This is a vast group of insects that includes houseflies, the most common of pests.
As insects, adult flies have three body parts, a head, thorax, and abdomen, and six legs. Adult flies are unique in the insect world because they have only two flight wings. This may be difficult to see with the naked eye, but a good magnifying glass can help you look at where the wings attach. Are there two points of attachment, or more?
Indoor flies come in a variety of sizes and shapes. The immature stage of all flies are maggots: eyeless, legless, worm-like organisms. Fly pupae, the stage between maggots and adults, are typically brown, cylindrical, and cigar shaped. They can be confused with rodent droppings.
Some species of non-biting flies can vector (carry) microorganisms and pathogens that could affect human health. Flies develop as larvae in wet situations where pathogens related to filth or food-borne illnesses may be present. Hairs on their bodies can pick up and transfer pathogens from one surface to another. Some fly species regurgitate previously ingested food items, which can contaminate food or food preparation and serving surfaces. These concerns make flies a significant public health risk.
In pest management, it’s said that “flies tell a story.” This means that each fly species found in and around structures seeks out specific conditions. Knowing what fly species you have tells you what conditions are present, just as examining those conditions hints at what fly is present.
Now, let’s look at these non-biting flies—what do they look like and why are they hanging around?
Metallic blue, green or bronze (variable size, 1/4 to ½ inch; 6 to 14 mm). Adults are metallic blue, green or bronze, and when found inside buildings usually indicate the presence of a dead animal. Outdoors, bottle flies feed on a variety of food items, from picnic food to pet feces.
Adult female bottle flies will lay eggs on small to large dead animals (ex. mice to deer), the carcass of which serve as food for larvae. Read more about bottle flies here.
Looks like a house fly except … a grey body with yellow hairs on thorax (1/4 to 3/8 inch; 8.5 to 10 mm). Adult flies enter homes in the fall for protection against adverse winter weather conditions. These flies do not feed or reproduce indoors, but cluster in protected areas, especially attics. Adult females lay eggs in the soil and larvae are parasites of earthworms.
These flies look like giant mosquitoes with long, spindly legs, but they are not mosquitoes and do not bite. The larvae of some crane flies are actually predators of mosquito larvae. Sizes vary (some larger than an inch; 30 mm). They normally live outside, but are highly attracted to lights and end up indoors. The larvae develop in wet or moist outdoor environments. One European species is a pest of turfgrass in the larval stage. Read more about crane flies here.
Small (1/4 inch; 5 mm or less) with a hairy body and wings. Drain flies resemble moths and often rest on walls resembling an upside down heart shape. As the name implies, the larvae of this fly develop in drains and pipes– specifically those with accumulation of organic material, including hair, and a layer gelatinous goo on the inside of pipes. Read more about drain flies here.
Large grey fly with three black stripes on thorax (3/8 inch to ½ inch; 10 to 14 mm); checkerboard pattern on abdomen ending in an orange tip. This large, grey and black fly indicates the presence of a dead animal the size of a rat, squirrel or larger, which provides the food source for larvae.
Fruit Fly, Dark-eyedDark-gray to black; rests on wall; hovering flight (larger than red-eyed fruit fly: 1/8 inch; 3 to 4 mm). Larvae feed on yeast found in decaying plant and animal material. They are commonly associated with wet food spillage in restaurants, coffee shops, and bars. Read more about fruit flies here.
Fruit Fly, Red-eyed
Tan to beige color; red-eyes; hovering flight (1/16 inch; 2 mm) Larvae feed on yeast found in fermenting fruit, including banana peels and spilled lemon and lime slices. Larvae can also breed in damp cloth items such as wet mop heads and wet dishrags, as well as sugar sources like soda fountains and beer taps. Read more about fruit flies here.
Small (variable sizes, up to 3/8 inch; 10 mm) and mosquito-like, they may have dark wings. The larvae feed on fungus that grows in over-watered potted plants, and sometimes in the overflow trap of sinks. Adults are attracted to lights, including the electronic screens of computers, tablets and cell phones, so they can appear to be flying around your face. Read more about fungus gnats here.
Four black stripes on thorax (where wings and legs attach), medium sized (1/8 to ¼ inch; 5 to 8 mm). Breeding typically occurs outdoors, with larvae developing in decaying organic material such as garbage, animal waste, and grass clippings. Adult flies enter buildings through open, broken or improperly installed windows, screens and doors, or if pulled indoors because of negative building pressure. Therefore, in most cases, these flies indicate poor exclusion or cultural practices of leaving doors and screen-less windows open. They may be attracted inside by food smells.
Mosquito-like, with large fluffy antennae. This outdoor fly is attracted to lights, often in large numbers. They vary in size (commonly from 1/8 to ½ inch; 5 to 12 mm) and do not bite. Larvae breed in moist to wet environments, including streams.
Small (1/16 inch; 1 mm), light tan with a dark region at front of wing. Look for four wispy veins/lines on main part of wing. Phorid flies exhibit erratic movement in flight and on foot. Larvae breed in rich organic material, notably septic waste, rancid dairy products and human remains in coffins. Adults can indicate breaks in a sewer pipe under a slab foundation, requiring specialized and often destructive investigative techniques to determine the source.
Find the breeding source
Fly management is simple in theory but may be difficult in practice. The oversimplification is that effective fly management depends on finding and eliminating the material in which they are breeding. However, finding the source can be a challenge. Knowing the fly species will tell you what condition to look for: a clogged drain, overwatered plant, dead animal, etc. However, some sources can be elusive, and fly problems get out of hand if the source is not obvious nor addressed during regular cleaning.
To home in on the source of a larger fly problem, think about the room or specific area where you see them most. Focus your inspection there because adult flies are often (but not always) found near the breeding source. Once you find the source, you can often eliminate it through cleaning to remove wet, organic material (see below). Note: one complication is that many flies are attracted to light, whether a light fixture, television, or window, so take this into consideration when looking for the source.
Once the source has been found, clean or remove it to eliminate the breeding site and provide long-term control.
Trapping can be used to detect and remove adult flies, but will not eliminate a fly problem. It is important to note that different fly species are attracted to different trap types. Here are a few different kinds of fly traps:
Light Trap: Available in a number of sizes and designs, from conspicuous to inconspicuous, professional to nonprofessional, many flying insects are attracted to ultraviolet light emitted from these devices, including: crane fly, midge, fungus gnat, house fly, bottle fly, and flesh fly.
Vinegar or Juice-Baited Traps: small vinegar traps with holes in the top are available for purchase or can be made at home to attract and capture fruit flies. See the Management section in this Cornell Insect Diagnostic Laboratory fact sheet, Fruit Flies, Vinegar Flies, and Pomace Flies.
Red Wine-Baited Trap: attractive to dark-eyed fruit flies and can be accomplished with the same cups described in the vinegar and juice-baited traps.
Yellow Sticky Cards: used primarily in greenhouse settings, these are attractive to fungus gnats in potted plants.
Drill Hole & Clear Cup: this specialized technique is used to determine if phorid flies originate from a sub-slab sewage leak. A construction professional equipped with the utility and floor plans can determine locations to drill small openings in the floor and cover each with a clear plastic cup. Adult flies emerging from the space under the slab will take the path of least resistance and head toward the light of the room, but get trapped in the cup. This can confirm or rule out a broken sewer line and help pinpoint the location of the problem.
Keep in mind that the source refers to the location where larvae are feeding and growing into pupae and then adults. This is different from adult feeding sites, which might include any number of liquid spills on a variety of surfaces. Removing the item that sustains the larvae – the source - is what leads to effective control.
Breeding sources tend to be moist. In restaurant kitchens and other locations where food spillage is a common breeding source, moisture can be kept to a minimum with the use of fans directed at problem areas. This can include dishwashing areas and locations where floors are wet washed.
“No Pest Strips” are not intended for fly management in places where people are present for more than four hours or where food is prepared or served. As with all pesticides, the label is the law, and the Directions for Use section states where and when these products can be used. For more information about these devices, see Pest-Strips: a Kitchen No-No.
Commercial Kitchens: some commonly overlooked issues that lead to fly problems include:
Floor drains that are not properly maintained can lead to problems with flies and other pests (including cockroaches). Drains should be clear of debris and have visible water in the J-trap. For more details on proper functioning of floor drains, see Pests of the Pipes.
Grout is used to fill the space between floor tiles. Repetitive washing of floors with liquids and cleaning chemicals can erode tile grout, leaving gaps where water and food spillage collect. Tile grout should be replaced if eroded and cleaned to prevent accumulation of food spills.