Growing Wisdom

How to prevent winter moth caterpillars from damaging trees and shrubs

This is a good weekend to spray your Japanese maples or other ornamental trees for winter moth caterpillars. These are the larvae which hatched over the past few weeks after those moths laid the eggs in the fall.

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Winter moth is known as Operophtera brumata. Winter Moth will eat the young leaves of trees as the leaves are emerging from the bud in spring. Many deciduous plants are hosts for the winter moth including oaks, maples, basswood, white elm, crabapples,


apple, blueberry, and cherry. It is important to treat for the pest early as the damage is done while the buds are breaking. By mid-May to early June the pests are pupating or becoming dormant until they emerge in late fall as moths. The larvae are loopers or inch worms and are green. They are very small when they hatch and difficult to see with the naked eye. The larvae hatch in spring when temperatures are around 55F. Young larvae tunnel into buds, especially the flower buds of fruits, and feed inside buds. Once the bud has been devoured the larvae will move to another bud to feed. In areas with large infestations winter moth larvae can completely defoliate host plants. If your tree gets defoliated and is weak it can kill a tree after years of damage.

There are several ways to combat winter moth. You can spray the bark of trees to suffocate the eggs. Use horticultural oil in early spring when temperatures are around 45F. Bacillus thuringiensis can be used on young caterpillars. Bt, is a bacterium which must be ingested to work. The insect will stop feeding and die of starvation.

Spinosad is another organic control that is derived from a naturally occurring soil bacterium. This product is organic and works by harming the nervous system of the insect. Homeowners can find any of these products at a nursery or garden center. Call a certified arborist to treat larger trees.



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