Pest Identification - Cluster Flies

Cluster Flies are a very common pest throughout North America.  They are a medium sized fly (1\4 to 3\8 inch long) which are usually dark gray in color with golden hairs located on the thorax. 

The female lays eggs in the soil near the burrows of earthworms. The tiny maggots that hatch from the eggs seek out earthworms to feed upon. The maggots of cluster flies have never been reared on any other food but living earthworms. When full grown, the parasitic maggot leaves the body of the host and enters the soil. There are about four generations a year.

  

During most of the year, cluster flies are not a problem in structures.  In August and September however, as evening temperatures begin to drop they seek protected places in great numbers to over-winter.  In many cases, this is within walls, attics and storage rooms of houses.  

       They are attracted to warm walls (usually South and West facing) where the sun shines the longest.  They then squeeze through amazingly small cracks and crevices to access the warmer interior of the structure.  They can even get around the weather stripping of expensive windows  Screens offer little protection since these flies prefer to crawl in through small openings elsewhere around the building or force their way around the edges of where the screen fits the window.  For this reason they are extremely difficult to keep out of houses.  Isolated houses in the country are especially prone to invasion, since they offer the only warm shelter for miles around.            

If they have not invaded the living space directly they eventually make their way down from false ceilings, attics and wall voids into living and working spaces.  They can number anywhere from a few dozen to many thousands of flies creating a very uncomfortable living or working environment.

Control is best achieved by preventing the flies from entering the structure in the first place.  This is accomplished by treating all areas at risk to cluster fly infiltration in the early fall.  This treatment must be timed in anticipation of the flies seeking over-wintering sites.  With specialized equipment, an application of a residual insecticide, targeted to critical areas will kill the vast majority of the flies as they attempt to infiltrate the structure.