The winter moth caterpillar was brought to North America sometime after 1950 but wasn’t a major problem until recently. In the past few years areas of eastern Massachusetts have experienced a great deal of damage from this pest, and it has been found in Rhode Island, parts of Connecticut and Long Island.
From late November through early January the male winter moths fly on warmer evenings, looking for the females who wait at the base of trees. The females lay their eggs in the trees and the larvae hatch when the temperatures average around 55 degrees.
Winter moth caterpillars eat the foliage of many deciduous plants but cherries, apples, blueberries, maples, oaks, ash, blueberries and roses are especially vulnerable to damage.
What to do? Products containing Bt or Spinosad are effective on the winter moth caterpillar. Bt is best used early in the season and on any plant that is flowering because Spinosad can harm foraging bees. Before and after flowering, however, Spinosad is a very effective treatment. There are several products that contain Spinosad as the active ingredient – ask at your local garden center. Be sure to spray Bt or Spinosad at bud break and repeat as the foliage grows. Continue spraying until mid-June.
Trees defoliated by the winter moth caterpillar will usually have enough reserves to produce a new set of leaves the first year they are attacked, but defoliation year after year can kill these plants. The additional stress of drought will further weaken shrubs and trees that were stripped early in the summer. If spraying with Bt or Spinosad is impossible you can help strengthen plants by watering them deeply every ten days to two weeks during periods of drought.
The Massachusetts Cooperative Extension has an excellent handout about winter moths available at: