Box Elder Bug

Behavior
It is found primarily on boxelder trees, as well as maple and ash trees. The adults are about 12½ mm (½ in) long with a dark brown or black coloration, relieved by red wing veins and markings on the abdomen. Nymphs and immature bugs are bright red.
These insects feed on the softer plant tissues, including leaves, flowers, and new twigs. Unless the population is exceptionally large, the damage to plants is minimal.

During years when their population soars, they can damage useful shade trees.
In autumn, they can become household and hotel pests. The adult insects seek wintering hibernation locations and find their way into buildings through crevices. They remain inactive inside the walls (and behind siding) while the weather is cool. When the heating systems revive them, they begin to enter inhabited parts of the buildings. In the spring, the bugs leave their winter hibernation locations to lay eggs on maple or ash trees.
In late spring and early summer groups of 50-200+ bugs may gather on the sunny side of the house siding or brick. A month or two later there may be pairs of them mating, connected end to end, also in groups of three and four.

Infestation Treatment
These insects can be killed with a dilute mixture of soap and water — 2 tablespoons per gallon — sprayed on them directly. This procedure can stain or discolor siding however. A small strip of duct tape can also be an effective way of killing these insects, as they seldom will fly away when approached. Unable to escape from the adhesive backing, they can then be disposed of. They can also be kept out of the home, to a degree, by putting boric acid and/or diatomaceous earth in places they would gather to enter, as well as by using weather stripping and other means to seal the house better.
Another well proven technique is to spray them with streaming Wasp and Hornet insecticide. The aerosol cans allow one to surprise them from a distance, and will kill them instantly. Not all flying insect sprays will kill them; products specified for wasps should be used. This technique is most effective when they are gathered in large groups in the Spring.
The best way, however, to eliminate and prevent them is to hire a professional to treat the outside of your home. They will put a residual down on the outside of the home that will not only eliminate current bugs crawling on the outside, but will kill any future boxelders that land on your home. The best time for this treatment is spring and fall when the boxelders are most active.
Citric (especially orange) based disinfectants have been found quite effective to clean the areas where they congregate, keeping them away.