Winter Moth – Operophtera brumata

These small brown moths are non-native invaders, so far confined to New England, eastern Canada, and the Vancouver/Seattle area of western Canada and the northwest U.S.  They were probably introduced from Europe sometime before 1950.

Adult Winter Moths emerge from pupation in the late fall.  In the Boston area they always seem to appear in the second week of November.  Only the males have wings.

Operophtera brumata male

On evenings in November when the temperature is above freezing, they flutter weakly in the chilly air in search of the wingless females.

Operophtera brumata female

After dark, the females climb the trunks of trees and emit sexual pheromones to attract males for mating.

The temperature when these photographs were taken was about 40° F (4.5° C).

Operophtera brumata mating pair

Mated females lay eggs in the crevices of bark during November and December. The whole generation is dead by January. The eggs overwinter, safe from predators, and the tiny green caterpillars hatch in late March or early April, depending on the weather.

The caterpillars have a distinctive set of markings: a dark dorsal median line flanked by a parallel white line on each side. They also have two stumpy prolegs near the rear of the body. They are so numerous in the Boston area during the spring that they have become by far the most commonly seen vernal caterpillars. You can walk along a path through a woodland without touching any vegetation and still find a few in your hair or collar afterward.

caterpillar 1 edited

Operophtera brumata larva

The caterpillars of O. brumata can severely damage deciduous trees and shrubs, and this tends to upset people. To help exert some ecological balance, in 2005 entomologists from the University of Massachusetts released one of the principal parasitoid species (a fly, Cyzenis albicans, Diptera: Tachinidae) that exploit the caterpillars as a host in Europe. The adult females of C. albicans lay eggs on the food plants of O. brumata caterpillars, which eat the eggs along with the vegetation. After a fly larva hatches inside a caterpillar, it lodges in the caterpillar’s salivary gland and waits for it to pupate in the soil. The larval fly then eats the contents of the pupa. After feeding, it concludes its exploitation of the host by pupating in the caterpillar’s pupal shell, which the caterpillar, of course, conveniently placed in the safety of the soil. After the larval fly has finished its development, it overwinters in the pupa and emerges as an adult during the following spring.

Words and photos by Matt Simon, Boston, Massachusetts
Editing by Marcie O’Connor