Bright 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.