Adult boxelder bugs are about ½ inch long, dull black with red markings along the front edges of their wings.
They are predominantly attracted to the female boxelder tree, also called an Ash-Leaved Maple, Acer Negundo. Female boxelder trees can be identified by their winged seed pods. The boxelder tree is their food source and that explains the attraction to it.
During spring, boxelder bug adults and nymphs can be seen emerging from hibernation sites, flying to a boxelder tree or gathering on the sides of buildings. At this time the overall size of boxelder bug infestations on trees and buildings can be considerable smaller (differing as much as tenfold) than boxelder bug infestations in the fall. Female boxelder bugs emerging from hibernation begin to lay eggs in April and May.
The temperature of surfaces found around buildings can contribute to infestations of adult boxelder bugs in the spring and fall. Research data, obtained by measuring the body temperature of boxelder bugs and the temperature of different surfaces, shows that boxelder bugs prefer surfaces with temperatures that are much higher than a boxelder bugs normal body temperatures.
Several cold nights followed by an Indian summer cause boxelder bugs to begin congregating on homes as they search for where to spend the winter.
In the fall there is an increase in the number of boxelder bugs on host trees, and this increase is associated with development of the ovules on female boxelder and maple trees. There are much fewer boxelder bugs around buildings in the fall; most are found on trees. Adult boxelder bugs move to overwintering sites during October. Although the same overwintering sites are not usually used by boxelder bugs every year, there are some that are frequented on a regular basis.
Boxelder bugs travel short or long distances (as much as two miles) from a boxelder tree to a favorable hibernation site. Boxelder bugs spend the winter in leaf litter next to buildings, under bark in protected areas or in other spaces in which large numbers of boxelder bugs can gather for a few months.
Normally, October is the month in which boxelder bugs become a pest. Boxelder bugs begin to mass in large numbers on boxelder tree trunks to prepare to overwinter. From the tree the boxelder bugs move to wood and rock piles, fallen leaves, and the cracks and crevices of buildings. They are especially attracted to the sunny side of light colored homes. Boxelder bugs creep into siding, window and door frames, porches, and cracks in masonry.
Boxelder bugs do not bite, reproduce indoors, cause any structural damage, or cause health problems to humans or pets. Smashing boxelder bugs can stain fabrics and wallpaper. Boxelder bugs are merely a nuisance.
Moderate daytime temperatures are confusing to boxelder bugs; it is normal to have a recurrence of the boxelder bugs in the middle of winter when it is mild. Boxelder Bugs come out of hiding and become active because it is so warm during the day, it seems like spring to them. As long as it is mild boxelder bugs will congregate on boxelder trees and light colored homes.