Pests Common Houseflies

Published on October 5th, 2016 | by Henri


Fly invasions in winter and summer

Q.   We have problems with flies in our New Jersey house. In the winter, every day as the sun rises and shines through this window, you can see at least five black flies moving about on on the same window. Some die but they keep coming. Do you think these flies are hatching in the woodwork around the window? I can’t seem to locate any area they are coming out of. But meanwhile, this summer I observed big black flies coming out of the cinder block in the garage. They were coming out one at a time sort of slow moving almost like they were just born. Any help or suggestions would be greatly appreciated.

A.  The winter flies are cluster flies. They get into houses through any small crack they can find to seek shelter as cold weather approaches. They hibernate within walls or attics and come out when the sun is shining, buzzing on window glass. They are not “dirty” flies, but they are annoying and, if crushed, smelly and messy.

One way to get rid of them is to vacuum them up. But the best way to prevent an infestation is to caulk and seal every joint between different materials – around windows, doors, beam projections, cantilevered joists, etc. it is also essential to caulk the joint between the foundation and the mudsill if it does not have a sill-sealer (a strip of foam or other material set on top of the masonry foundation to seal any gap between it and the wood mudsill of the floor framing). You may need to do this from the basement or crawlspace.

Spraying the siding on the south and west side of the house in late August in colder climates or September in moderate climates with an appropriate insecticide is also helpful.

As to the big black flies that came out of the cinder blocks during summer, I wonder if they were carrion flies that had just cleaned up a dead mouse. If it’s a one-time occurrence, it is likely to be the case.

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