I suspect the inevitable was inevitable. They most popular — or at least the most often asked — question at the Utah State University Extension Service was, "What are these horrible orange and black bugs that are invading my home, school, businesses or other buildings?" If they knew the name of the pest, they phrased it, "Why are these boxelder bugs picking on me?"
The time we field the most questions of this type is in late summer and fall when they start to invade the interiors of homes and other buildings. The time when they actually do any damage would be spring and then early summer.
Immature and adult boxelder bugs are plant juice fluid feeders with piercing-sucking mouthparts. They are true bugs. Insects are divided into large groups called orders. While all the bugs are insects, only a few insects are true bugs.
Bugs have incomplete metamorphoses, meaning once the eggs hatch the tiny nymphs resemble the adults. Adult insects are about 1-11/2 inches long, flattened on top, with an elongate-oval shape. They are a slate gray in color but are distinguished by three red lines that run the length of the first body segment with a pair of legs, attached to the base of the head.
They have tough, leathery wings so they can fly, and the forewings are rimmed with red so they are colorful at all of their life stages. That gives rise to their common name of firebug.
To say that boxelder bugs cause no harm is erroneous. They can feed on many different plants, including maple, ash, stone fruits (cherry, plum, peach), apple, grape, strawberry and grass.
The typical damage is noticed in the fall as the fruit is being harvested. It will be distorted into a puckered or shriveled shape called cat facing, which makes it unattractive for sales and may reduce the shelf life.
However, their primary food source is the seed pods of the female boxelder trees where large numbers are typically found as nymphs and adults feed on developing seeds.
They do have their favorite collection points. South-facing walls are preferred, with taller, two-story homes winning over lower homes. Darker homes are preferred to those that are lighter in color. Wood has a slight preference to brick, probably because it can accommodate their sheltering easier.
Although boxelder bugs are active throughout the summer, many people don't notice them until they start "sunning" themselves on structures, particularly the southern-facing walls. As temperatures start to decrease in the late summer and fall, large numbers of adults will move from plants and congregate on heated buildings.
Boxelder bugs are not usually found on ornamental plants, but have been known to damage fruiting trees during the late summer. Homeowners do not necessarily have to have a boxelder tree to notice adults around the home, because the flying insects are mobile until they die from winter cold.
The most common question is what damage they cause. When they move inside, they can stain carpets, drapes and other fabrics, but the risk is low. They mostly are just very annoying as they collect around the windowsills and other areas.Comment on this story
If you consult any good insecticidal reference, it will list numerous pesticides that are registered for controlling boxelder bugs. Typically these are not needed as there are better ways to manage the pest.
The small nymphs, which have no wings, are easily controlled with insecticidal soap. Spray those when you see them and they will not reappear.
Caulking to keep the pests from coming inside your home is a more efficient method of keeping them out. However, if they do come inside, a vacuum cleaner will quickly and cleanly collect them. Winter weather will take care of the problem outside — at least until spring.
Larry A. Sagers is a horticulture specialist for the Utah State University Extension Service at Thanksgiving Point.