Article: The Dreaded Winter Moth

Created on: Saturday, March 24, 2012

THE DREADED WINTER MOTH
March 2012
What are they and where did they come from?
Winter moths, in their caterpillar form feed, on a number of deciduous trees and shrubs found in Massachusetts, including blueberry and rose bushes, cherry, apple, crabapple, elm, ash, maple and oak trees. The damage they do can be substantial.
The winter moth is an invasive species introduced to North America from Europe, first recorded in Nova Scotia in the early 1930s. It has since spread all over coastal areas in the western and eastern United States but Massachusetts appears to have the largest and most damaging populations of this pest.


Why should I worry about this now?
In late March and early April, winter moth larvae hatch into smooth green inchworms (caterpillars), identified by narrow white stripes running lengthwise on each side of the body. These caterpillars emerge from eggs deposited in November and December on bark, in bark crevices, on lichen or elsewhere. If visible, the eggs appear green at first, then red-orange, then to bright blue, and finally a very dark blue-black just prior to hatching. Some of the caterpillars will then start to feed on both flower and foliar buds of the host tree. Others will crawl up tree trunks and spin a strand of silk, which, with the help of air currents, takes them into tree canopies of either the host tree or nearby trees and plants in a dispersal method known as “ballooning”. Once there, the damage to the tree or shrub begins as the caterpillars work their way into tree buds, leaves and fruit in order to feed.

What can happen to my plants?
Caterpillars can kill a tree, over time, and can lead to a greatly diminished harvest of fruit crops as the caterpillar moves from bud to bud, and then on to developing foliage. If not treated properly, and soon, emerging leaves could be riddled with holes, and complete defoliation may ultimately kill maple, oak, apple, cherry, linden and ash trees in as little as four years.
The many outbreaks observed last year and the large number of winter moths seen this winter is an early warning sign that spring outbreak may be high.

What can I do?
In late March/early April, a dormant horticultural oil spray such as Bonide All Seasons Horticultural Oil applied to trunks and branches will help kill the over-wintering eggs before they hatch and prior to buds swelling and the leaf emergence. This spray smothers the eggs while the trees are still dormant.
Once the foliage appears, it is too late to apply a dormant oil spray. At that point, a product such at Bacillus thuringiensis (B.t.) or a product containing Spinosad, such as Captain Jack’s Dead Bug Brew can be used. Spinosad is an organic, natural bacterium that targets the nervous system and the caterpillars will stop feeding and die within 2 –3 days. A second application may be needed if trees are sprayed as the leaves are unfolding.
A word of caution, however. Most pesticides are toxic to honey bees. Do not spray when trees and other plants are in bloom and when bees are active. Once a spray has dried, the threat to bees drops significantly.
Call Pine Tree if you have larger trees at risk that are difficult to treat or have a large number of trees that need attention. We work closely with a certified arborist and can arrange treatment.
For additional information please contact us at 508 432 8878.



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