You’re most likely seeing Asian lady beetles, which look like ladybugs. They gather on sun-warmed walls in the fall, ideally light-colored and sunlit, as they look for places to stay in the winter.
Native ladybugs mostly overwinter outdoors under leaf litter and bark and want nothing to do with your siding. The wall-swarmers are Asian lady beetles, look-alikes introduced decades ago for crop pest control, and unlike the natives, their overwintering instinct points them at cliffs and crevices. In the typical Massachusetts suburb, the closest thing to a sunny cliff face is the southwest wall of your house.
The beetles start heading for large, sun-heated surfaces after the first cold snap eases off. This is usually in October. They prefer walls that are light-colored and facing southwest. It will likely start as a few beetles landing on the wall. Then these release pheromones to call over other beetles. And from there, gatherings get really big really quickly. Those chemical markers linger too, so don’t be surprised if you see the beetles gathering in the exact same spot next year.
Gathering on the wall is more or less harmless. What happens next isn’t harmless, as beetles work their way into gaps around window frames, siding seams, soffits, and utility penetrations. This lets them settle into wall voids for the winter, and then reappear in your living room and foyer during January thaws.
The best thing you can do is to seal cracks and gaps along that favored wall. Focus on window trim, siding junctions, and vents, using rope caulk and screening. This makes it harder for them to get indoors. The best time to do this is before they start swarming in October. Perimeter treatment of exterior surfaces also helps because it creates another obstacle between the bugs and the inside of your house.
If your sunny wall has become an annual beetle convention, Mosquito Squad of Boston Metro South can help. Mosquito Squad's home and yard coverage addresses overwintering pests at the perimeter. This can help you protect your house before the fall rush arrives.
