This story was originally published in December 2017.

A male cluster fly, Pollenia sp. Photo by TristramBrelstaff via Wikimedia

With the holidays upon us, a lot of Mainers are opening up rooms for guest accommodations that may have been closed off and unheated since fall.

For some, it also can mean feeling like they are in the middle of a scene from “The Amityville Horror,” as these warming rooms suddenly come alive with flies seemingly out of nowhere.

In that 1979 cult classic based on the 1977 book of the same name, homeowners are plagued by a number of paranormal activities in their new home, including nonstop and unexplainable invasions of houseflies.

But while annoying, there is nothing satanic nor particularly mysterious about the invasion of winged insects from the family Pollenia — commonly known as “cluster” or “attic” flies in Maine this time of year.

Cluster flies resemble house flies but are larger, with yellowish hairs on the thorax.

Their name derives from their behavior of amassing or “clustering” out of sight behind household siding, shingles or other cracks and crevices in attics, lofts or other wall voids.

“The reality is, they are a part of nature,” Dr. Kathy Murray, entomologist with the Maine Department of Agriculture, Conservation and Forestry, said. “They overwinter in our homes in a nice warm place. On any warm, sunny day they come out.”

It’s a particular issue in agriculture areas or anyplace with rich, damp soil, with heavy earthworm populations.

“Cluster flies are parasites on earthworms,” Murray said. “As part of their life cycle the flies feed on the earthworms, so any rich soil that benefits earthworm populations you are going to have cluster flies.”

According to a University of Maine Cooperative Extension fact sheet on cluster flies co-authored by Griffin Dill, pest management specialist with the Cooperative Extension, cluster flies often go unnoticed during the summer as they search for their earthworm host.

The adult female cluster fly lays her eggs — up to a 130 at a time — in earthroom-rich soil. When the fly larvae hatch, they will find and follow subterranean tunnels made by the worms until they find a live earthworm.

The larvae will then burrow into and feed on the earthworm. Once the host worm is dead, the larvae moves on to the next one until they reach adulthood.

“In the fall about the time we are getting our first hard frost the cluster flies start aggregating on the sunny side of houses, especially on the upper stories,” Murray said. “Then they will sneak into any crack or crevice and get into the attic or upper stories inside.”

Evolutionarily speaking, moving into a home is a mistake on the part of the cluster fly, Murray said.

“They will remain dormant during the cooler periods inside, but when things warm up on a sunny day or the heat gets turned on in a room that had been cold, they think it’s spring and wake up,” she said. “They will gather at windows and try to get out and end up dying of dehydration.”

The best way to control the flies, Murray said, is to prevent them from getting inside in the first place by sealing up cracks or placing screens over attic vents and soffits.

Large gatherings of the flies can be vacuumed up and released outside and commercially available products like insects strips and sprays can help.

But it’s not all bad.

“As far as we know, they don’t carry any diseases or pathogens and don’t bite,” Murray said. “And they are a good indication of a healthy earthworm population.”

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Julia Bayly

Julia Bayly is a reporter at the Bangor Daily News with a regular bi-weekly column. Julia has been a freelance travel writer/photographer since 2000.