The Asian lady beetle and cluster flies (also called attic flies) are two insect species that Iowans have been finding inside their homes in the past few weeks. Actually, these insects have been inside your home all winter. They have spent the winter in your walls or attic, waiting for spring. As warm weather returns these insects become active and begin crawling around, attempting to go outside. Unfortunately, they just as often wander into the living areas of homes.
Asian lady beetles and cluster flies are considered a nuisance because they annoy humans. They do not feed or reproduce indoors. They do not sting or spread disease, and do not damage our homes or furnishings.
Asian lady beetles feed on plant pests, such as aphids, so they are considered a beneficial insect species. Asian lady beetles first became established in the late 1980's in Louisiana. Since then they have been spreading all over the U.S. and entered Iowa sometime in the mid-1990's. Homeowners often mention that they never used to have a lady beetle problem, and this is because until the recent arrival of the Asian lady beetle we had no species of lady beetle in Iowa that would aggregate in homes to overwinter.
Cluster flies are large, black flies that show up in bedrooms and on windowsills from late fall through early spring. They are called cluster flies because of their habit of clustering in large numbers inside attics. Their abundance varies from year to year, possibly in relation to the amount of rainfall through the summer. Cluster flies do not reproduce indoors, and homeowners bothered by these pests do not need to fear the flies are "hatching" from a dead animal or other unpleasant material within the attic or walls. Cluster flies develop in the summer as parasites inside the bodies of earthworms.
Unfortunately for the besieged homeowner, there is nothing that can be done to stop these invader insects in the spring. The insects are emerging from hidden areas that are inaccessible, and so they cannot be treated with insecticides. These insects do not survive long indoors and can be disposed of with a vacuum cleaner.
Fall is the best time to try to reduce the numbers of insects entering your home. Seal exterior gaps and cracks around windows, doors, eaves, roofs, siding and other points of access before the insects appear to prevent insects from entering the house. Experience suggests, however, that comprehensive pest proofing is time-consuming, often impractical and usually not 100% effective. Spraying pyrethroid insecticides to the outside of buildings when the lady beetles appear may help prevent pest entry. Homeowner insecticides other than pyrethroids usually do not provide satisfactory prevention. Long-term relief from the Asian lady beetles may come from planting trees that will grow up to shade the south and west sides of the house. Insecticides can also be used on the outside of the house in mid-September to reduce the number of cluster flies entering your home if you have a persistent problem with them. However, the outdoor treatment with residual insecticides such as garden sprays labeled for exterior house treatment or cattle barn fly sprays is difficult and potentially messy.
This article originally appeared in the April 1, 2003 issue, p. 31.