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Controlling the winter moth

Those moths dancing around your holiday lights are male winter moths, which have infested the Cape during the past three years. According to experts, this insect pest is expected to remain a problem here until an effective biological control can be found. The winter moth has no known natural predator to keep its population in check.

“There’s nothing in the environment that provides a control for them,” says Michael Talbot, a certified horticulturalist and arborist in Mashpee. “People should know, if they’ve had a problem, it’s going to be worse next year—until some community-wide biological control can be instituted.”

So far, Falmouth, Sandwich Village and West Barnstable have been hardest hit. But the moth is expected to migrate into other areas of the Cape.

Introduced from Europe, where they are currently a problem in the United Kingdom, winter moth caterpillars are capable of defoliating whole stands of trees. They emerge in spring from eggs laid by the wingless female in winter to feast on leaf buds and flower buds of fruits.

Research in Canada, where the winter moth has been a problem for years, has shown that four consecutive years of complete defoliation of host trees will lead to the death of those trees, while partial defoliation can lead to the death of branches. In certain regions of Nova Scotia, this pest is responsible for a 40 percent red oak mortality in forested stands, according to information provided by Cape Cod Cooperative Extension of the University of Massachusetts.
Because they must put out a second flush of growth in order to survive, trees heavily defoliated by winter moth caterpillars become severely stressed. So water is critical to their survival. Supplemental watering of trees is necessary if little rainfall occurs. Experts advise against fertilizing trees that have been defoliated because it makes them work harder.

If you feel you must fertilize, Talbot suggests compost teas and liquid seaweed applied in fall. Deep watering once a week will help the stressed trees. “If you have a lawn sprinkler, it should be set to water 45 minutes to an hour every five days,” Talbot says.
Many different deciduous plants are susceptible to the winter moth. These include oaks, maples, basswood, ash, crabapples, apple, blueberry and certain spruces, such as Sitka. The caterpillars appear to favor fruit trees. They move from bud to bud as they feed. Delayed bud opening due to cool weather conditions can lead to bud death, as the caterpillars have a longer time to feed.
Once leaf buds open, the small green caterpillars can be found within tight clusters of new leaves during the day. At night, they often depart these clusters to become “free feeders” beneath the trees. They may drop to herbaceous plants, such as perennials and roses, beneath or near the trees.

The caterpillars emerge from eggs laid by the moth in winter on tree trunks and branches, in bark crevices, under bark scales and loose lichen. A female winter moth is capable of laying up to 250 eggs. The eggs hatch in spring when temperatures average around 55 degrees. The caterpillars feed voraciously on buds until mid-June and then migrate to the soil for pupation. They can be seen dangling from trees on silken threads. In late November, the adult moths emerge. The males are often seen flying around outside lamps and holiday lights. They can be active into January.

PEST CONTROL
In the states of Washington and Oregon, as well as in Canada, various natural controls have been introduced to combat this pest, with varying levels of success. These include parasites such as flies and wasps. Certain native beetles, such as ground beetles and oil beetles, may act as predators of this pest as well. At the USDA facility on Otis Air National Guard Base, a team of researchers from the University of Massachusetts is currently experimenting with parasitic insects that might be used to control the winter moth population.

Application of dormant oil spray to the trunks and branches of trees may be helpful in killing the over-wintering eggs before they hatch. The horticultural oil, available at local garden centers, should be sprayed in early spring. Some egg clusters under bark flaps and loose lichen may be protected from oil sprays, however.
According to the University of Massachusetts Extension Service, insecticidal soap may be effective against the younger caterpillars, but only when they are exposed on the host plant, according to horticulturalists.

Certain chemical insecticides, such as Sevin and Malathion, are labeled for these pests. But the application of these poisons is probably best left to professionals.

Talbot recommends dormant oil spray in early spring, followed by spinosad when the caterpillars are feeding. He believes insecticides are “overkill” because of their adverse effects on the environment.

WHAT TO USE

• Dormant oil spray – when applied to the trunks and branches of trees in early spring, horticultural oil may help kill winter moth eggs.

• Baccilus thuringiensis (B.t. kurstaki) – this bacterium specific to caterpillars of butterflies and moths works well on the younger larvae while they are free feeders (not in the bud).

• Spinosad – commercially available as Bulls Eye BioInsecticide, Monterey Garden Spray, Ferti-lome Borer and Bagworm and Tent Caterpillar Spray, this compound works well against winter moths.

• Insecticidal soap – this may be effective against younger caterpillars but only when they are exposed on the host plant.

• Chemical insecticide – many compounds, such as Sevin and Malathion, are labeled for these pests. Consult your local supplier and always read, understand and follow label directions for pesticides.  Information provided by the University of Massachusetts Extension service.

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