The boxelder bug (BEB) is a common and well-known insect in Iowa that is most abundant after summers when the month of May is very warm and July is very dry. However, the abundance varies greatly from place to place as well as from year to year. There are some BEB problems even in years when a widespread outbreak does not occur.
During the summer months, BEBs live, feed and reproduce on trees, shrubs and other plants (including boxelders, maples, ashes and others). They feed on sap from their host plants but do not cause significant damage. BEBs become nuisance pests in the fall when they leave the plants to find hiding places for the winter. During their random search, they congregate in the sunshine on the south sides of buildings, trees and rocks. From there they stray into houses through cracks in the foundation and siding, gaps along windows and doors, and other small openings. BEBs within walls or attics remain inactive while they are cold. The nuisance occurs when the ones warmed by heat from the furnace or the sun become active during the winter and crawl into the rooms.
BEBs do not reproduce indoors. They only lay eggs on trees and other plants. BEBs do not feed indoors. They are sap feeding insects with a beak that can only suck liquid food (sap) from the twigs and seeds of selected species of trees and shrubs. BEBs are harmless as they can not damage the house, its furnishings or occupants. They can be, however, a considerable nuisance.
There is no easy way to determine when and where there will be a problem until it starts. By then it may be too late for effective treatment. Bugs could be controlled on the trees in mid summer with insecticides labeled for use against boxelder bugs on trees, but the effectiveness is limited. Spraying large trees is difficult and tree spraying is usually impractical.
The best deterrence against BEBs and similar invaders (e.g., crickets and attic flies) is to prevent entry by caulking and sealing possible entry sites (cracks and gaps). Secondly, spraying to reduce the number outdoors may limit the number that will get into the house. A lawn and garden insecticide or soapy water spray (5 tablespoons of liquid detergent per gallon of water) can be used outside on masses of bugs perched on and along the foundation in the fall. Additional insecticides are available to professional pest control operators for exterior treatment in the fall. Repeated applications are usually necessary especially when using a soapy water spray. For more information on insecticides available to homeowners please see this article.
Unfortunately, there is no easy cure for eliminating BEBs already inside the house. They are generally not killed by the aerosol household insecticide products, and most residual insecticides are not of much benefit. A sure control for bugs already in the house is to remove them as they appear by vacuuming, sweeping or picking them up and discarding. Treatment by professional pest control operators may be more effective than what homeowners can do using generally-available household insecticides.