The Short Answer: Duke, there are soldier beetles (family Cantharidae) that look vaguely like your picture (http://bugguide.net/node/view/457009 to see one), but what you have is an Eastern boxelder bug (Boisea trivittata – the nearly identical Western boxelder bug is Boisea rubrolineata). As you mentioned, soldier beetles prey on other insects, but boxelder bugs feed primarily on the seeds of certain trees and use the tree sap as a source of liquids. When feeding on seeds, boxelder bugs use their piercing mouthparts to puncture the seed. Then they inject digestive juices and suck out the seed contents. When congregated, they will sometimes cannibalize each other. This typically happens when a boxelder bug has just shed its exoskeleton in transition from one nymph stage to the next. Boxelder bugs have also been known to feed on other insects, if they come across them in a vulnerable post-molting state.
Boxelder bugs are part of a larger group of insects called the soapberry bugs (subfamily Serinethinae). Soapberry bugs feed on the seeds of trees in the Sapindaceae family, which include soapberry trees, from which people around the world have long made soap. Other trees in this family include maples (genus Acer) and the boxelder tree (Acer negundo), a slightly oddball maple (in fact another name for it is the boxelder maple). Rather than solitary leaves with three or five lobes like your standard sugar maple (Acer saccharum) or red maple (Acer rubrum), the boxelder has a compound leaf with less obvious lobes. Although boxelder bugs do especially love boxelders, they seem willing to feed on the seeds of any maple. So anywhere you find boxelders and other maples, you are likely to find boxelder bugs.
Why are you finding them in your house? When the weather becomes too cold to feed and grow, boxelder bugs find a place to overwinter in safety. If there were no houses around, they would typically hide in the bark and leaf litter at the base of the trees they feed on. But they also seem to be attracted to sunny southern and western exposures that would be likely to stay a bit warmer during the winter. And it turns out that the sunny sides of houses fit their needs quite well. They often crawl underneath the siding or into attics and other spaces and happily wait for the spring. An especially warm and sunny day can fool them, however, and they may begin to move around. If there happen to be cracks in your walls or window frames, they may end up inside your house.
Since there are probably no boxelder or maple seeds in your house, to a boxelder bug, the inside of your house is a dead end … and one from which they aren’t likely to find their way back out. So if you wanted to be especially kind, you could simply scoop them up and deposit them outdoors. If you have quite a few, or are especially squeamish about insects, you could vacuum them, but make sure you dispose of the bag, since they have been known to come crawling back out. If you are prone to squishing bugs, be forewarned that the insides of boxelder bugs can be pretty smelly and may leave a stain. Joseph Schwarz, a biologist who studies boxelder bugs, also told me “If they become too active inside the home, they may cannibalize each other and then defecate on furniture and textiles, leaving brownish-red protein stains that are hard to remove.” Not an attractive prospect.
If you want to limit the number of boxelder bugs that make it inside your house, there are a couple of things you can do. One is to eliminate as many entry points as possible, by checking weather stripping and caulking gaps wherever possible. I read one suggestion that you rake up any maple seeds when they fall and dispose of them. A more drastic measure would be to remove any maple trees in your yard, especially boxelders. If you have an especially popular boxelder, this may cut down on the number of boxelder bugs that are attracted to your house. Because boxelders have male and female trees and only the females produce the seeds boxelder bugs feed on, if you can figure out which sex your trees are, you can leave the males and only remove the seed-producing females.
Boxelder bugs will travel some distance to find a good winter nesting place, however, so removing trees from your backyard is not a guaranteed solution. You may be stuck with the bugs, unless you want to move. One thing to know is that boxelder bugs are completely harmless, and they won’t kill your trees, so it’s probably best just to find a way to live with them.
Thanks to Joseph Schwarz for his help in reviewing this article, and to Crystal Perreira of Soapberrybug.org for her help as well.
Cornell Cooperative Extension, accessed on 12/5/2013, http://ccesuffolk.org/assets/Horticulture-Leaflets/Boxelder-Bug.pdf