Box elder bugs are a small flying insect that flocks to female box elder trees and other trees in the elder family that include ash and maple trees. Box elder bugs do not significantly damage mature trees, but are considered a nuisance pest due to their overwintering activity. Box elder bugs search for a warm place to overwinter as the weather cools in the fall. Wood chips used as mulch around the base of trees and homes is a common place for box elder bugs to overwinter.
Box elder bugs are common insects found throughout North America. They go through complete metamorphosis, and nymphs and adults are frequently found in mulched wood chips in home landscapes. Adults are 1/2 inch long and have a dark brown or black base coloration with red to orange markings on the edge of their wings. Nymphs appear as smaller versions of adults but have a brick red coloration on their abdomen.
Box elder bugs emerge from mulch and other protected winter sites in the spring when temperatures rise. The bugs feed on various grasses and seeds in the landscape until box elder trees produce seeds. The bugs then flock, in mass, to female box elder trees to feast on the seed pods and reproduce. As the weather cools in the fall and trees drop their leaves, nymphs and adult box elder bugs search for a warm place to overwinter. Landscaping mulch below a host tree or mulched gardens in front of southern walls are ideal winter habitats for the bugs.
While the sight of a cluster of box elder bugs can be unsettling, the bugs themselves do not cause any damage to landscaping mulch or plants. Mulched areas near your home may increase the number of box elder bugs that congregate on walls and find their way into your home during the fall and winter. The bugs are considered nuisance pests due to their ability to discolor walls with their excrement.
While box elder bugs do not cause significant damage to garden plants or trees, large numbers of the bugs in your landscaping mulch can be unsettling and make it difficult to work in your garden. You can apply residual insecticides to problem areas, but they are not always effective at killing box elder bugs--especially if they are already hiding under the mulch. Removing mulch from sunny areas that remain warm through the winter and also from around the base of box elder, ash and maple trees on your property reduces the suitable habitats on your property for the bugs to overwinter.