Last November/December my phone was abuzz with reports from many clients in Southeastern Massachusetts, letting me know of these little “whitish” moths flying around porch lights during the evening hours. I noticed on several properties, as we raked leaves, moths swirling about at waist height and quickly retuning to the ground. This is a sure sign of the presence of winter moth. It appears we will be having a heavy infestation of these non native caterpillars this spring.

The male moth is the one we see flying about in the evening hours. He is seeking a female to mate with. The female is virtually wingless, so she walks up the stems of trees depositing her eggs wherever she may in a hap hazard manner in cracks and crevices, under lichen and along branches. These tiny eggs will begin to hatch at around 50 growing degree days, usually in early April. It appears that hatching will occur as buds swell on the host plant. They then hang on a thin silk strand and get transported on a breeze upward into the canopy of the tree, where they wriggle their way into the leaf or flower buds and begin eating. To me the caterpillar at this point looks a lot like a green eye lash. The longer it takes for the bud to open the greater the damage they can do before a spray can be applied to stop their feeding. If the flowers are eaten then there will be no fruit. These non native caterpillars have no natural enemies so the damage they inflict can be tremendous. They feed on a variety of deciduous plants like blueberry, cherry, apple and crab-apple, maple, oak, etc.

To control these caterpillars can be very challenging prior to their entering the free feeding stage, which is when the leaves unfold and begin growing. Horticultural oils can be applied prior to bud break in an attempt to kill off the eggs by smothering them. This is a most environmentally sound treatment and we have had very good results on ornamental trees that were not flowering due to winter moth infestations in the past. The oil spray reduced the infestations and flowering occurred, much to the clients delight. It is much more difficult to have similar results on very large trees because it becomes harder to gain complete coverage.

Once the winter moth is free feeding Bacillus thuringiensis (kurstaki), B.t.k. can be used with great effectiveness and safety on the younger instars stages of growth. One must remember that the B.t. must be ingested to work and only the leaf material that was covered will have the B.t. on it, as the leaf continues to grow. Spinosad then becomes the next most effective and least toxic material for control of larger winter moth. Caution must be used around bees, as it is highly toxic to bees, till the material is dry. At full maturity the winter moth will be about 1 inch long.

If trees have been seriously defoliated, watering will become critical to the plants survival. Care should be taken to properly water weekly during the dry part of the season. I would recommend deep slow watering once per week. Do not use chemical based fertilizer as they are salts.