Coping with Winter Moths

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For the past few weeks, a quick look out your windows or a drive down the street in the early evening might have left you with the first impression that it was snowing.  But the fluttering wings passing your windows are almost certainly winter moths (Operophtera brumata), one of the newest invaders to decimate our trees and gardens.

An unwelcome immigrant from Europe, male and female winter moths emerge over a period of several weeks beginning in late November and continuing through December. During this time period, the moths mate and the females lay eggs on the trunks and branches of their host plants. The eggs are visible as tiny ovals.  Initially, they’re green and loosely scattered along the bark, in bark furrows and on larger branches. After a short time period, the eggs turn a pinkish-orange color and thus become more visible. Within days prior to hatching in the early spring, winter moth eggs will turn very dark in color.

Because many winter moth eggs are exposed on the bark, horticultural oil spray can potentially be used to control the moths on small trees and shrubs. Typically, dormant oil sprays are applied in the very late winter or very early spring depending on temperatures. When applying oil sprays, it is necessary to have temperatures above 45° F and to avoid applying oil when temperatures may dip below freezing for 24-48 hours after application. Below this threshold, there’s a considerable potential for injuring the plant.

Oils work by suffocation and can be effective on the eggs, but the oil must completely cover the eggs at the time of application. Eggs that are protectively hidden within crevices and under lichens will not be covered by the spray and so will live to hatch. Given the phenomenal numbers of winter moth eggs in Massachusetts this year, oil sprays will most likely only achieve limited results; eggs are virtually everywhere on trees and shrubs and new caterpillars will quickly migrate from untreated areas to the oil-treated plants.

In general, winter moth caterpillars commonly feed on maples, oaks, apple, crabapples, ash, fringetrees and blueberry bushes.  This pest has been known to drop from trees and feed on perennials such as roses and hosta. It is not been commonly seen feeding on magnolia and kousa dogwood.

The best estimates are that, in Massachusetts, winter moth eggs hatch between 20-50 Growing Degree Days, a period that can occur anytime from late March (during atypically warm springs) into the second or third week in April (cool springs). The tiny (less than 1 mm) caterpillars then spin a small silk strand and become air-buoyant and are carried upwards on air currents into the tree canopy where they then try to “weasel” between the bud scales, bracts, etc. to get into the buds.

They do not chew their way into buds via an entrance hole. If buds are not yet swollen enough for them to gain access, these small larvae will then spin down from the tree on a silken thread and be carried away by the wind, which is a dispersal process known as ballooning.

This is the stage where the highest levels of injury to the plant can occur. The longer that the buds stay swollen but unopened, the greater potential for feeding injury. Winter moths will enter both leaf and flower buds.

Once the buds open, the larvae are known as “free-feeders” given that they are now on the foliage and free to move readily from one area to another. Winter moth will be in this stage until late May or early June whereupon they drop to the soil and almost immediately spin a cocoon and pupate. While still on the host plant, however, they are exposed and very treatable with a variety of products.  Here are two:

1. Bacillus thuringiensis (kurstaki), also commonly known as B.t.k. This product is a bacterium that is specific to lepidopteran larvae (butterfly and moth). It must be ingested to be effective. Through a somewhat complicated biological process, this product will eventually form toxins that become lethal to the caterpillar. A few notes about its use:

  •   B.t.k. works best on the younger instar stages of caterpillars;   older ones are much less affected.
  •   This product fits well into any IPM program in that it doesn’t   pollute the environment, harm the applicator nor does it affect beneficial   organisms such as predators and parasitoids, when used as directed.
  •   If applied while the buds are still expanding, any new foliage   that emerges days after application of this product will not be protected.
  •   Caterpillars that ingest B.t.k. will stop feeding almost   immediately.  However, they may not die   for one to three days. It is often disconcerting to still see live   caterpillars days after treatment but even though moving, they are not   feeding.
  •   Although some may be concerned about B.t.k’s effect on native   lepidopteran species, it usually poses a limited negative effect given the   relatively small areas being treated (e.g. individual trees and not entire   forests).

2. Spinosad Products: The first version of this product in the U.S. was only available to commercial licensed pesticide applicators and was trademarked as Conserve SC™ (Dow AgroScience).  Since 2004, homeowner spinosad products have become available and two common ones are known as Monterey Garden Insect Spray™ and Bull’s-Eye Bioinsecticide™. These products are derived from a bacterium that is subjected to a specific fermentation process to derive the active ingredient(s) for the insecticide commonly known as spinosad. It works on the insect nervous system in a novel manner and can be effective as a contact spray as well as by ingestion.  A few notes on its use:

  •   Spinosad products work well on caterpillars of all ages.
  •   Although fairly safe by not harming parasites and predators, the   label does warn that it can be highly toxic to bees at the actual time of   application. Once the spray has dried, however, the toxicity to foraging bees   is much reduced.

2. Spinosad Products: The first version of this product in the U.S. was only available to commercial licensed pesticide applicators and was trademarked as Conserve SC™ (Dow AgroScience).  Since 2004, homeowner spinosad products have become available and two common ones are known as Monterey Garden Insect Spray™ and Bull’s-Eye Bioinsecticide™. These products are derived from a bacterium that is subjected to a specific fermentation process to derive the active ingredient(s) for the insecticide commonly known as spinosad. It works on the insect nervous system in a novel manner and can be effective as a contact spray as well as by ingestion.  A few notes on its use:

There are other products that are available only to commercial licensed pesticide sprayers.   If you believe you have a serious infestation affecting many trees on your property, you should consult an arborist or reputable tree service.

Other solutions have been tried but found generally ineffective.  Physical barriers such as sticky tape are among these.  The problem is that the number of winter moths are so great that, while the first moths are trapped, later moths will simply crawl over their bodies.  Also, sticky tape doesn’t address the caterpillars that have ballooned into the trees upper reaches.

One of the greatest problems is that of timing.  Because the caterpillars do not all hatch at the same time, and because they are protected inside leaf buds while they do damage, there is only a narrow window when treatments are effective

 Posted by at 12:39 am