Small Fly Management for Homes

by David Moore
Manager of Technical Services and Board Certified Entomologist
with contributions by Eric Smith, PhD, BCE

Warm weather is here and along with it comes many flying insects, and especially bothersome are flies. In this blog, we’ll discuss only the small flies because we discussed the larger (filth) flies in the last blog.

The 4 most common of the small flies that are found in homes are the fruit (small) or vinegar (genus Drosophila, family Drosophilidae), phorid, humpbacked, scuttle, or coffin flies (several species, family Phoridae), drain, moth, or sewage flies (several species, family Psychodidae), and dark winged fungus gnats (several species, family Sciaridae).

How can each group be identified?

Because all of these flies are very small in size, it is best to have any preliminary identification verified by a pest management professional who may have to take specimens back to their office for examination under a microscope for positive ID.

1. Fruit (small) or vinegar flies. Adults are about 1/8″ long. Their color is dull, tan to yellowish to brownish black, and they have bright red to dark reddish brown eyes depending on the species.

2. Phorid, humpbacked, or scuttle flies. Adults are about 1/64-1/4″ long with a very characteristic humpbacked appearance when viewed from the side. Their color is black, brown, or yellowish. Run about is an active erratic manner.

3. Drain or moth flies. Adults are about 1/16-3/16″long, and are very delicate and fuzzy in form. Their color is yellowish to brownish gray to blackish depending on the species. Their wings are broadly oval with hairy margins.

4. Dark winged fungus gnats. Adults are about 1/32-7/16″ long, slender, and long-legged, somewhat mosquito-like. Their color is usually black.

Why is their control desirable other that the fact that they are a nuisance?

All of these flies may be of medical concern as possible mechanical disease vectors because they develop in moist decaying organic material such as that found in drains, mop heads, sewage, etc. In addition, the larvae of humpbacked flies have been found in open wounds on people.

Where do these flies breed?

1. Fruit (small) or vinegar flies. The common/wild species (has bright red eyes; Drosophila melanogaster) prefer fruits and vegetables in the early stages of decay or fermentation. The dark-eyed fruit/vinegar flies (Drosophila repleta group) prefer decaying organic matter such as the biofilm (slimy substance) of dirty drains. They are also found the moist dirty mop heads.

2. Phorid, humpbacked, or scuttle flies. These breed in moist organic matter including that found in drains, grease pits, sewage under slabs, and some species breed in human corpses.

4. Drain or moth flies. These breed in the biofilm (slimy substance) of dirty drains, sewage pits, and wet organic matter.
5. Dark winged fungus gnats. The larvae feed primarily on the fungi growing in soil and on moist decaying organic matter.

How fast can they develop?

In the summer, most of these flies can go from egg to adult in about 7-28 days. Fruit (small) or vinegar fly females average about 500 eggs which require only about 6-10 days. Phorid, humpbacked, or scuttle fly females lay up to 740 eggs which require 11-28 days depending on the species. Drain or moth fly females lay 30-100 eggs and they require from 7-28 days depending on the species. Little is known about dark winged fungus gnat biology other than several to many generations per year are typical, depending on the species.

How are these small flies managed?

The fact that they are small flies indicates that they are most probably breeding within the home. So, it is critical to correctly identify which group(s) of small flies is causing the problem because this will indicate the likely source. The key to control is correct identification, finding the source, and eliminating the source. You may need the services of a pest management professional to do this.

Fungus gnats

• Overwatered potted plants. This is the most common source 99% of the time.
  • To confirm this, put a clear plastic bag over the plant and secure it to the pot for 24 hours. If gnats are in the bag, this is the source.
  • For infested potted plants, remove the top 1″ of soil and replace it with sterile soil.
  • Do not water the plant until the soil is dry. If the water overflows into the saucer below the pot, empty this until it stays dry.
• Overwatered mulch on the outside.
  • First, the mulch should be no more than 2″ deep. If deeper, remove all mulch down to the soil and then add new mulch. If removal is not practical, the mulch should be thoroughly treated with an appropriately labeled pesticide.
  • Mulch should be watered only when the top 1/2″ of the mulch is dry.

Small fruit or vinegar flies

• Look for spoiling fruit or vegetables and any wine, beer, cider/apple juice, milk, ketchup, or soda containers being held for recycling. Remove any found to an outside trashcan. Opened containers can be temporarily stored in the refrigerator, and also fruits and vegetables if appropriate.
• Dish rags and mop heads. These must be laundered at least weekly.
• Floor and sink drains including garbage disposals.
  • To check, put a clear plastic bag over the drain overnight and secure it. If flies are in the bag in the morning, this is a source.
  • Drains must be kept clean down through the p-trap. Disposals must also be cleaned under the header gasket. Microbe cleaners work best.
• Keep the trash cans clean, lined with plastic bags, and emptied at least weekly.

Drain or moth flies

• Inspect all drains in the immediate area. These are weak fliers and rest on vertical surfaces near the source. They can be seen hovering over the infested drain about sundown each day.
• Floor and sink drains.
  • To check/confirm, put a clear plastic bag over the drain overnight and secure it. If flies are in the bag in the morning, this is a source.
  • Drains must be kept clean down through the p-trap. Microbe cleaners work best.
• Occasionally, an infestation can involve leaking or broken sewage pipes in crawl spaces and below slabs. A pest management professional should be consulted.

Phorid, humpbacked, or scuttle flies

• See the advice for small fruit or vinegar flies above. However, they do prefer moist organic matter in a more advanced stage of decay.
• Occasionally, they infest overwatered potted plants; see above.
• Occasionally, an infestation can involve leaking or broken sewage pipes in crawl spaces and below slabs. A pest management professional should be consulted.

If you don’t want to or can’t solve your small fly problem

• Yes, individual rooms can be treated by applying an aerosol pesticide formulation according to label directions; but do not over apply. Unfortunately, this will only temporarily reduce the number of adult flies present, which will be quickly replaced. The source(s) must be found and eliminated for control.
• It is advisable to call a pest management professional (PMP) who will inspect your house and yard. They will identify and advise you about fly breeding sites, problem sanitation areas, and routes of fly entry.
• Remember that correct identification of the problem fly/flies is critical to finding the source(s) of infestation.
• PMPs can also offer traps for harvesting some species of adult flies, and can apply pesticides if, where, and when required to provide temporary relief.
• PMPs also have microbial formulations for cleaning out the biofilm (slimy gunk) in drains where several of these flies breed.
• Remember that fly reduction is a cooperative effort and that you must do your part. Also, flies are a very common and mobile insect that can be introduced or reintroduced into your home on multiple occasions. So, you should expect to see an occasional fly inside.