Bactrocera dorsalis Hendel Diptera Tephritidae

Natural History

Distribution. Oriental fruit fly occurs in tropical regions of Asia, including some Pacific islands. It became established in Hawaii about 1946. On occasion, oriental fruit fly appeared in California and Florida, but always was successfully eradicated. The environment in southern California, Texas, and Florida probably would be suitable for this pest, so it remains a threat. The taxonomy of this insect is uncertain; there likely are several species within the oriental fruit fly "complex." Thus, many elements of the life history, including host records, are suspect until confirmed.

Host Plants. Over 120 plants have been reported to serve as hosts of oriental fruit fly larvae, though many are attacked only during population outbreak conditions. Principal hosts are fruits such as avocado, apple, mango, peach, pear, citrus, coffee, and especially guava. Oriental fruit fly is so highly attracted to guava, and so effective at utilizing this host, that it has displaced Mediterranean fruit fly, Ceratitis capitata (Weidemann) as the principal pest of guava, and significantly lowered the overall density of Mediterranean fruit fly in Hawaii. Among vegetables, pepper, tomato, and watermelon are reportedly attacked.

The adult flies feed on secretions of extrafloral nectaries, honeydew, rotting fruit, bird dung, and other liquefied items. The adults survive only three days without water, and six days with water, but no sources of carbohydrate. The ability of flies to disperse long distances to obtain food is present in this species. However, unlike the clearly defined dispersal of melon fly, Bactrocera curcurbitae (Coquillett), between adult feeding and oviposition sites, it is uncertain how often oriental fruit flies relocate.

Natural Enemies. In the absence of natural enemies, which was the situation in Hawaii immediately after introduction of the fly, very high pest densities are attained. Searches were conducted in many countries for beneficial insects, and a predatory rove beetle (Coleoptera: Staphylinidae) and several hymenopter-ous parasitoids attacking the egg and larval stages were introduced. Apparently some of the wasps contributed materially to reducing population densities of this damaging fly, because fruit infestation levels declined by 1951 (Bess and Haramoto, 1961). The principal beneficial parasitoid is Biosteres arisanus (Sonan), but Diachasmimorpha longicaudata (Ashmead) and

Biosteres vandenboschi (Fullaway) (all Hymenoptera: Braconidae) also are considered important. Over a seven-year period on Oahu, Bess and Haramoto (1961) found that 68-79% of fly larvae in guava fruit were parasitized, though Vargas et al. (1993) reported only about 40-50% parasitism. Destruction of eggs by B. arisanus was sometimes considerable, up to 80%, probably owing to infection of eggs by bacteria and fungi following damage to the chorion by the parasi-toid (Bess et al., 1963). Rearing methods for B. arisanus have been developed (Harris and Okamoto, 1991). Parasitoid monitoring technology has also been studied (Stark et al., 1991; Vargas et al., 1991). Predation, especially by ants (Hymenoptera: Formicidae), was noted but not considered appreciable. Overall fruit fly densities were also determined significantly by availability of fruit and warm weather.

Life Cycle and Description. Oriental fruit fly can complete a generation in about 30 days. In tropical climates, many overlapping generations per year are reported. Fruit fly abundance typically coincides with availability of ripening fruit, though they tend to be most common in summer and autumn (Vargas et al., 1989; 1990).

  1. Oriental fruit fly eggs average about 1.17 mm long and 0.21 mm wide, which is slightly smaller than melon fly. The female may puncture fruit and deposit her eggs, or she may take advantage of cracks or other wounds, including the oviposition punctures of other flies. The principal reason that Oriental fruit fly is believed to have successfully displaced Mediterranean fruit fly in Hawaii is that Oriental fruit fly larvae are more competitive when both species of larvae inhabit the same fruit. Eggs may be deposited at a depth of 56 mm in soft fruit, whereas they may be very near the surface in hard fruit. The upper- and lower-developmental thresholds for eggs are estimated at 38° and 12°C, respectively (Meats, 1989). The average time for egg hatching is 1.6 days (Vargas et al., 1984), but hatching may be extended to 20 days in cold weather.
  2. Oriental fruit fly larvae are typical in form for tephritid fruit flies: cylindrical and broad posteriorly and tapering to point at the anterior end. There are three instars; all are whitish in color. The first instar ranges in size from about 1.2 mm to 2.3 mm, whereas the second ranges from 2.5-5.7 mm and third ranges from 7.0-11.0 mm. The upper- and lower-developmental thresholds for larvae are estimated at 34° and 11 °C, respectively (Meats, 1989). Larval development generally requires about 7.8 days, though its development time can range from 6 to 35 days.
  3. Mature larvae leave infested fruit and enter the soil, usually at the base of affected trees, to pupate. The puparia are 3.8-5.2 mm long and vary in color from tan to brownish-yellow. Pupal development requires about 10.3 days.
  4. The adult fruit fly has a yellow to orange abdomen marked with a black "T". The thorax is predominantly black but bears two-yellow stripes laterally. Oriental fruit fly lacks cross bands on its wings, and therefore is easily differentiated from melon fly. The dark abdominal markings serve to distinguish oriental fruit fly (and melon fly) from Mediterranean fruit fly. After adults emerge, a period of 6-12 days normally elapses before oviposition can occur. Copulation persists for 2-12 h. Males expel pheromone in a visible form resembling smoke (Ohinata et al., 1982), similar to pheromone production by melon fly. Mating occurs at dusk in aggregations called "leks". Mating normally occurs at 4-5 day intervals. The adults continue to produce eggs for about two months. The female oriental fruit fly is more fecund than the related tephritids melon fly and Mediterranean fruit fly, and she produces an average of over 1400 eggs per female during a life span of about 80 days (Vargas et al., 1984). The oviposition rate is reported to be about 130 eggs per day.

Keys for distinguishing all life stages of these species were provided by Hardy (1949), White and Elson-Harris (1992), and Foote et al. (1993).

Damage

The adult flies sting fruit during the process of ovi-position. The presence of larvae is, of course, highly objectionable to consumers. Even if the larva fails to develop, fruit deformities may occur. Also, the ovipo-sition wound is frequently a site for invasion by bacterial and fungal diseases. Other insects such as fruit flies (Hymenoptera: Drosophilidae) and sap beetles (Coleoptera: Nitidulidae) may attack fruit infested by larvae. As many locations lack Oriental fruit fly, quar-

Bactrocera Dorsalis
Adult oriental fruit fly.

antine restrictions are frequently imposed that restrict the sale and transport of valuable produce.

Management

  1. Liquid bait traps such as the McPhail trap (McPhail, 1937; 1939) have long been used in fruit fly detection efforts, but they require considerable maintenance so there is a continuing effort to develop effective but inexpensive trap technology. Methyl eugenol is highly attractive to oriental fruit fly. Only males are attracted (Cunningham, 1989). Yellow and white sticky spheres may be useful for population monitoring (Vargas et al., 1991). Growers are advised to spray crops when flies become abundant in traps.
  2. Male suppression has been used to eradicate oriental fruit fly on some islands. Males are lured to substrate that is impregnated with the attrac-tant methyl eugenol, where flies also come into contact with a lethal dose of insecticide. Baitinsecticide relea-sers are air-dropped or suspended from trees (Steiner et al., 1965b; Koyama et al., 1984). Release of sterile male insects also can be used for eradication (Steiner et al., 1970), alone or in combination with other means of suppression. Insecticides are also mixed with protein hydrolyzate baits and sprayed on foliage to affect fly suppression (Roessler, 1989).

Cultural Practices. Fruit can be protected from flies by being wrapped in netting or bags, as is sometimes done for melon fly. Obviously this approach is limited to small-scale fruit or vegetable production.

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