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Nature Notes: Winter Moth

Ed Robinson
October 28, 2017

November finds us enjoying the last of the foliage season, but it can be a busy time getting our homes and lawns cleaned up for winter. It is also a time for action if you have valued deciduous trees. Once the weather turns colder, we will begin to see large numbers of the invasive winter moths (Operophtera brumata). Harpswell is a hot spot for this pest, and the damage caused in our town, and others nearby is worrisome.

These moths are from Europe, but crossed the Atlantic as early as the 1930s when they were confirmed in Nova Scotia. For many years, they were a local issue, spreading slowly in the early 2000s to Massachusetts, Connecticut and Rhode Island. But more recently, the population has exploded in many coastal towns of Maine from Kittery to Bar Harbor. The moth has also been confirmed in places as far flung as British Columbia, Oregon and New Hampshire.

Recent mild winters have facilitated an upswing in winter moth populations, as have indiscriminate movement of plants with attached root balls, where the pupae reside until late November through January. As temperatures drop, the moths emerge and breed, with the females laying eggs on the bark and lower limbs of favored tree species. The primary targets seem to be oak, maple, basswood, ash and white elm trees, but crabapples, apple and cherry trees are also at risk. Even blueberry plants have been targeted in some areas.

The light green and white striped caterpillars, around ½ inch long, emerge from the eggs in the spring and attack tree buds just as new leaves begin to develop. This can be quite damaging to a tree if the resulting defoliation is extensive, since the tree has little foliage left for the vital process of photosynthesis. After eating their fill, the caterpillars can use silk to float on air currents to new locations or drop to the ground where they spend the summer and autumn underground wrapped in their silk cocoons. If people then move shrubs and plants to other locations, the root balls may contain the cocoons, thus aiding the spread of the winter moth to new areas.

If there is any good news here, it is that the brownish gray colored females are nearly flightless, with short stubby wings, so they are limited by the distance they can walk or climb before breeding. The males can fly, and when evening temperatures are above freezing this winter, you will likely see them on your windows and doors when you look outdoors from lighted rooms. Their wings range in color from grayish yellow to a light brown, with darker bands. The females emit pheromones, which draw the males to them in the trees. The eggs begin life with a green color, which make them hard to spot on trees that have mottled bark and/or lichens. As the eggs begin to mature, they turn light orange, making them more visible for treatment or removal.

The eggs are susceptible to treatment with horticultural oil sprays applied by licensed professionals. These sprays suffocate the egg by denying oxygen to the developing caterpillars. Insecticides may also be appropriate to kill newly hatched caterpillars before they can attack the tree’s foliage.

Another approach for partial control is to band your trees so that you can catch the female moths before they can climb up your tree for breeding and egg laying. I have had good success in recent years using a product called Tanglefoot, available from local nurseries or online from companies like Amazon. The product includes a heavy duty paper tape that is wrapped tightly around each tree trunk, combined with a sticky material spread with a putty knife (wear gloves or you will need mineral spirits to clean your hands). I’ve learned to stretch the paper tape before tacking or stapling it to the trees since it may sag when it gets wet, especially on bigger trees. The key is to make sure the moths cannot climb under your banding, or over excess numbers of captured moths if you have a heavy local population.

It is obvious that an exploding winter moth population cannot be contained long term through spraying or banding. Because the winter moth is an invasive pest, there seem to be no natural enemies here that will feed upon the moths or caterpillars. Fortunately there has been some progress made in using a biological control method, a parasitic fly (Cyzenis albicans). In the 1950s, the flies were released in British Columbia and Nova Scotia with slow but gradual results. Massachusetts began releasing the flies in 2005 and after a few years began to see an impact on winter moth numbers. Thanks to federal funding, Maine began releasing thousands of the flies in Harpswell and Cape Elizabeth in 2013, followed by Kittery Point, Vinalhaven and Peaks Island in subsequent years.

Let’s hope the flies are successful in Maine, since aerial surveys and data collected from residents has shown extensive defoliation with resulting tree losses in hard hit communities, with Cape Elizabeth already losing hundreds of old oak trees. When combined with the heavy recent Browntail moth activity, our local trees are at risk. Trees that suffer extensive defoliation for three or more years are quite likely to suffer severe branch die backs or death.

Now is the time to consider banding your trees. Another option is to contract with a qualified company for spraying your trees in the spring before the eggs hatch. It is also very important that you refrain from moving plants and landscaping materials from infected areas during the months of June through December to ensure that you are not spreading the cocoons to new areas.

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