Winter Moth

Order: Lepidoptera
Family: Geometridae
Species: Operophtera spp.

Photo courtesy of S. Fitzpatrick

Description: The family name is based on the strikingly angled wings of the adults; however, this characteristic is not as evident in winter moths. Eggs that are deposited onto the bark along the lower areas of the shrub or tree are relatively large, circular and white. The larvae are a light green with longitudinal white stripes along the abdomen. The larvae have the classical inchworm form. Adult males are small moths with light brown to tan colored wings. Both the forewing and hind wing are fringed with small-elongated scales giving them a hairy appearance. Females of this species are, interestingly, wingless.

Life History: Flying and mating occurs between November and January, hence the name “winter moth.” Females will crawl up the trunk of the tree and call for males using their sex pheromone. In the evening, you can stumble across this courtship. A cloud of males can be seen fluttering around the base of trees waiting for the female to select the lucky guy. These insects spend the rest of the winter as eggs. Larvae will hatch as early as March to begin feeding on the emerging buds. This is where winter moth becomes a problem. Larvae can feed on a wide range of hosts but do a fair amount of damage to apples, cherries and other fruit and flowering trees in this area. Economically speaking, larvae are a big problem on blueberries. Other trees affected are oaks, maples and other broad-leafed deciduous trees. Since the larvae emerge so early in the season, fruiting buds and flowers are preferred food sources.

You may be putting the pieces together and wondering how these insects get around. The females don’t fly and most caterpillars don’t crawl very far, so how does your newly planted blueberry get winter moths? It is actually the early stages of the larvae that migrate or disperse into new areas. The young larvae will climb out to an open branch and send out a fine, silken web. When the wind picks up, the larvae can ride the gust for considerable distances. Upon finding a suitable food source, the larvae will continue to feed until mid-June. The inchworms drop down and pupate in the soil until late fall where it starts all over again.

Damage: A good indication of an infestation is early petal drop from your cherries and other early season flowering trees and shrubs. By then, it is usually too late to do anything. In early spring, look inside a few flowers and you may find the young inchworm feeding at the base of the petals. Other signs of infestations are dead and flagged buds. In years of late bud-break and large populations of winter moth, trees can be completely defoliated. This is not an uncommon occurrence in Northwest Washington.

Control: Dormant oil treatments are the easiest control of the egg stage and Bt treatments are good controllers of the larvae in late spring. If infestations are severe enough to warrant additional pesticide applications in the spring, please take note of the history of this insect. Always consult your PNW Insect Control Handbook before making a pesticide application.

History: Winter moth is one of many gifts from Europe. Winter moth has been in Washington and Oregon since 1950 but very few releases of beneficial insects have been made to biologically control winter moth. On Vancouver Island, however, a large campaign against winter moth was made in the early 1980s. The Canadians made several releases of parasitic flies and wasps to attempt to control the winter moth. Winter moth and its natural enemies moved over to the main land and slowly spread from there. Winter moth had been a big problem in Whatcom County for blueberry growers and home owners until the mid 1990s. One species of parasitic fly (Cyzenis albicans) has recently built up a viable population in the area and is impacting winter moth numbers. This fly is a smart one. It is able to smell areas damaged by winter moth and deposits an egg along the chewed margins of the leaves and buds. This egg is then accidentally eaten by the winter moth as it chews on the leaf. The fly maggot waits in the salivary glands until the caterpillar turns into a pupa when it then consumes the winter moth. It is probably because of this fly that landscapes have not suffered as compared to early years of infestations.