How to control cluster flies in winter

For many years, the cluster fly was thought to be a single species (Pollenia rudis). But recent research has identified six distinct North American species, with multiple species capable of overwintering inside the same structure simultaneously. Overwintering cluster fly populations can be quite large in attic or wall void spaces. There, they form aggregations during cold temperatures before returning outdoors in the spring to lay eggs in the soil, which hatch into larval earthworm parasitoids.

During winter months, cluster flies are often observed flying awkwardly around windows on warm days, prompting calls for service. Controlling cluster flies indoors can be difficult, because populations are frequently hiding in hard-to-reach voids or attics. Indoor insecticide applications are often impractical, and result in large numbers of dead flies that are attractive to secondary pests like carpet beetles (Dermestidae). Usually, exclusion is the best solution, but should be performed in the early autumn, before flies arrive.

Completely sealing every entry point is difficult, especially because cluster flies enter near the roofline through soffit or ridge vents. Exterior surface applications of appropriately labeled repellent insecticides can be helpful in deterring flies, but applications should be made before aggregations of flies begin to appear on the building. Clients can be instructed to vacuum any sluggish flies appearing in living areas. Because cluster flies are attracted to light, many technicians find that an insect light trap (ILT) placed in a ceiling void or attic space can capture hundreds of flies. Just remember to change the ILT’s glue board frequently, as it will fill up quickly.

Finally, sharing information about the biology and behavior of cluster flies can eliminate much of the “yuck factor” associated with indoor fly infestations, and can set the stage to begin preparing the home during the spring and summer to guard against next year’s invasion.

You can reach Dr. Fredericks, VP of technical and regulatory affairs for the National Pest Management Association (NPMA), at jfredericks@pestworld.org.