Monday, March 11, 2013

Adirondack Bugs: Cluster Flies

cluster-flies-1Bright sunny weather with little wind in March, such as the type that our region experienced this past weekend, not only triggers the flow of sap in maple trees and causes a case of spring fever in every normal human, but it also prematurely awakens several creatures that pass the winter in a deep state of dormancy.

Among those forms of animal life impacted by bouts of intense sun and temperatures above freezing during the latter part of winter is the cluster fly. After the sun starts to bake the south side of homes, barns and sheds, this insect rouses from its prolonged period of torpor and exits its winter retreat in the company of dozens to hundreds of its kin. As the late afternoon temperatures cool, this bug is forced to return to some hidden nook or cranny and wait until the next spell of spring weather awakens it again.

The cluster fly is often mistaken for the house fly; however, the two can be easily identified with some careful observation and a little effort. When seen at rest, the cluster fly folds its wings over its abdomen, allowing a major portion of these translucent structures to overlap. This creates a slightly elongated body shape and gives this bug a darker color. The house fly holds its wings off to its side when not in flight. This gives it a more triangular appearance and a lighter color. Additionally, the cluster fly has light, yellowish hairs on its thorax giving this body segment a slightly golden hue. When in flight, the cluster fly is slower than the house fly and can more easily be swatted.

The cluster fly gets its name from its habit of gathering together in a protected spot during mid autumn in clusters that typically number several dozen individuals. By huddling in a group, this insect reduces its chances of freezing, and limits the loss of moisture from its body to the exceptionally dry winter air.

Around towns and villages, the cluster fly seeks out cracks and crevices on south facing walls of houses. Attics, eaves and the space around windows are known as preferred overwintering sites. In settings well away from any manmade structure, the cluster fly will seek out a deep cavity in a large standing tree, or a partially collapsed chamber in an abandoned squirrel nest.

When the sun elevates the temperature within these places, individuals in the cluster gradually leave the mass of insects and begin to find their way outside. While most are able to discover the path leading out, some become disoriented and follow routes that bring them indoors, or to the space between a window and a screen.

When spring finally comes, these individuals disperse to the surroundings and begin the process of establishing the next generation of cluster flies. Females lay their eggs on the ground in places containing fairly rich soil. It is in such settings that earthworms abound and will be actively crawling on the surface as soon as the frost has completely thawed in the soil.

Cluster fly eggs hatch into tiny larvae that attempt to come into contact with the belly of an earthworm. Once it touches the thin layer of skin on the underside of this annelid, the larvae attaches itself and quickly chews its way into the body cavity of the worm. Once inside the worm, the larvae begin to consume the various cellular matter of the worm. If several larvae enter a single worm, they can inflict enough damage to this common soil dweller to cause its death. After maturing, the larva exits the body of the worm and pupates on the ground. Eventually, an adult hatches to begin the cycle again.

Soils that lack earthworms are unable to support cluster flies. This is why the cluster fly is common around towns and villages where humans maintain gardens and lawns that are periodically watered and fertilized, allowing for a healthy earthworm population.

In many places last summer, exceptionally dry weather prevailed, causing a serious reduction in worm activity on the surface of the soil for many weeks. This reduced the success of cluster fly larvae in finding a host to parasitize. By the time that cool weather arrived in autumn, there were very few adult cluster flies to establish a winter cluster.

This past weekend was a perfect time for being outside. It was also a perfect time to begin seeing large numbers of cluster flies basking in the sun on the south facing side of homes throughout the Park. Around my house I counted a mere 6 cluster flies. Ordinarily there should be a hundred. This is probably a result of the dry weather that occurred last summer. My hope is that the mosquito population will be just as seriously impacted. I know that this is not the case, but I can always hope!

Photo of cluster flies courtesy WSI Consolidated.

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Tom Kalinowski is an avid outdoor enthusiast who taught field biology and ecology at Saranac Lake High School for 33 years. He has written numerous articles on natural history for Adirondack Life, The Conservationist, and Adirondack Explorer magazines and a weekly nature column for the Lake Placid News. In addition, Tom’s books, An Adirondack Almanac, and his most recent work entitled Adirondack Nature Notes, focuses on various events that occur among the region’s flora and fauna during very specific times of the calendar year. He also spends time photographing wildlife. Tom’s pictures have appeared in various publications across the New York State.

3 Responses

  1. Charlie Stehlin says:

    Cluster flies.I know them well.They come out of the nooks and crannies in mom and dad’s house in Blue Mountain Lake when the timing is ripe.My mom used to swat them and flatten them dead,or throw them out a window into the frigid degrees,which was a death sentence fer shure.Then I came along and swayed her some,she saw me carrying them off and finding safe havens for them until the weather was ripe.I have a hard time killing any thing it is the nature in me.I am well aware that if flies were left unchecked the world would be layered in three inches of them in a fortnight or less,as they propagate at a rapid pace,so elimination is necessary.Nonetheless I have a hard time killing any thing.When I visit mom and dad and I see a cluster fly pop out from a breach in the ceiling or a wall or a gap in a floorboard I catch it and try to find a safe haven for it,versus throwing it out into the cold air.This is not always an easy thing to do in the winter as warm places for a fly are very limited in an Adirondack house,especially where it can go undetected.I am hopeless I know but it’s just the way I am,I have a hard time killing any thing as i have previously mentioned. Imagine if all the world had thinkers like me,imagine all of the life preserved. Some critters we could do without,but there must be a reason why any of them are here in the first place…otherwise why would they be here? Cluster flies! Maybe they are here so that when a nuclear or chemical bomb,or a virus,wipes us out,when our scattered bodies and decaying flesh is putrid in the air…maybe their purpose,in maggot form,is to slow down the decaying process,so that other species may propagate and carry on life on this once pure planet Earth.

  2. Bev Stellges says:

    Tom – great info about the cluster fly and enjoy all your articles! Question: Do pileated migrate south??? Seems we are now beginning to see them again in our north woods.

    • Tom Kalinowski says:

      Good Evening Bev and Thanks for reading the Almanack. The pileated Woodpecker does NOT migrate. It becomes a permanent resident of the area that it establishes with its mate. Like other birds, their numbers fluxuate, depending on food, predators, and habitat considerations. In some areas, as the forests mature, this giant among the woodpeckers increase in number. It is great that you are seeing this bird, however, since it feeds mainly on carpenter ants, it would indicate that these large, black ants are becoming overly abundant in your section of the forest. But that is all part of a healthy ecology!

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