffo gaukrogers
    What are the host plants for fruit fly drosophilidae?
    8 years ago

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