First things first-how are they getting into the attic? At my place, they came in through the space between the chimney and hot water heater exhaust pipe. The chimney protector had been damaged, and the hardware cloth in the attic had broken. Once those problems were fixed, a few rat traps were left up there to take care of any stragglers. That was a few years ago, and no squirrels in the traps yet.
