Blue Bottle Fly
Bottle flies, otherwise known as blow flies, are most often found indoors when there is a dead animal present. Dead mice, rats, birds, squirrels or other animal could have died in an attic, behind walls, under porches or crawl spaces. The adult flies are attracted to the odor of dead animals from a distance and quickly find and lay eggs on carrion. There are blue and black blow flies as well at metallic green blow flies. All have similar biology and control methods.
Blue bottle flies (Calliphora vomitoria) have a dark metallic blue abdomen in dorsal view. They are slightly larger than house flies. A large number of these flies often emerge from carrion at the same time and fly to the nearest light source, where they are often trapped behind a window in the home. During the first few hours these flies appear to be sluggish but they quickly become more active and are strong and fast in flight. When several of these flies are found during a short time period in the home, it is most certain that the flies emerged from either a dead animal or a section of meat caught partway in the garbage disposal unit.
A single female can lay a batch of more than 150 eggs. Blow fly eggs are deposited directly on the carrion or meat source where they quickly hatch into larvae or maggots. The maggosts feed voraciously and within three to ten days they become mature enough to metamorphose into adult flies. The maggots leave the carcass and often burrow down below where they change into adult flies. This process may take another week. All of the flies emerge as adults at about the same time since they are all progeny of a single egg laying event by a female fly. Just after emergence the flies are strongly attracted to light. After dispersing and mating these same flies then begin their search for dead animals as a food source.
If your response is rapid, you can immediately vacuum up the flies as they land on windows inside the home. As time passes and the flies become stronger, they are more difficult to vacuum or swat. If you can open the window, the flies will disperse into nature and not return unless there are additional dead animals present. Rodents or other animals recently controled with anticoagulant or other rodenticides often become the source of flies in a home or building. As many as 200 to 300 flies can emerge if more than one rodent carcass is present.
If you can find and remove the carcass, that is the quickest way to end the infestaion. A thorough search for dead animals may take more time but is well worh the trouble. Check the roof of the house for squirrel entry holes. Check to make sure rodent poison is not being used nearby. An electric light zapper or a light trap with a sticky board will capture many flies if they are coming from a basement where there is little competition with other light sources.
If no action is taken, within just a few days all of the flies will have emerged and died. As long as there is no remaining fresh carrion, the infestation will end. If unable to escape the building, all flies will end up dead near the brightest window. There is no need to spray each fly with a pesticide unless you cannot open a window or otherwise swat and vacuum up individuals.
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