I love warm rainy nights in December. Not because I particularly like rain, but because I love how winter moths (Operophtera brumata) emerge seemingly out of nowhere, to take advantage of the warmer weather. In fact, they’re one of very few Lepidopterans in the temperate region that are active in the late fall and early winter. On nights like we had this weekend, with light rain and temperatures close to 50 degrees, they can be so numerous that they remind me of large fluffy snowflakes as they light up in my headlights when I’m driving. 

But other than enjoying the fluttery show they put on, I never thought much more about them until doing a little research to write this post. Two things surprised me. First, winter moths are invasive. They were introduced to Nova Scotia from Europe in the 1930s. From there, they spread to the United States by the 1950s, and are now well established in Massachusetts.  The second winter moth fact that surprised me is that only the male is able to fly. The female lacks true wings and instead climbs to the base of a tree or building and uses her pheromones to attract a male. Unlike the males, which are strong fliers with their four broad light tan to yellowish wings fringed with small elongated scales that give the hind margins a fringed appearance, the flightless female has a brownish-grey body with vestigial wing nubs. 

After mating, females lay a cluster of approximately 100 to 150 eggs under tree bark or in tree crevices, where the eggs will over winter. Hatching coincides with the emergence of leaves in the spring, when the small green inchworm-like caterpillar will have ample food. Early in the season, the larvae feed on expanding leaf buds and can actually burrow into the bud itself. Later in the season they feed on fully developed leaves. If the available food runs low on the tree where they hatched, the caterpillars can produce silken strands to float to nearby trees, an action referring to as ballooning. The destruction they inflict on tree canopies ends in mid-June when the caterpillars migrate into the soil to pupate.