Cluster Fly Biology
Cluster Flies usually live outside during the warm spring and summer months, feeding off of flowers or fruit. As the cooler fall weather approaches, cluster flies find their way into buildings through small cracks and crevices. Once inside, the flies hide in clusters in dark corners, wall voids, and virtually any tight space they can find-- beneath curtains, behind pictures, underneath furniture, inside door and window casings, and under baseboards. Here they will stay over winter until spring. When the warmer weather returns, they will leave their hiding places. The problem arises when cluster flies emerge inside of the home instead of outside and become a nuisance in our living spaces, which is often the case.
Cluster Fly eggs are laid singly in the summer and deposited in to cracks in the soil, where they will hatch within three days. The maggots that emerge then embed themselves inside of an earthworm to complete the larval stage, which lasts between 13 to 22 days. The pupal stage lasts between 11 to 14 days. This process will repeat itself to four generations each summer.
Cluster Fly Prevention and Control
In early summer, before cluster flies have entered the structure:
- Exclude flies from entering by sealing any visible crack or hole on the exterior with caulk. This should be done in early summer before cluster flies seek a place to overwinter. Sealing cracks during the fall or winter is not advised since it will force flies already living in wall voids to emerge on the inside of the structure.
- Apply a residual pyrethroid-based insecticide such as Suspend SC or Demon WP on the exterior of the building in late August or early September, just before adult cluster flies begin to overwinter. They will land on these surfaces and die before they can enter. These products may be applied using a one-gallon sprayer to the undersides of eaves, around doors and windows, and on the sunny side of the building.
Cluster Fly Elimination
Okay, so those pesky cluster flies have already taken up residence in your home, and are showing up in large numbers now that the spring weather is here. Here's how to get rid of them:
- Use a vacuum hose to remove single flies or clusters of adult flies as they appear in attics, basements, crawlspaces, or false ceilings.
- If vacuuming is not convenient, a contact pyrethrum spray like CB-80 can be kept on hand and sprayed lightly as needed for contact kill.
- Inspect around windows for live or dead flies. Any void where cluster flies are found or suspected should be treated using a residual dust insecticide such as Delta Dust or Drione Dust. Apply using a hand duster or small power duster to any enclosed void including underneath baseboards and behind electrical outlets and window and door frames. Some enclosed wall voids in critical areas may need to be drilled, dusted, and then resealed.
- A residual spray insecticide such as Suspend SC or Demon WP may also be used to treat areas where flies are frequently seen after they emerge in the spring, such as attics, basements, storerooms, closets, and other areas.
Fly Light Traps
Installing insect or fly lights in attics and false ceilings is an effective approach to dealing with cluster flies when the overwintering site cannot be determined. When the cluster flies "awaken" in the spring, a fly light will attract and capture a large portion of the cluster fly population before they are able to inhabit living spaces.
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