Hundreds of Small Black Bugs in Montana Home: Fall Plant-Feeding Bug Invasion

The ~¼" black bugs appearing by the hundreds in your Montana home in fall are plant-feeding true bugs (Hemiptera)—likely a species that breeds in agricultural fields (alfalfa, grains, weeds) and migrates to structures as crops dry down.

Behavior

• Mass invasions in fall = seeking overwintering shelter.
• Similar to boxelder bugs but smaller and different host plants.
• Harmless: don't bite, sting, breed indoors, or damage anything.

Control

• Exclusion: Seal cracks, screen vents, weatherstrip doors/windows.
• Vacuum indoors. Discard bag outside.
• Exterior perimeter spray (pyrethroid) in late summer may reduce numbers, but sealing is more reliable.
• No indoor treatment needed.

They'll leave in spring. It's an annual nuisance, not an infestation.