The insects that are commonly referred to as “bottle flies,” “greenbottle flies,” and most frequently, “blow flies,” belong to the Calliphoridae family in the Diptera order of insects. More than 1,200 Calliphoridae fly species have been documented worldwide, and they are between .3 and .5 of an inch in body length, making them slightly larger than common house flies. While blow flies closely resemble house flies, the former is unique for the metallic body colors that many species possess. Like most common fly pests of homes, blow flies are categorized as “filth flies” due to their habit of breeding on decaying sources of organic matter.

While fruit flies prefer to breed on rotting fruit, most blow fly species favor decaying animal carcasses and excrement as their primary breeding material. Blow flies become problematic for homeowners when the pests breed on indoor materials, or when they enter homes in large numbers during the fall to overwinter. The most common blow fly pests of homes include screwworm, bluebottle fly, greenbottle fly, and cluster fly. The cluster fly is one of the few blow fly pest species that is not a filth fly, as they do not breed on pathogen-rich materials, and therefore, do not pose a disease threat to humans. However, cluster flies are one of the most common overwintering pests within Massachusetts homes.

The presence of a large number of blow flies within homes is often perplexing to homeowners, and even pest control professionals, as their breeding sources are usually located within inaccessible indoor areas. For example, female blow fly adults often lay eggs on dead rodent carcasses that have collected within wall voids or within crawl spaces, and their presence in a home does not become obvious until they take flight into interior living spaces after reaching adulthood. However, many blow fly species are well known for entering homes in large numbers during the fall in order to overwinter within protected locations, mainly wall voids, attic spaces, and beneath siding. Once they gain access to a protected location, blow flies enter into a state of quasi-dormancy for the duration of the winter. The arrival of spring warmth prompts these flies to reemerge in an effort to get back outside. Due to their habit of hiding within inaccessible areas, eliminating overwintering blow fly infestations is tremendously difficult. The most reliable treatment consists of insecticide applications within all wall voids and other concealed indoor locations. The best way to avoid blow fly pest issues is to prevent them from entering homes in the first place. This can be done by sealing cracks, crevices and other potential entry points on the exterior wall of homes, and installing mesh screens over attic and crawl space vents.

Have you ever experienced a blow fly infestation during the summer?