For a few days, we’ve had these big blue iridescent flies buzzing around our house, usually ending up at the living room window. We recently moved an old, unused recliner chair in our basement and found lots of brown cylindrical shells underneath and a few of the flies. Is there a connection?

J. B., Brookline, MA

First thing, you should collect and save a couple of the flies and some of the “shells.” Second thing, give Colonial Pest a call and ask for one of our technicians to come out for an inspection and pest identification.

HAVE YOU NOTICED A BAD SMELL FROM A DEAD ANIMAL?

The big flies you mention sound like what are commonly called blow flies, or in this case bluebottle flies. These are one of the groups of so-called filth flies that feed on everything from rotting garbage to animal feces to decaying carcasses. They can sometimes spend the winter hiding inside homes (see Blow Flies Can Move Indoors) but when you find stages other than the adult inside, the food source is usually a dead animal hidden away in a wall void, ceiling space, attic or other unseen location. You may have noticed a putrid smell.

A female blow fly will quickly home in on a dead animal and will lay her eggs on the carcass if she can get to it. The hatching larvae or maggots feed on the rotting flesh. When mature, the maggots wander away from the carcass to find a place to pupate before turning into adult flies. The brown shells that you saw are likely the pupal cases inside which the adult fly develops. Some are probably broken open on one end, indicating that the new flies inside have already emerged, as you well know.

TO GET RID OF THE FLIES, FIND THEIR FOOD SOURCE

The key to eliminating an indoor blow fly infestation is to find the larval food source and remove it. That’s where Colonial Pest comes in. There are many possibilities to eliminate. For example, there could have been a mouse nesting in the stuffing of the chair, or mature maggots could simply have crawled to the protected space under the chair from a carcass in a nearby wall void. If the basement is little-used and you have a pet, maybe there’s an accumulation of pet poop in a corner. We heard a story of a delivery of packaged fresh meat destined for a basement freezer but left stacked on a workbench instead. Guess how it was discovered many days later!

If the larval food source is a larger animal like a squirrel or a forgotten bag of garbage, the blow fly infestation could continue for weeks more… and you don’t want that. Give Colonial Pest a call today.