Understanding Blow Fly Behavior and Biology Key to Their Control

Among large filth flies, blow flies are easily recognized by their shiny metallic blue, green or black appearance. They typically breed outdoors but may occasionally enter structures, drawn by light near garbage bins or areas with fecal and decaying organic matter. When a pest management professional (PMP) is called to address blow flies indoors, the first assumption is often correct: a dead animal, usually a mouse or squirrel, is hidden in a wall void or another inaccessible space. Locating and removing breeding sources in and around a structure is essential to effective control. A comprehensive fly management program should combine chemical and nonchemical tactics, along with monitoring and record keeping.

SANITATION AND EXCLUSION. Nonchemical control methods should form the foundation of any blow fly management plan. Successful sanitation and exclusion require coordination among all involved parties, including residents, property managers and PMPs. When executed properly, these practices can prevent infestations or keep populations at manageable levels.

Fly-proofing measures include installing 16-mesh screens on windows and doors, placing air curtains above frequently used entrances and ensuring doors remain closed when not in use, ideally with self-closing mechanisms. Sealing gaps along exterior walls and adding door sweeps further reduces entry points.

Equally important is identifying and removing breeding sources. Clues such as the number of flies present and the timing of activity can guide inspections. A sudden surge of flies indoors often signals a dead animal in a wall void or accumulations of fecal or decaying organic material in areas like garages, attics, chimneys or crawl- spaces. Smaller, sporadic numbers typically indicate flies entering from outdoor breeding sites through open doors or windows.

Indoor inspections should include checking for forgotten rotting produce such as potatoes, onions or apples, as well as old garbage containing meat or fish. Attics and crawlspaces should be examined for animal feces. Outdoors, inspect for manure, carcasses and decomposing vegetation, including grass clippings and compost piles. Insect light traps can be used indoors to monitor adult activity and help track population trends over time.

KNOW SEASONAL BEHAVIORS. Blow flies generally overwinter as fully grown larvae that pupate briefly before emerging as adults in spring. However, the black blow fly, Phormia regina, behaves differently in temperate regions and may overwinter as an adult. As a result, black blow flies observed during fall or on warm winter days are not typically associated with breeding sources such as carcasses or organic waste.

Instead, these flies behave similarly to cluster flies, seeking sheltered indoor locations like attics and crawlspaces to survive colder months. Recognizing this distinction can prevent unnecessary and time-consuming searches for nonexistent breeding sources during off-season encounters.

USE PESTICIDES STRATEGICALLY. Once breeding and feeding sources are eliminated, targeted chemical applications may be appropriate. Spot, crack and void treatments with residual insecticides can help control larvae in breeding areas and adult flies in resting or entry points. When fly populations exceed acceptable thresholds, space treatments may be used for rapid knockdown of adults.

Always follow label directions carefully to ensure both effectiveness and safety. This includes removing people and pets during application and covering or cleaning food preparation surfaces and utensils. Many products also require specific holding and aeration periods before reentry.

Fly baits can also play a role in control programs. These products attract flies to treated areas where they are exposed to the active ingredient. However, baits must be used with caution. They should be placed out of reach of people and animals, and care must be taken to avoid drift onto non-target surfaces, water sources or food areas. Baits are not appropriate for use in food processing facilities, classrooms or patient rooms.

If blow flies are known to overwinter in a structure, they should be managed similarly to other fall-invading pests such as cluster flies, boxelder bugs and multicolored Asian lady beetles. Applying a long-lasting residual insecticide to the building exterior, especially on sunlit surfaces, can reduce entry. Pay close attention to eaves, soffits and rooflines — areas that often provide access to attics and wall voids.

TAKE A PRACTICAL APPROACH. Effective blow fly management begins with a clear understanding of the infestation. The number of flies present and the time of year can help pinpoint the source and guide corrective actions. In many cases, solutions are straightforward: remove a dead rodent, clean areas with fecal or decaying organic material, increase garbage collection frequency or seal entry points.

When chemical treatments are necessary, set clear expectations and prioritize safety. Preventive measures taken in early fall can also reduce overwintering populations before they establish themselves deep within structures.

Ongoing monitoring and accurate record keeping are essential components of any successful program. Together, these practices ensure that blow fly issues are addressed efficiently and prevented from recurring.

The author is technical training director for Adam’s Pest Control, Medina, Minn.