Larvae of two types of flies (families Drosophilidae, called drosophilids and Tephritidae, called tephritids) commonly infest certain fruits or nuts. Both fly types are called fruit flies.
Adults can be observed on and flying around hosts. They can be captured in various types of monitoring traps, and the best traps use a bait that attracts the flies. Eggs and larvae live hidden in fruit or nuts, as may pupae. Pupae can also live in organic litter and topsoil under infested host plants. Eggs, mature larvae, and pupae of tephritids are twice or more the size of those of drosophilids.
Drosophilid flies. Also called pomace or vinegar flies, the adults and mature larvae are 1/12 to 1/8 inch (2 to 3 mm) long. Adults are brown to yellowish flies with red eyes. Adults of most species have clear wings and are not readily distinguished to species. Larvae are pale orangish, translucent, or whitish. They have an indistinct, forked breathing tube at the rear and dark mouthparts in front. Pupae are generally brownish and have forked breathing tubes at their rear end. They are found in, on, or near damaged fruit where they fed as larvae.
The species important for home fruit growers is the spotted-wing drosophila (Drosophila suzukii). Males of spotted-wing drosophila have a dark blotch near the tip of each wing. Where female spotted-wing drosophila lay eggs they leave a tiny depression in fruit with a hole in the center.
Tephritid flies. Adults are about 1/4 inch (6 mm) long and mature larvae are up to 1/2 inch (12 mm) long. Adults can generally be distinguished to species by their coloration and wing markings. Eggs are long, slender, whitish, and less than 1/25 inch (1 mm) long. Larvae are rounded at the rear and tapered toward the front. They have indistinct, dark mouthparts and overall are translucent to whitish. Larvae can take on a pale coloration of the fruit on which they feed. Pupae are pale brownish to orange, oblong, rounded at the rear end, and tapered toward the front.
Pest tephritid fruit flies established in California include apple maggot (Rhagoletis pomonella), olive fruit fly (Bactrocera oleae), and walnut husk fly (Rhagoletis completa). Western cherry fruit fly (Rhagoletis indifferens) occurs in Washington and some other states.
Other exotic tephritid species have been repeatedly introduced into California and eradicated. These include Mediterranean fruit fly (Ceratitis capitata), melon fly (Bactrocera cucurbitae), Mexican fruit fly (Anastrepha ludens), and Oriental fruit fly (Dacus dorsalis).
Fruit flies develop through four life stages: egg, larva, pupa, and adult. After hatching, larvae develop through three increasingly larger instars. Mature larvae (prepupae) generally emerge from fruit to pupate but may be found in or on fruit.
Fruit flies overwinter as prepupae or pupae on the ground. Adults emerge the following spring or summer. The number of generations varies by species and growing location. For example, spotted-wing drosophila can have up to 10 generations per year. Olive fruit fly can have up to 5 generations per year. Walnut husk fly has 1 generation per year.
Drosophilid or vinegar flies, such as the common Drosophila melanogaster, as larvae feed in damaged or fermenting, overripe fruit of all kinds. Most species do not feed in undamaged fruit. Fruit fly larvae that feed in undamaged fruit directly damage the fruit and cause it to become contaminated with microorganisms that contribute to decay.
The introduced spotted-wing drosophila is a serious pest because it feeds as larvae in otherwise undamaged, ripening fruit. Host fruits of the spotted-wing drosophila include ripening blackberry, blueberry, cherry, raspberry, and strawberry. It occasionally attacks boysenberry, Japanese plum, nectarine, and plumcot.
The tephritids established in California are host specific. They primarily infest the fruit or nut crop of their common name. The exotic tephritid flies that have been repeatedly introduced and eradicated from California (e.g., Mediterranean fruit fly, melon fly, Mexican fruit fly, and Oriental fruit fly) each feed inside numerous species of fruits.
Take any tephritid flies or maggots you find in fruit or nuts to the local office of the county agricultural commissioner for identification unless you are confident the species found is already known to be established in California. For example, compare the wing markings of the species found to those of the tephritids that are known to occur here.
Use sanitation to help suppress the abundance of fruit flies. Remove and dispose of overripe fruit, such as that dropped on the ground. For certain species, specialized traps can help to reduce fly abundance but these will not provide complete control.
Spinosad applied at the proper times can control these fruit flies. For example, the formulation GF-120 NF Naturalyte Fruit Fly Bait combines spinosad with an attractive bait for spot spraying to control fruit flies. Pyrethrins, or pyrethrins sold premixed with piperonyl butoxide, can also be applied to quickly reduce the abundance of vinegar flies.
For species-specific management information see Apple Maggot—Rhagoletis pomonella, Managing Western Cherry Fruit Fly in the Home Garden (PDF), and Pest Notes: Olive Fruit Fly, Spotted-wing Drosophila, and Walnut Husk Fly. For a well-illustrated guide on how to recognize exotic tephritids that are not established in California but are periodically introduced, see Field Guide to Target Insects in California Pest Detection Programs (PDF). For detailed technical information see Pest Fruit Flies of the World.
Adapted from the publications above and Pests of the Garden and Small Farm: A Grower's Guide to Using Less Pesticide and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
Adults of a tephritid (left) and drosophilid (vinegar) fruit fly.
Adult, male spotted-wing drosophila.
Larva of a vinegar fly, Drosophila species.
Adult apple maggot flies, male (left) and female.
Larvae of apple maggot feeding and contributing to fruit decay.
Adult olive fruit fly.
Adult walnut husk fly.
Adult western cherry fruit fly.