What to do about the Winter Moth

Winter moth caterpillar - Image (c) Burlington Science Center - www.bsciencecenter.wordpress.com
Winter moth caterpillar – Image (c) Burlington Science Center – www.bsciencecenter.wordpress.com

Submitted by Melinda Chamberlain Dietrich, of the Bedford Arbor Resources Committee

An earlier article discussed the appalling extent of the danger posed by the winter moth, with its voracious appetite for our native northeastern trees and shrubs—especially red maples and oaks—and its ability to “balloon,” or float, from treetop to treetop,  as well as their appearance and behavior. This addresses what to do about them in greater detail.

When to treat

Timing the treatment in spring when the larvae first appear is critical, because this is when they are exposed and vulnerable.  Treatment while the insects are small also ensures protection for the growth of new buds. Note that in heavy infestations, larvae may hatch over a period of time.

Dormant oil sprays typically applied in the fall are useless for the winter moth, because they work by completely covering eggs to suffocate them. The female winter moth has protected her eggs by laying them in crevices and under lichen.

Do it yourself or hire a professional?

It depends. Do-it-yourselfers can be successful treating low-growing shrubs and perennials where sprays can completely cover buds, both sides of the leaves and stems. With mature trees and heavy infestations, a commercial spray company is recommended to reach and saturate the tops of branches.  Contact local companies now to review options and contract terms for treatment in the spring. Prices are usually based on the number and size of trees as well as the overall property area.

Doing it yourself

Two important cautions for homeowners who prefer to handle the problem themselves are

  • Follow directions on the product label;
  • Be sure to apply the correct chemical to the correct plant.

The chemical products:

Off-the-shelf insecticide products are available at local retail stores. Directions on the product label must be followed! Robert Childs, UMass Center for Agriculture, warns it is extremely important to know which chemicals can and cannot be mixed together to avoid serious injury to plants, to the environment—and to the person doing the application. Products listed by UMass Center for Agriculture for winter moth treatment are Bacillus thuringiensis (kurstaki); spinosad; and pyrethroid.

Bacillus thuringiensis, or “B.t.k.,” is a bacterium specific to Lepidopteran larvae (butterfly and moth) that forms toxins lethal to the caterpillar through a biological process, working best on the younger instar stage. Used as directed, it does not pollute the environment and is benign to the applicator and beneficial organisms, like predators and parasitoids. Limitations include that if applied while the buds are still expanding, foliage that emerges after application will not be protected. Also, it is useful for treating relatively small areas but not entire forests.

Spinosad products for the homeowner include two common ones, Monterey Garden Insect Spray™ and Bull’s-Eye Bioinsecticide™, derived from a bacterium subjected to a fermentation process which creates the active ingredient. It works on the insect’s nervous system at any age and is effective both as contact spray and by ingestion.

The concern is its high toxicity to bees from application until the spray has dried, when toxicity is much reduced.

The pyrethroids are the most conservative chemical option, best when applied at egg hatch. Since timing a hatch is difficult, pyrethroids are mostly used on free-feeding caterpillars after the buds have opened. The “knock-down” effect occurs within a number of days, which varies depending on the product used and the conditions after application, such as, weather, temperature, or added stickers.

The sticky products:

Sticky products wrap around tree trunks to prevent caterpillars from climbing up trees. They consist of a fiber-batting band that wraps around the tree trunk and is covered with a doublewide band whose one sticky side is placed inward around the batting. Half of the double-wide band hangs below the batting, held away from touching the tree. Its limitation is that caterpillars can “balloon,” floating from treetop to treetop, by-passing the sticky-bands altogether.

 Avoid targeting the wrong, possibly beneficial, moths:

Be sure to distinguish Operophtera brumata from other beneficials. The winter moth looks similar to the fall cankerworm, Alsophila pometaria, and the related bruce spanworm moth, Operophtera bruceata, at all life stages. Spring cankerworm, Paleacrita vernata, larvae also appear at the same time in spring as fall cankerworm and winter moth larvae.

For information on how to identify and manage caterpillar pests prevalent in Massachusetts, go to UMass Extension at https://www.umassgreeninfo.org/fact_sheets/defoliators/wm_overview_07.pdf

                   Better solutions for the future?

Progress is currently being made at UMass Amherst Extension to develop a biological treatment to replace chemicals. The USDA Forest Service announced in 2007 that Joe Elkinton, researcher at Umass, has successfully reared and released small flies, Cyzenis  albicans, at two locations in eastern Massachusetts, “… in hopes of ending a persistent winter moth outbreak,” with support from the Forest Service, APHIS, and the Commonwealth,  hoping for the same success experienced in Nova Scotia in the 1950s and the Pacific Northwest in the 1970s. C. albicans, which looks like a small housefly, is very successful at finding areas where winter moth larvae are numerous.  They lay their eggs on leaves where winter moth larvae feed, then kill the host after it pupates. Not only has C. albicans reduced winter moth populations, but it is very host specific and will not attack even closely related inchworm species. According to an article in the Boston Globe, “The parasitic fly, a natural enemy of the winter moth, was expected to destroy more than 200,000 of the winter moths…”


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