The Smaller Flies, what you can/should do


Flies are typically separated into 2 major groups, the large flies and the smaller flies. The larger or filth flies (those we’ll cover in the May 2013 blog) are typically about 1/8-3/8″ long, breed outdoors, and come from the outside (e.g., house fly). Then there is a whole group of tiny flies that most people call "gnats.” Although they can commonly be problems in homes, more often they are problems in commercial accounts, especially offices, food handling establishments, food processing plants, and grocery stores.

Why they are of concern. Yes, their presence is a nuisance, but more importantly consider that they all breed in some kind of decaying organic "goo.” Hence, they can carry disease-causing pathogens and can affect human health and food safety. For example, consider the following:

  • Many of these flies breed in very unsanitary conditions and may mechanically act as disease vectors.
  • Some, once dead, because of their delicate bodies can cause bronchial asthma via inhalation of their body parts.
  • Many have a short development time (egg to adult) and can build into large numbers within a short time; e.g., the small fruit fly’s developmental time can be 8 days.

Not only are these “gnats” typically found indoors causing problems, but they also typically breed indoors. Hence, the source of these flies will almost always be indoors. Finding and eliminating the source is the key to solving the fly problem.


Correct identification is the key.

  • Knowing which kind of "gnat" tells one where to look for breeding sites. Although they all breed in some kind of decaying organic "goo,” each has preferred sites. So, let’s get to it.
  • A 10-20-power hand lens or a microscope may be required to positive ID, a good reason to have your identification confirmed by a pest management professional. However, their flight habits and/or the breeding sites you may find are good clues.
  • The most common groups of these "gnats" belong to 3 families of flies. Being flies means that they have only1 pair of wings, belong to the insect order Diptera (di = 2 and ptera = wings), and have complete metamorphosis (egg → larva(e) →pupa → adult). Pupation takes place in the last larval skin, which hardens and turns reddish.

Small Fruit/Vinegar/Drosophila flies (family Drosophilidae).

  • The common name of small fruit fly (often just called fruit flies) comes from their fondness for fruits as egg laying and developmental sites. The name of vinegar fly comes from the fact that they develop in the briny or vinegar-like liquids at the top of imperfectly sealed canned fruits and vegetables. The name drosophila comes from the family and genus (Drosophila) to which they belong.
  • Recognition.
    • Size: About 1/8" (3 mm) long.
    • Color: Tan to brownish yellow or brownish black. Eyes usually red.
    • Antennae: Very short, with a feathery bristle (arista).
    • Flight: Hovers in small circles.
    • Other features: Small robust flies with large bright or dark red eyes.
  • Biology.
    • Females lay their eggs (about 500) near the surface of fermenting fruits and vegetables.
    • The larvae develop in the briny or vinegar-like liquids of the fermenting materials where they feed primarily on the yeast.
    • Prior to pupation, the larvae crawl to drier areas of the food or elsewhere and then pupate.
    • The life cycle (adult to adult) may be completed in 8-10 days at 85° F / 29° C.
  • Habits.
    • Because of their small size, many species are able to penetrate ordinary screens. Smaller screen mesh size may be required.
    • They are attracted primarily to fresh fruits and vegetables and those fermenting because of yeast. Materials lose their attractiveness when they begin to decay because of bacteria and fungi.
    • Newly emerged adults are attracted to lights.
    • Because of their extremely short life cycle of 8-10 days, they can exploit many temporarily available developmental sites such as sour dishclothes and mopheads, fruit under a table or left in a bowl, etc.
    • Typical breeding sites (* = most common) include the following:
      - overripe fruits & vegetables*
      - overripe fruits & vegetables*
      - dirty garbage can/trashcan*
      - fermenting canned goods*
      - stagnant drain trap
      - bagged onions, potatoes, in cracks & crevices & other vegetables
      - sour floor mop
      - food disposal unit
      - refrigerator drain pan
      - containers held for recycling*
      - discarded wine, beer, milk, or ketchup bottles*
      - spilled wine, juice, beer, etc. in cracks & crevices
      - dishwasher
      - old, forgotten dishcloths
      - compost pile

      Note that such things as dishwater and mop water are full of food particles which can accumulate on surfaces and/or in crevices (e.g. under and behind baseboards) and ferment, providing ideal fly breeding conditions.

Moth/Drain/Filter/Sewage flies (family Psychodidae).

  • These flies get their common name of moth fly from their fuzzy appearance due to the wings and bodies being very hairy. The names of drain/ filter/sewage fly are from their typical breeding and developmental sites.
  • Recognition.
    • Size: 1/16-3/16” (1-5-5 mm) long.
    • Color: Pale yellowish to brownish gray to blackish.
    • Antennae: About half the body length, beadlike.
    • Wings: Broadly oval but pointed apically, veins and margins hairy.
    • Flight: Short hops, or hovers above drains.
    • Other features: Body very hairy, fuzzy.
  • Biology.
    • Females lay their eggs (about 30-100) in irregular masses on the surface of the gelatinous film which lines the water-free portion of drains or covers the filter stones of sewage treatment plants.
    • Both larvae and pupae live in this gelatinous film with their breathing tubes projecting through the film.
    • The larvae feed on the algae, bacteria, fungi, microscopic animals, and sludge of this film.
    • Developmental time (egg to adult) is 7-28 days.
    • Adults live about 2 weeks.
  • Habits.
    • Because of their small size, moth flies are able to penetrate ordinary screens.
    • They are weak fliers.
      • Indoors during the day they typically rest on vertical surfaces near drain openings. If they fly, it's a short hop.
      • Outdoors, they can be carried 300 feet or so by the wind.
    • Their greatest activity period is in the evening when they can be seen flying above drain openings or sewage filter beds, etc. outside.
    • Typically, only a few adults are seen at a time indoors because adults live only 2 weeks, but they are continually replaced with new emerging flies.
    • Typical breeding site or sites (* = most common) include the following:
      - Slimy drains*
      - Sewer leak or backup
      - Dirty garbage cans
      - Clogged roof gutters
      - Clogged storm drains
      - Saucers under potted plants
      - Bird baths or feeders
      - Moist compost
      - Air conditioners
      - Cooling towers
      - Rain barrels
      - Septic tanks
      - Sewage treatment plants

      Note: If large numbers of flies are present, there may be a nearby sewage treatment plant, especially upwind from the structure.

Dark-winged fungus gnats (family Sciaridae).

  • Recognition.
    • Small, slender, long-legged, somewhat mosquito-like flies, with their abdomens usually pointed.
    • Size: 1/32-7/16” (1-11 mm; usually 5 mm or less) long.
    • Color: Usually black, sometimes brownish or yellowish.
    • Antennae: Typically long, threadlike, usually 15- or 16-segmented.
    • Wings: Front vein (costa) thickened almost to wing tip; usually with 1 longitudinal vein branched/forked; wings typically darkened.
    • Flight: Prefers to run in a jerky manner.
    • Other features: Eyes meet above bases of antennae; with somewhat elongated coxae, tibiae with 1 or 2 apical spurs.
  • Biology.
    • Females lay their eggs on decaying vegetation, fungi, or excrement on/in which the larvae feed.
    • Developmental time (egg to adult) can be as short as 10-12 days.
  • Habits.
    • Because of their small size, some species of these gnats can penetrate ordinary screens.
    • Weak fliers, they prefer to run in a jerky manner.
    • They are attracted to decaying vegetation, fungi, or excrement.
    • Because of their short development time, a large infestation can develop in a few weeks.
    • Very common flies outdoors in shaded, moist situations.
    • Adults are attracted to lights and collect at windows.
    • They do not bite.
    • Their small size allows many species to come through ordinary window screens. If the flies are coming from outdoors, then reducing the screen mesh size can be helpful.
    • Typical breeding sites (*= most common) include the following:
    • - overwatered potted plants*
      - decaying vegetation
      - overmulched beds
      - overwatered lawns
      - overwatered lawns

The application of IPM methods and tools. Due to limited space, we’ll review examples of IPM pest control strategy and procedures as they can be used primarily in single-family homes and apartments.

  • Overall, preventing fly entry. It is usually far easier and more cost effective to prevent the entry of flies into a structure than it is to eliminate them once inside.
    • Physical barriers.
      • All holes in exterior walls sealed or vent holes screened with screens in good repair.
      • All windows which open are screened, screens in good repair.
      • All doors tight-fitting and fitted with self-closures.
      • Screen doors with self-closures used where appropriate and screens in good repair.
    • Removal of fly attractants.
      • Proper location and maintenance of trashcans and dumpsters.
      • Proper grounds maintenance.
      • Proper lighting.
  • If the flies are small fruit flies (fruit flies) or vinegar flies.
    • Inspect the list of common infestation sites above. Look for spoiling fruit or vegetables and discarded soda, wine, beer, cider/apple juice, milk, or ketchup containers. Remove any found to an outside trashcan.
    • Sour mop heads, dishcloths, or cleaning rags. These must be laundered at least weekly because these flies can develop (egg to adult) in only 8 days.
    • Deteriorating baseboards in the kitchen that are not tight-fitting to the floor and/or wall. Remember that mop water full of food particles that accumulate under the baseboards.
      The replacement of the mop water/solution with a biocide is recommended.
    • Floor drains. If there is gunk in a floor drain and it is suspect, cover the cover/grate with duct tape for 24 hours, but leave the center 1/2″ uncovered. If flies are coming from the drain, they will be stuck to the tape.
      Apply a biocide foam (microbes and/or enzymes used to remove the gelatinous layer inside the pipe just above and below the water line where these flies breed) weekly for the first month and then on a regularly scheduled monthly basis; bleach does not work.
    • Trashcans. Be sure the inside of trashcans are clean (especially underneath the liner) and emptied daily.
    • Counter laminate that is peeling up and scum/goo is building up under the laminate. Until repaired or replaced, treat this buildup with a biocide foam to remove the gunk/goo where these flies breed on a regularly scheduled monthly basis.
    • There are special traps made to capture/harvest these adult flies which have a vinegar based lure in them. However, these flies will outbreed the catching ability of any trap.
  • If the flies are drain or moth flies.
    • Inspect all drains in the immediate area. These flies are weak fliers and rest on vertical surfaces near the infested drain. They can be seen hovering over the infested drain about dusk (sun down) each day.
    • If there is gunk in a floor/sink drain, it is suspect even though no flies are observed in the immediate area. One can verify infestation by covering the drain cover/grate with duct tape (except for the middle 1/2″) for 24 hours. If flies are coming from the drain, they will be stuck to the tape.
    • Apply a biocide foam (microbes and/or enzymes used to remove the gelatinous layer inside the pipe just above and below the water line where these flies breed) weekly for the first month and then on a regularly scheduled monthly basis; bleach does not work.
  • If the flies are fungus gnats.
    • Over-watered potted plants are the most common source, the problem 99% of the time. To confirm a suspected potted plant, place a plastic bag (see-through kind are easiest to use) over the plant and secure it with tape to the pot for 24 hours. If flies are caught within the bag, the source is obvious.
      Note: Also check the immediate outside area to be sure that the plants and/or mulch are not being over watered. Freshly installed mulch could be the source.
    • For quick relief, the infested potted plant can be taken to the outside until the soil completely dries.
    • If this cannot be done, then:
      • Remove any mulch and the top inch or so of soil where the larvae breed, if practical.
        Replace with sterile soil.
      • If removal of soil is not practical, apply an appropriately labeled pesticide to the treat the top 1” of soil as a drench.
        Note: First be sure that the pesticide will not harm the plant by checking the label and with a florist or cooperative extension agent.

So, start with the correct identification (have this verified by a pest management professional), then inspect to find and eliminate the source(s), or employ the services of a pest management professional to do the inspection and work with you on eliminating the source(s).

Eric H. Smith, PhD, BCE
Dodson Bros.