Cluster Flies

Summary: The sudden invasion of flies inside a house can indicate the presence of cluster flies. Exclusion is the best method of controlling a cluster fly investation.

A reader asks: My mom has lived in her condo for 35 years. Last year she started having problems with flies. First it was regular sized house flies, but towards the end of summer it looked like horse flies. One night we counted twenty-two flies. Needless to say we are worried. I'm looking for a do-it-yourself solution. Can you help?

Dear Reader: Let's see if I can help. First and foremost you need to have the fly identified. Catch a few of the flies in a bottle and take them to a nearby reputable pest control company or county extension agent. They will not charge you for the identification, but they'll probably suggest using their service. If you are not interested, simply say thanks, but no thanks and that will be the end of it. Once you know the fly you are dealing with you can take positive action.

Now, lets talk about the Horse flies. These flies breed in very wet areas. They are active during the day and their bites are quite painful. If you have Horse flies you are going to know it because they will be biting you

and you won't be able to ignore it. Since you didn't mention being bitten and based upon the time of year being early Spring, it is more likely you are encountering Cluster flies.

Cluster flies entered homes in August through any tiny cracks and crevices they can find as they walk up the outside wall of a house. They are attracted to light colored walls or walls that are reflecting bright light from the sun. It is most always the south or west facing wall because of the warmth created by the sunlight.

Cluster flies over-winter in attics and ceilings and suddenly enter homes on warm days in the winter or early summer. They fly towards windows or other light colored surfaces.

Cluster flies are dark grey in color and are usually ¼ to ¾ inches in size. They have no metallic coloring, as do many other common flies. Another characteristic is they fold their wings over their back when at rest.

These flies breed in the soil, but treating the soil around your house is a waste of time. Once they emerge from the soil they can fly hundreds of feet in their search of a warm wall.

Exclusion is the best method of control. That means lots of caulk and climbing ladders. Sealing up around windows and doors and under eaves it a major job and does not guarantee success. In addition to caulking you must treat the exterior south and west facing walls, window frames, door frames and eaves with a pesticide. You are going to need a compressed air sprayer that you can purchase for around $15 at Home Depot. You are also going to need several gallons of insecticide. Read the label! It must specifically list Cluster flies as one of the target pests. Otherwise, don't buy it.

Before you go to all this effort you might seriously want to think about hiring a professional pest control company to do the work. It's safer, less time consuming and probably comes with some type of warranty. You might also be surprised that the cost is reasonable, too.