I have often been asked how does an exterminator stay busy in the winter time? Surely there are very few bugs to contend with and the mouse calls can only go so far, what do you do? Living here in Florida I’m not asked that quite as much even though we recently had a record snow fall of 0.006 inches of snow. Some companies affected by the freezing weather will do ad on services such as Christmas lights and even snow plowing to generate income but I can assure you that there is no shortage of work in the urban areas and when I did work in the white stuff my days were filled with roaches, carpet beetles and a host of other pests not to mention mice and rats. Actually there are some bugs that can really become a problem in the dead of winter and worried homeowners will rush to the phone just as quickly as they would for a swarm of termites in the spring.
Cluster flies are one such pest that often makes its debut while there still may be 3 feet of snow on the ground and come out by the hundreds if not thousands.
The cluster fly is a slow flying insect who is just a bit bigger than a house fly. They produce 3 to 5 generations per year and will lay their eggs in soil that has earthworms in it. The larva hatch out and find the earthworms and enter their bodies where they will finish their larval stages and pupate. Adults emerge and begin the process again and this takes about one month.
In the fall the adults will begin to look for a place to over winter much like the lady bug and it is quite common to find whole hordes on the sunny side of a home. The 
flies search for a place to enter which may be a loose siding board, gap under a window or perhaps into the attic through a hole in the vent screen. These flies can build up in great numbers in the voids of your walls or attic and stay undetected for several months. On warm winter days the cluster flies thinking spring has sprung will begin to make there exits. What happens most often is they don’t go out the original hole they came in but now find there way through a crack in the baseboard, gap in the window trim or the space around loose fitting lights in the ceiling. Attracted to light they swarm to the windows in mass and buzz around in a sluggish manner. For one or two flies I’m sure this would not cause a panic but the sheer numbers at times can make bug squeamish homeowners break out the phone book in one hand while dousing the blackened window with Raid.
Treatment for the cluster fly
Besides your aerosol for those that have emerged it is really not wise to treat with chemicals for the mass that may still be in the voids waiting to come out. The reason is that the dead flies will attract carpet beetles who when they are done with the fly smorgasbord in your walls will be happy to invade your home and you will have created a bigger more destructive problem. To keep more flies from coming out it is best to seal the access points with caulk or some other suitable method. This will force the flies to find other areas to 
escape and hopefully that is back outside where they will pose no problem. In the spring you should work on sealing up the exterior of the home with caulk, tightening up loose boards or replacing screens. Since the flies are attracted mainly to the sunny sides of structures that should be the area you concentrate on most.
In warmer states this fly is not a problem that I’m aware of but in places like Vermont they can be the winter times life blood of a pest control company. Recently my family and I took our annual ski vacation there and as I always do, I look at the yellow page ads to see what other companies around the country are doing. Each ad in the directory listed cluster flies in bold and promised a quick response. I can only assume they wanted you to call them before you checked your most favorite pest control website and found the answer. I was hoping for a mass exodus of the fly in our condo just so I could get some great pics but alas it barely got over 12 degrees. Our village was set up with some nice Christmas lights in the trees however & I wondered if a cold but determined bug man somewhere set those up just for me.
