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Browntail Moth Info


Browntail Moth Primer

Of a million species of insects so far identified only about one percent are occasionally pests for humans, and only one thousand species considered seriously damaging. But when one species arrives uninvited and starts to impact summertime enjoyment, all that is irrelevant. The innocent sounding browntail moth is such an insect. Found last summer on quite a few properties in the watershed, the caterpillar form of the insect is causing problems and not just because they can defoliate trees in yards and forests alike. The caterpillars could be called armed and dangerous because they are covered with irritating and toxic hairs that break off and persist even after the caterpillars are gone. Depending on a person’s sensitivity to exposure to the hairs, and to the numbers of insects present, they can cause a rash similar to poison ivy and even respiratory problems due to inhalation.

This insect is native to Europe and other places but was accidentally introduced to Massachusetts in 1897. For a history of the Browntail Moth in New England go here. Recently it has once again expanded its territory away from coastal areas in southern Maine (where they have been found historically for years as well as on Cape Cod). Much of Knox, Lincoln and Waldo counties have infestations in places. Removing winter nests where they can be reached is the place to start because each web can hold up to 400 caterpillars!

The most important thing right now is to read about these pests and to be aware so that you can learn to recognize them and how to do an inspection of your property before spring. The best resource is from the Maine Department of Ag, Conservation and Forestry. If you read nothing else, read this: https://www.maine.gov/dacf/mfs/forest_health/documents/browntail_moth_brochure.pdf.

At this time of year, the caterpillars that hatched last fall are overwintering in nests found at the tips of tree branches. It seems unfair but many of the trees and shrubs we favor in our area are the very trees that they choose to eat, a feast seemingly specifically chosen for this pest (or vice versa): oak, apple, crabapple, shadbush, rose, including rugosa rose. But as soon as the tree starts to leaf out, the caterpillars emerge and start eating, returning to their nests at night. They are tiny in the early spring and it takes a magnifying glass to see they have two rows of white dots along their sides and two bright red dots on their back. Once they reach a certain size, these identifying are more easily seen as they go on the march, looking for new food sources that permit them to grow to about 1 1/2 inches in length. Their foliage eating habit that can weaken ornamental trees and shrubs, but also on taller forest trees, as distressing as it is, is not the most concerning problem.

Many people develop skin rashes similar to poison ivy to exposure to the hairs of the caterpillars. Depending on the individual’s sensitivity, the rash can be severe and some people can develop breathing issues due to inhaling the hairs. Besides breaking off, the hairs are shed by the caterpillars as they form their cocoons before coming adult moths and also are found later in the season, in July, as covers for the female moth’s egg masses and in the web nests of the caterpillars. And the hairs not only are found everywhere where the caterpillars are found – on the ground, in the lawn, on outdoor furniture, in the air on windy days – but can persist up to three years. Activities such as raking leaves, mowing the lawn, other gardening activities stir up the hairs and perpetuate the problem even though it is worst in the summer months.

What can be done? Right now, the number one top suggestion is to check your property for the winter nests.  Some pictures of infestations from around the watershed are here.  They are noticeable and their placement at the tips of tree branches is practically diagnostic. On ornamental trees or shorter forest trees, these nests can and should be cut down and immediately soaked in soapy water before disposal. Wearing gloves is mandatory. Here is a short video on to prune and dispose of the nests: https://vimeo.com/192349741

Physical removal of nests in taller trees is harder. Licensed arborists can climb trees or use bucket trucks to get at nests at the tops of tall trees.  Obviously this method is labor intensive, it may be feasible for a few trees but a major infestation on a number of properties may be problematic.  A list of some arborists in the area that are willing to remove nests is found here.  The time to do this is in March and April, before the leaves come out.

Chemical pesticide control of the growing caterpillars in the watershed areas are limited to few products that need to be applied by licensed and experienced arborists.  There are also no spray setbacks close to the shores of our lakes, ponds, and streams.  Unfortunately spraying can also affect non-target and possibly beneficial insects. Injecting trees with systemic insecticides can avoid issues of drifting spray but may also kill the important caterpillars and other insects that are necessary bird food for all the garden and forest birds such as chickadees, woodpeckers, warblers and for their offspring. Birds do not eat the browntail caterpillars because they too don’t like the hairs. The adult moths are strongly attracted to light which may eventually lead to a control method as may parasitic wasps or flies. Licensed pesticide applicators can be found here.

The nests are not large, and removing as many as possible before the caterpillars start emerging is the most effective control for home owners right now. Every nest taken down foils the potential for between 25 and 400 inch and half leaf-eating machines lining the branches of trees and blanketing the surface of any structure as they do before they form cocoons, both horrifying sights, believe me. Ten little nests, 4,000 possible caterpillars! You will be glad you did.

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