The Natural Enquirer: The annoying cluster fly

Cluster flies (Pollenia rudis), also known as the Buckwheat fly, is found from British Columbia to Nova Scotia, all of the United States as well as Europe and Africa. It looks like a house fly, but is larger and darker, with the wings overlapping on the back when at rest. There is a white spot between the eyes, the thorax (the middle part of an insect) is dark with golden short crinkly hairs under longer black hairs, the abdomen is checkered silver and black. The cluster fly is sluggish, and flies with a buzzing sound. At temperatures above 50 degrees, these insects seek light and avoid contact with each other; however, below 50 degrees, they avoid light and seek close contact with other flies-giving them the name of "cluster flies".

Several species of flies enter buildings in the fall to find a protected place to spend the winter. The cluster fly is the most common and annoying of these over wintering flies, and on warm winter and spring days, they emerge from hiding and plague the homeowner. Although they resemble house flies, they are not related at all, nor are they alike in their habits. They do not breed in buildings, nor are their young (maggots) found in dead animals. We believe these old world aliens came to the North America in colonial days in the soil used for ships ballast that contained earthworms.

Cluster flies mate in late winter and early spring, around here probably in March and early April. The adults then leave the house and lay about 100 eggs in the soil. After the eggs hatch, the larvae (maggots) enter the bodies of earthworms and feed upon them as parasites. After spending about two weeks feeding on the worm, they leave and pupate in the soil, again for about two weeks, emerging as adults. Around here they probably have three to four broods a year

As fall approaches, cluster flies congregate on the warm sides of buildings, and when the sun goes down and the temperature cools, they enter the structure through cracks and crevices. They then gather together and hibernate in clusters. In the spring, the flies break their dormancy and want to fly to open areas, but are trapped inside the house and can't get out.

Cluster flies are not attracted to food in the home, and do not present any recognized health hazard. When crushed or mashed, the fat in their bodies may stain fabric, and an odor is given off the smells like buckwheat honey, thus their alternate name of buckwheat flies. Buckwheat honey has a strong odor that some people find distasteful. The adults feed on the nectar of flowers and juices of ripe fruit during the summer months.

Many of these flies die during the winter, and these fly carcasses provide food for the larder beetle, also known as the dermestid beetle, which is a dark brownish black beetle with a yellowish band across the middle of the body which is about 3/8ths to a 1/2 inch long. This insect will feed on "people" food, and also destroys your mounted deer trophies and other mounted specimens. Though these insects are almost impossible to keep out tight screens and storms, caulking and closing all cracks and openings to the inside will decrease their numbers. Spraying the outside of the house with fly controlling chemicals containing cypermethrin in the fall (not to the point of runoff!) may reduce their numbers. Some people have gone to the extent of applying Ficam to the soil to kill earthworms, but this I feel is overkill, as earthworms are so beneficial. We just vaccuum them up about every other day.

This might be worth a try-ZIP Lock Bags, believe it or don't...

I think I will give this a try. We don't have many flies this year, but one is one too many! We went with some out of town friends to a fast food place for breakfast recently, and we sat in an enclosed booth section inside. We happened to notice a couple of zip lock baggies pinned to a post and a wall. The bags were half filled with water, each contained four pennies, and they were zipped shut. Naturally we were curious! We were told us that these baggies kept the flies away! So naturally we were even more curious! We actually watched some flies come in the open window, stand around on the window sill, and then fly out again. And there were no flies in the eating area!

Peter Dring is a retired nature biologist and phenologist who lives in the Land 'O Lakes area. Questions or comments for Dring can be sent to starjournal@jcpgroup.com.