Winter moth in Southern New England 2015

We expect winter moth eggs to start hatching in Southern New England in early to mid April. The average date of egg hatch in RI is April 10. The hatching date depends on future Spring weather. Last November, I set up tree wraps at 5 locations: three in RI, one in Pawcatuck, CT and one in Acushnet, MA. The tree wraps encouraged female winter moths to lay eggs just below the tree wraps. Over the next week I’ll remove all the tree wraps and look for eggs to monitor. I removed the tree wraps at URI last week and found hundreds of eggs to monitor. Winter moth eggs start out orange, but then turn blue a few days before hatching. Very handy for monitoring egg hatch!

For landscape trees it’s not important to control winter moth just when hatching, but for apple and blueberry growers it’s very important. Once eggs hatch, winter moth caterpillars wriggle into swollen buds and begin feeding. For apple trees and blueberry bushes, swollen buds are primarily flower buds and once caterpillars are inside buds they are protected from insecticide sprays until just before bloom. By this time many flowers may have been damaged or destroyed, destroying the crop. Landscape trees, on the other hand, can withstand early winter moth feeding damage. To save trees from being defoliated, insecticides can be applied after trees leaf-out, but before excessive feeding damage has occurred.

Dormant oil can be sprayed before eggs hatch, but this may not be very effective if there are unsprayed trees near where you are applying dormant oil. Winter moth caterpillars are pretty easy to kill, provided they are not inside closed buds. Insecticide choices for newly hatched caterpillars for blueberry and apple growers include, but are not limited to, spinosad (Delegate), imidan, sevin, and synthetic pyrethroids such as Asana. Adding a dormant oil may be useful for the first spray. Once buds are open, B.t. kurstaki products (Bacillus thuringiensis kurstaki) such as DiPel and Biobit work well. For landscape trees, winter moth caterpillars can be controlled once trees leaf out with spinosad (Conserve), B.t. kurstaki (Dipel Pro, Javelin, and others), as well as synthetic pyrethroids such as bifenthrin.

Winter moth caterpillars continue to feed and grow until around the end of May. Once mature they drop to the ground, dig down a few inches, and pupate. Pupae will remain in the soil until November or December when winter moths emerge as adults. It’s male moths that more and more Rhode Islanders are seeing at their porch lights and headlights, especially on warm evenings between Thanksgiving and Christmas. Females are rarely seen because they don’t fly.

In collaboration with Joe Elkinton from UMass, we have released a parasitic fly that attacks only winter moth caterpillars. The fly, Cyzenis albicans, has successfully controlled winter moth outbreaks in Nova Scotia in the 1950’s and the Pacific Northwest in the 1970’s. Cyzenis albicans lay eggs on leaves of winter moth host plants. When eating leaves, winter moth caterpillars accidentally eat fly eggs too. A fly egg hatches and the larva develops inside a caterpillar body. When a parasitized caterpillar drops to pupate, it digs into the soil but instead of a winter moth caterpillar pupating, the fly pupates instead. The fly pupa remains in the soil until the following spring when it emerges as an adult fly at the same time winter moth eggs hatch.

Parasitic flies have been released in Massachusetts since 2006 and in Rhode Island since 2011. In Massachusetts, some of the early release sites are already seeing winter moth populations decline due to high rates of parasitism. In Rhode Island, we recovered flies for the first time in 2014 in Goddard Park. In a few years we hope to start seeing winter moth controlled by Cyzenis albicans.

Heather Faubert