Winter Moth Alert by Gail Anderson

Experts on all fronts are expressing concern that the warm winter and early spring will bring a devastating outbreak of winter moth larvae. This is the caterpillar of the small white moth that was so prevalent late last fall. The moth’s eggs overwinter and hatch into small green “inch worms” just as leaf and flower buds are unfolding. They can quickly defoliate many deciduous trees and shrubs, especially roses. If a woody plant is defoliated, it will be forced to expend energy producing a second set of leaves after the larvae pupates. Over several years, this can compromise or even kill a tree. We can only hope that the larvae will hatch early in the warm weather and then starve before leaves and flowers emerge.

The key is to treat the larvae early—as soon as they begin to munch. They may be very small at first, so inspect your woody plant material carefully. Home gardeners can treat smaller shrubs and trees. Large trees and shrubs should be treated professionally. Be sure to engage a reliable arborist early. Last year there was a waiting list.

Winter moth caterpillars are pale green with a faint white longitudinal stripe running down both sides of the body. At maturity, they will be approximately one inch long. In late May and early June, they stop eating leaves and flowers and drop to the soil for pupation.

Deb Lambert at Corliss suggests home gardeners treat the emerging winter moth larvae with a naturally occurring bacteria Bacillus thuringiensis subs. kurstaki (Bt). It goes under the trade name of Thuricide made by Bonide. This is a concentrate that is diluted at time of use and sprayed on.

For larger larvae, Deb suggests a biological pest control called Spinosad. Spinosad is a bacteria that was—of all things—discovered in fermented soil near an old rum distillery in the Caribbean. The home gardener’s version goes by the quirky name of “Captain Jack’s Deadbug Brew,” also manufactured by Bonide. This is applied as a spray and may need to be reapplied as more larvae develop.

If you wish to be as organic as possible, Deb suggests Bt for early-stage larvae and Neem oil for later stages. Neem oil is a vegetable oil pressed from the fruits and seeds of neem (Azadirachta indica), an evergreen tree found on the Indian subcontinent and in other tropical areas. It is a natural antifungal, antibacterial, and antiprotozoan agent.

While the three products above are fairly environmentally friendly, they can be toxic to beneficial insects, especially bees. Deb suggests applying the products late in the day after many insects stop being active. The so-called “organic” products may also take longer to work, up to five days for Bt, for example. Early application is key.

It you hire a tree company, determine your comfort level with the spray product(s) they use. Some companies use Conserve, a commercial version of Spinosad. Others companies use pyrethroids, because of their effectiveness. However, phyrethroids are neurotoxins and not as environmentally friendly. Natural pyrethrum—which, according to Mayer Tree, is difficult and/or expensive to obtain, is a compound with insecticidal properties made from extracts of plants in the chrysanthemum family. Bifenthrin, sometimes used by tree companies, is a synthetic phyrethrum, but it can be highly toxic to fish.

For more information:
http://extension.umass.edu/landscape/fact-sheets/winter-moth-overview