Cluster Flies Indoor Settings

Cluster Flies in Indoor Settings

Adult cluster fly showing alternating pattern of shiny abdominal
patches (top). Adult cluster fly showing diagnostic yellow hairs
on thorax (bottom).

A common fly found indoors throughout the winter months is the cluster fly (Pollenia rudis). Like the boxelder bug, they overwinter as adults in attics and voids, becoming periodically active during warm winter spells and early spring. These warm periods trick the flies into thinking that spring has arrived and they attempt to leave the house, gathering at windows.

You will not find eggs or larvae of cluster flies indoors. They lay their eggs outside, in soil cracks. When the eggs hatch, larvae seek out earthworms to parasitize, and can take from 27-39 days to develop into adults. There can be up to 4 generations per summer. During summer, adult flies can be found on flowers and fruits. Cluster fly adults don’t become a nuisance until temperatures cool down in the fall and the flies enter the home, congregating in the attic, voids, or any out-of-the-way place.

Fortunately, cluster flies are not considered a pest of health concern. They do not bite, and since they do not reproduce in feces or filth they do not transmit diseases.

Control of cluster flies in the home should begin in early fall when temperatures drop. Monitor south- and west-facing walls for flies, as they will begin to congregate there to find heat. Once they are detected, spray the exterior walls with a pyrethroid insecticide to reduce fly numbers entering the structure. In Utah, there are over 250 products labeled for control of cluster flies on “household or domestic dwellings: outdoor.” Common active ingredients from Group 3 (pyrethroids) include permethrin, deltamethrin, pyrethrin, lambda-cyhalothrin and cyfluthrin. Reapply the insecticide as stated on the label to provide long-term control, until cold temperatures curtail fly activity. Choosing a product with a long residual (time period before reapplication) might limit the number of applications to only one well-timed spray.

A solely chemical approach should not be taken for cluster fly control. Cluster flies must enter the home through cracks and crevices in the foundation, siding, fascia, window/door frames, etc. Inspect the exterior of the house and seal all cracks, crevices, and openings. Unfortunately, cluster flies can fit into very small spaces and total elimination of all entry points is not always feasible. Do the best you can to reduce entry points and target sprays for the times when flies are most abundant. Using exclusion techniques for flies will also keep out unwanted boxelder bugs and other nuisance pests.

Once cluster flies are in the house, insecticide treatment is not recommended. Cluster flies are not of medical concern and occasional presence should be tolerated until the following spring when they leave the structure. If large numbers of flies are killed in wall voids or attics, carcasses can attract worse pests such as carpet beetles.

If you can find groups of cluster flies, use a vacuum to suck them up. Since the flies are usually sluggish in winter, they are easy to capture or kill. Light traps can be used in attics or voids to collect flies, and other traps such as “Cluster Buster” can be used inside, attached to windows.

Treating lawns, fields, soil, etc. is not recommended for control of this fly. Cluster flies can be found throughout the environment; treating your lawn or attempting to treat favorable habitat in the surrounding areas will not have a significant effect on their numbers. Focus on protecting your home instead of trying to eliminate these flies from the environment.

-Ryan Davis, Arthropod Diagnostician