Winter Moth

Lepidoptera: Geometridae (Operophtera brumata)


Newly hatched caterpillars are blackish in colour and 1/10 inch in length. The grown caterpillars are pale green with a faint white longitudinal stripe on both sides of the body and about ¾ inch in length. The winter moth caterpillars are loopers because they have two pairs of prolegs which make them move with a looping gait.

Female and male adults of winter moths are sexually dimorphic. The females are visually wingless and cannot fly while the males are fully winged and fly strongly. Females have brownish-grey bodies but males vary in colour, from tan to grey. Both female and male adults are approximately 1/2 inch long.

Life Cycle

Winter moths have four life stages: egg, larva (caterpillar), pupa, and adult (moth). Adult moths emerge from pupae in the soil usually in late fall and may be active until the temperature falls below zero. The wingless female emits a sex pheromone and attracts a cloud of males. After mating, the female deposits eggs mostly on the bark or in bark crevices. Each female may lay 150 to 350 tiny eggs. The eggs overwinter and hatch just before the bud break of most of the host plants. The newly hatched caterpillars crawl up tree trunks and attack the opening buds, producing a shot hole. Older larvae feed in expanding leaf clusters and defoliate trees when abundant. Fully grown larvae drop into the soil and pupate. Winter moths complete one generation per year.


Several hardwood trees (such as oak trees and apple trees), ornamental trees, and shrubs can be attacked by winter moths. The caterpillars severely defoliate the trees a nd quickly run out of food. Then, they drop to the ground by the thousands looking for new host plants which cause a nuisance in urban areas. If the trees are attacked over four consecutive years, partial defoliation can cause mortality in infested branches and twigs, and full defoliation may kill the whole tree.



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