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All of the flying Bruce Spanworm moths you see are males, as the females lack developed wings. In October (through to early November, depending on the weather), the males seek out the flightless females which then lay eggs in places like bark crevasses. These eggs overwinter and hatch out in the spring.
The larvae (known as spanworms or inchworms), like all caterpillars, can do some damage to tree leaves but usually don't cause permanent harm to the trees themselves.
As always, click on the photo for a larger version.