Milkweed, boxelders and other bugs are invading Colorado houses this winter

My house is being invaded. It happens to some extent in fall of most years, but this is the most intense we have experienced, and on the first day of February we are still heaving cadavers and actives insects into the back yard. One day I caught eight invaders. Most years we have just one species trying to get in, but this year it is three.

Let me introduce the combatants, shown in the triptych. On the left is the small milkweed bug, Lygdaeus kalmi, a boxelder bug, Boisea trivitatti, is in the middle, and a western conifer seed bug, Leptogossus occidentalis, is on the right. These three insects have much in common. For example, all of them are looking for a safe, warm place to spend the winter so they can reproduce in spring. More about this later. None of the three has cryptic coloration, they all have aposematic or warning coloration and each has a chemical defense. They all suck sap from green plants. None of them bites or stings or carries diseases that can be passed to us.

The aposematic colorations advertise to predators that they are wielding chemical defenses. The colors and patterns of the three species make them easily identified, and the foul and poisonous fluids make any encounter poisonous and memorable. Small milkweed bugs, like monarch butterflies, sequester cardiac glycosides that they take from the sap of the pods and seeds. Boxelder bugs have abdominal glands that release a foul smelling, disgusting tasting liquid when they feel threatened. The western conifer seed bug has glands between its legs that release a repulsive, pungent smell taken from the seeds of Douglas fir, western white pine and lodgepole pine. Their bright colors, backed up by an awful taste with sickening effects, adequately protects these three bugs from predators.

I find it interesting that monarch butterflies, large milkweed bugs, milkweed leaf beetles and milkweed tiger moths, all feeding on milkweed, have adopted the orange and black aposematic coloration. They undoubtedly gain protection from the legions of herbivores, similarly colored, all carrying similar cardiac glycosides synthesized by milkweeds.

All three of these insects eat by sucking fluids sap or fluids inside green leaves and developing seeds. Their common names from their most common source of sap: small milkweed bug, boxelder bug, western conifer seed bug. Boxelder bugs favor the sap that they get from developing boxelder seeds, though they grow adequately by feeding from silver maples in Boulder.

By far the most common of these bugs that I encounter inside the house are the boxelder bugs. At first it was puzzling that such a high proportion of them, approaching 50%, are lifeless chitinous sheaths lying on the floor. This observation reminded me of the reason that ladybugs, Hippodamia convergens, fly to the tops of mountains, such as Green Mountain and Bear Peak, as winter approaches.

Ladybugs head to high elevation peaks for winter so that they can go into an undisturbed dormancy until spring. If they try to overwinter at lower elevations, they stir and fly about on warm sunny days in the winter. They fly about and search for food when none is available. They might die of starvation while searching for food, or they may exhaust lipid stores that they need to lay eggs in spring.

Natural selection favors those that leave the most offspring, so ladybug genes that favor prolonged hibernation are most common. The insects trying to get inside houses should talk to ladybugs! I asked a neighbor about boxelder bugs and he responded that “all their lifeless bodies scattered around the house.”

Bugs that get inside have a comfortable environment, but they need more water and food to remain active inside, where familiar sources of water and food are not available. Natural selection needs more time to teach boxelder bugs the lesson that ladybugs have learned.

Jeff Mitton is an emeritus professor of the University of Colorado Boulder Department of Ecology and Evolutionary Biology.