Why are there big black ants in my house in winter?

Rutgers puts the outdoor carpenter ant season in the Northeast at May through October, with peak activity in July. In winter, an outdoor colony is dormant. A heated house is a different story: a colony inside a warm wall void stays active, and the workers forage into living space looking for water and food. That's why winter sightings are more diagnostic than summer ones. In July, ants in the kitchen could be wandering in from a stump in the yard. In February, they live with you.

Carpenter ants nest in wood that is moist or partially decayed (Rutgers). Inside a house that means the wood around a leaking window, under a bathroom or kitchen with a slow plumbing leak, porch posts, deck framing that meets the house, and the sill plate where the foundation sweats. Cornell Cooperative Extension calls carpenter ants an indicator organism: their presence in the home indicates a moisture problem exists somewhere. Look for fine sawdust with insect parts below trim and window frames; that's the excavated wood.

The North Shore's older homes with mature trees and cedar trim, and any house near the water, are the classic setup. We find winter nests most often under bathroom windows in 1950s and 1960s splits and in the porch framing of prewar colonials in places like Oyster Bay, Manhasset, and Huntington. Killing the ants you see doesn't touch the colony; the nest has to be found and treated, and the leak has to be fixed or they come back.