Fruit flies are small, stocky-bodied flies (<1/8 inch long), with short antennae, and a dull, dark-gray to tan color. They often have small red eyes. They might easily be confused with other small flies, except for their hovering behavior around ripe fruits, decayed vegetables and syrups around food preparation areas.
Phorid flies are small flies up 1/8 inch long. These flies can be recognized by the distinct ”hump” or arch of the fly’s thorax, enlarged hind femurs and reduced veins (no cross-veins) in the wings. They often fly and walk with a quick, darting motion.
Drain flies are 1/16 to 1/4 inch long, pale yellowish to brownish gray to black in color, with broad wings held in a characteristic V-shaped position over the back. Wings are have parallel, hairy veins and margins. They are often seen resting on walls and other vertical or horizontal surfaces.
Fungus gnats are another small (1/8 inch long) fly, more common in classrooms and office areas. They can be distinguished from the other small flies by their slender, mosquito-like appearance (much smaller than a mosquito) and long, hairless antennae. They become a nuisance when they hover around faces and computer screens.
Fruit flies (Drosophila species) are often present in the kitchen areas and food service areas of schools. They are also called pomace or vinegar flies, and are sometimes confused with other small flies, especially humpbacked flies (Family Phoridae). Fruit flies are strongly attracted to, and breed in, fermenting fruits or liquids. Large numbers of fruit flies may indicate unsanitary conditions including spilled or spoiled fruit and vegetables; poorly maintained garbage containers; accumulation of organic matter around drains, grout or broken tiles; and wet areas under and behind equipment.
Phorid flies feed on, and breed in, wet areas containing decaying organic matter. Common breeding sites include drains, garbage areas, animal carcasses, and contaminated soil. Phorid flies may also be associated with decaying organic matter in the bottoms of garbage cans, or trapped in cracked floor tiles or under the bases of kitchen equipment. Chronic, heavy infestations of phorid flies may be a result of a broken, underground sewage line. When other causes have been ruled out or corrected and a phorid fly problem persists, sewage lines should be inspected by a plumber. Any breaks should be fixed and contaminated soil removed to eliminate the infestation.
Drain flies are also called moth or sewage flies. Adult female drain flies deposit eggs in in drains, garbage disposals, grease traps, and even sewage treatment plant filters. Larvae feed on bacteria mats inside drains and on decaying organic matter in a variety of sites, and can survive in extremely wet conditions. Most infestations are generated from within the school, including food service areas and custodial closets. Drain flies can carry bacteria and other microorganisms from egg-laying sites to food and food contact surfaces and high populations should not be tolerated.
Larvae of fungus gnats feed on fungi and plant rootlets, and are most commonly found emerging from the soil of potted plants. Fungus gnats typically do not harm healthy plants but their presence can be an indication of over-watering. High populations may feed on plant roots and adversely affect plant growth, especially young plants, if preferred food, including microorganisms, is not available. Fungus gnats may also carry plant disease organisms from one plant to another.
None of these small flies bite, although their presence may sometimes be associated with employee complaints of bites. Any association of these flies with bites is incidental, or possibly psychological in origin (e.g., fungus gnats look like very small mosquitoes, and may induce people to imagine that they are being bitten).
The key management strategy for control of these flies is to identify and eliminate the breeding sites. Control of adult flies with sprays, traps, etc., will be only temporary unless the source can be eliminated.
Suggested thresholds will likely be based on complaints, or observations from sticky cards and glue boards. More than two flies (of any species) observed per visit, or collected per sticky trap or glue board indicate a need for more thorough inspection of the area to identify and eliminate potential breeding sites. Where employee complaints about fruit flies occur more than once a month, fruit fly traps that use liquid attractants should be employed as a supplement to breeding site elimination. Office complaints of fungus gnats are more difficult to assess, and thresholds should be based on consultations with the campus principal.
Routine visual inspection for fly breeding sites should be made during each IPM service visit. Inspections should be made of tile flooring, under kitchen equipment and floor drains. Sticky cards, UV light traps and glue boards can also be used to monitor drain flies and fruit flies. Kitchen staff should be trained to inspect incoming produce for fruit flies; infested produce should be discarded or covered and places in cooler or refrigerator until it can be more closely inspected and bad produce culled.
When drain flies or phorid flies are suspected to be emerging from floor drains, a piece of duct tape, or a tent formed from a sticky trap or glue board may be placed over the suspect floor drain. These should be checked within the next day or two for flies.
Indoor plants can be gently lifted and or shaken to determine if fungus gnats are present; yellow or blue sticky traps can also be mounted on stakes placed in potted plants to monitor for fungus gnats.
Indoor (UV light) fly traps should be numbered with the location noted on a list or ideally on a schematic diagram of the facility and dated and initialed each time they are checked or replaced. For drain and fruit flies, ideal placements include locations near plumbing fixtures, dishwashers, under prep tables and in trash or recycling storage areas. Electrocuting type fly traps should not be used in kitchens as exploding insects can contaminate food preparation surfaces. Bulbs on UV light traps should be replaced annually.
Specific monitoring for fruit flies, including fruit fly traps, should not be required on an ongoing basis if the proper management practices are in place to prevent conditions conducive to fruit fly infestation.
Cultural, physical and mechanical management options are the best strategies and include posting notices to encourage the cleanup of spills, proper food storage and trash/recycle handling, elimination of standing water, fixing plumbing leaks, drying mops, emptying mop buckets and inspecting incoming produce and rejecting any infested or overripe product
Always read and follow the label. The label is the law. Pesticides must be used in accordance with federal, state and local regulations. Applicators must have proper credentialing to apply pesticides and should always wear personal protective equipment (PPE) as required by the pesticide label during applications. All labels and Material Safety Data Sheets (MSDS) for the pesticide products authorized for use in the IPM program should be maintained on file.
When using pesticides in schools, appropriate notification and waiting intervals should be observed. For more information contact your state regulatory agency.
Populations of small flies, especially in kitchens, can fluctuate greatly in a short period of time. Follow-ups to service calls for small flies should be conducted approximately one-week following treatment. Authors: Compiled from publications by PMSP, Janet Hurley, Mike Merchant