This Spring’s Most Un-Wanted: Winter Moth

April 6th, 2010

Wanted For: defoliating & weakening trees ultimately causing death after several years

Description: young are 1” green inchworms with white stripes on sides; adult males are grayish brown with 1″ wingspans and appear hairy; females have no wings

Hangout: ornamental trees (crabapples, cherries & birch), fruit and shade trees (oaks & maples)

Kryptonite: Horticultural Oil, Bt, Spinosad

Research from UMass Amherst is indicating that Winter Moth is expected to be in very large numbers this spring and the potential for defoliation this year might surpass last year. Begin peeling the buds open and start inspecting for the presence of tiny larvae, which wriggle into swelling buds to feed.

Horticultural oil spray should be applied now when temperatures are above 45°f. Hort oil is an organic control that works by suffocation. Eggs that are protectively hidden within crevices and under lichen will not be covered by the spray thus will not be killed.

Winter moth eggs hatch anytime between late March and early April. This is the time when the most damage occurs. There is great potential of injury when the buds stay swollen but unopened. Winter moths will enter both leaf and flower buds.
Once the buds open, the caterpillars are known as “free-feeders” given that they are now on the foliage and free to move easily from one area to another. Winter moth will be in this stage until late May or early June. While still on the plant, however, they are exposed and very treatable with Bt or Spinosad.

Bacillus thuringiensis, Bt, is a bacterium which must be ingested to be effective. Bt produces proteins that paralyze the digestive system of the winter moth. The infected insect stops feeding within hours thus starving to death in a couple of days. Bt works best on the younger stages of caterpillars; older ones are much less affected.

Spinosad is another organic control that is derived from a naturally occurring soil bacterium that was collected from soil in an abandoned rum distillery in the Caribbean by a scientist on vacation. Spinosad kills damaging insects by causing rapid excitation of the nervous system. Spinosad is fast acting – the insect dies within 1 to 2 days after ingesting. It works well as both a contact spray as well as by ingestion. Spinosad products work well on caterpillars of all ages.

The winter moth will then be out of sight until adults emerge in late November or early December when they are seen fluttering in your headlights or by your back door light.

-By Maggie Oldfield Thayer Nursery

Tags: Caterpillars, garden pests, Lawn and Garden, Problem Solvers
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