Cluster Fly Control
Prevent Cluster Flies
Prevention is the best measure to keep away Cluster Flies. Fall months are the times to use a good residual sprays before they enter the building and over-winter. Spray the outside walls of the building where cluster flies are likely to land and rest before entering the building.
Spray recommended residual insecticides, such as LamdaStar UltraCap 9.7 or Cyper WSP. Apply these insecticides outdoors to window frames, door frames, soffits and eaves as well as any other areas that are vulnerable to entry. Spray beneath the eaves and around the windows and other areas where flies are likely to seek entry. Apply in the late summer or early fall.
Application of to the surrounding soil of the structures with LamdaStar UltraCap 9.7 may also help minimize Cluster Fly populations and later entry.
Active Cluster Fly Infestations
Cluster fly problems can be partially prevented, but after the fall little can be done. All of those currently found within the home had infiltrated walls months ago. A "mini-vac" is the best control of the individual flies that are currently present. If this is not successful, an aerosol contact pyrethrin aerosol can be used as a space spray to get some immediate relief. Recommended pyrethrin aerosols are CB-80 or PT 565 contact aerosols.
FAQ: It is winter time, but we have had some warm days recently. When should I spray if I see them after the fall time, is it still too late? When should I seal up the area, there are still many flies inside?
Answer: If you seal them up at this point, the cluster flies probably won't know how to get out. The time to seal up is in early fall before they start to get inside. Wait a couple of months to seal but spray now (for the ones that have come out already).
You can also use a dust D-Fense Dust. Dust this into all cracks and crevices. The dust will flow into the void areas. Pay attention to the west and south walls and caulk any openings.
Identifying the Cluster Fly (Pollenia rudis)
The cluster fly averages between 1/4 to 3/8 inch long. They are dark gray, never metallic blue or green. When crushed, they give off an odor like buckwheat honey. Cluster flies closely resemble house flies, but they are usually larger and have a yellowish sheen on the thorax. They move slowly. They gather in cluster or large numbers, particulary around windows.
Cluster Fly Image Source: Steve Jacobs PSU Entomology
Biology and Habits of Cluster Flies
The cluster fly is a parasite of earthworms and breeds outdoors in lawns and fields during the spring and summer. You can find cluster flies almost everywhere in the United States and Canada, except for the Southern states bordering the Gulf of Mexico. They do not cause a health concern, because they do not lay their eggs in human food.
Female Cluster Flies lay their eggs in cracks in the soil, which hatch in three days. The larvae use earthworms as a food source. The larvae feed for about 22 days. After that, they go into the pupae stage, which lasts 11-14 days before emerging as adults. Adult flies feed on flowers. There are about four generations hatched per summer.
When fall approaches, the cluster flies begin to enter structures in large numbers. Problems with cluster flies begin in late August as they move to winter quarters to over-winter. The cluster fly is seeking warm sites with protective cracks for shelter, crawling back as far as they can get. It is important to consider treatment before this happens.
Cluster flies have been known to squeeze around the edges of windows that are weather-proofed. As the number of cluster flies attracted to the building increases, large clusters of flies huddle inside wall voids, attics, and false ceilings. Most infestations occur in the upper regions of buildings, such as the attics of homes. In multi-story buildings, the cluster flies can be found in the upper two or three floors, and almost always of the south and west sides of the buildings.
If you have unseasonably warm weather in the late fall or winter, the cluster fly may emerge thinking it is spring, going for the warmer air outside. Cluster flies fly very slowly when they just wake up. They are strongly attracted to light, so they are usually found around windows. At night, they are attracted to lamps.
Cluster Fly Inspection
Check around windows for live or dead flies. If you can find the voids in which they are over-wintering, you can treat the voids with a dust or aerosol, but that is not an easy task. In most cases, the voids can't be located.
To locate the voids, start with an inspection of cracks and crevices on the southern and western exterior walls. Usually the only accessible voids are the attics, crawls paces and false ceilings.