Operophtera brumata

Moths (the adult stage of winter moth) emerge from pupae in the soil usually in mid-late November (usually before Thanksgiving) and may be active into January, whenever the air temperatures are mild (typically when above freezing). The small (0.79-0.98 inch wingspan) male moths are light brown to tan in color and all four wings are edged with small elongate scales that give the hind margins a slightly hairy or fringed appearance. The male moths are strongly attracted to lights and can often be found flying around outdoor lamps or holiday lights. The female (0.31 inch) is gray, almost wingless (brachypterous) and, therefore, cannot fly. She emits a sex pheromone that often attracts clouds of male moths. Females are usually found at the base of trees or scurrying up tree trunks, but can be found almost anywhere.

After mating, the female deposits loose eggs on bark, in bark crevices, under bark scales, on lichen, etc. Each winter moth female may produce 150-350 tiny eggs, which are very difficult to see. The adult moths then die and the eggs over-winter. Eggs are green at first, but turn red-orange soon thereafter. In March, prior to hatching, the eggs turn a bright blue and then a very dark blue-black just before hatching. Eggs hatch when temperatures average around 55º F. It is believed that egg hatch in Massachusetts occurs when 20–50 Growing Degree Days (base 50º F) have accumulated, which can be anywhere from late March into early-mid April, depending on the year and the location. (The Elkinton lab at UMass, uses a base 40º F when completing Growing Degree Day calculations for this insect and suggests that hatch occurs between 177 and 243 GDD (base 40°F, January 1 start date, double sine method).

In most years, egg hatch occurs just at, or right before, bud break of most of the host plants and delayed bud opening due to cool weather can lead to caterpillar death. The newly hatched caterpillars crawl up tree trunks and wriggle between bud scales of newly swelling buds of such hosts as: maple, oak, ash, apple, crabapple, blueberry, and cherry, etc. and begin feeding. Winter moth caterpillars cannot chew their way through closed bud scales, but as the bud scales open, the caterpillars can scrape away at the soft leaf tissue below, producing bullet-hole like damage to the leaf, even before it fully expands. The young larvae also produce strands of silk, which make them air buoyant and this larval dispersal method is known as "ballooning" (like bungee jumping, but not staying tethered to the point of origin). In certain situations, given topography and wind patterns, ballooning winter moth caterpillars can arrive in areas where they have not been expected to be a problem. The caterpillars feed in both flower and foliar buds and, once a bud has been devoured from within, the caterpillar will migrate to other buds and repeat the process. At maturity, the caterpillars will be approximately one-inch-long, whereupon they drop to the soil for pupation. Pupation occurs in late May/early June depending on the geographic location. Areas including Cape Cod are often a week or two behind inland locations.

Winter moth caterpillars are very tiny when newly hatched, less than the size of an eyelash and they are blackish in color at that time. As they feed and increase in size, they are pale green with a faint white longitudinal stripe running down both sides of the body. They are "loopers" or "inchworms" and have just two pairs of prolegs. Winter moth caterpillars are often found in association with both the fall and spring cankerworms, as well as Bruce spanworm (Operophtera bruceata), which are very similar in appearance and have similar feeding patterns to that of the winter moth caterpillar. Fall cankerworm caterpillars have “2 and ½” pairs of prolegs: two pairs of the prolegs are longer than the third, much shorter pair which is counted as number 3 when starting your count from the rear end of the caterpillar. Bruce spanworm is native to the Northeast and is rarely a problem.