Box Elder Bug a Nuisance, but Not Harmful


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Box elder bugs cause concern in the autumn when they gather in considerable numbers on the warm outside walls of homes and sometimes find their way into houses looking for a suitable place to over winter.

When they gain entry to buildings through cracks or other openings they remain in wall cavities and will occasionally emerge inside the home in the spring. They will not breed indoors, so there is no danger of starting an “infestation”.

They cause no structural damage whatsoever, but they can “spot” interior furnishings with their droppings. They can’t bite, they don’t eat anything on the inside of your house, including house plants, and they won’t harm you, your family or your pets.

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Identification and Life Cycle

This bug is about half an inch long and one-third as wide. It is black with three red lines on the thorax, a red line along each side, and a red line on each wing. The wings lie flat on the back when at rest. Box elder bugs are sometimes confused with another insect called milkweed bug.

Eggs are a rusty red colour and are not often seen as they are deposited on box elder trees. The young nymphs are red and gray. The population of bugs may number into the thousands. They resemble adults, but do not have fully developed wings and are not able to reproduce. The change from nymph to adult is a gradual one.

The adult bugs lay eggs on the host trees in the spring and the nymphs emerge in a few days. The nymphs are small and show more red than adults. These nymphs develop into adults during the summer, then mate and lay eggs which hatch into the nymphs of the second generation. In the summer, box elder bugs normally feed on the leaves, flowers, and seed pods of the box elder tree, silver or red maple. The bugs cause little damage to trees. Activity of nearly fully grown nymphs is noticed in August and September when they gather in large numbers on the trunks of box elder trees. The migration of the adults begins at this time.

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Prevention and Control

An obvious way to avoid infestations by this pest in residential properties is to get rid of near-by female box elder trees. If this species is to be planted as an ornamental or shade tree, male trees should be purchased from the nursery. They are propagated by cuttings from staminate trees. Chemical control can best be obtained by spraying the nymphs on the host trees before the adults have had a chance to migrate. Power spray equipment is usually required and a professional should be hired to do the job safely. Caulking windows and doors, and repairing window and door screens will prevent bugs from entering a home. A pest control professional can apply a residual insecticide to exterior walls in the fall where the bugs are congregating – this will tend to discourage them from landing.

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Box Elder Bug        vs.     Milkweed Bug