Browntail Moth Update #9: June 16, 2023
At many of our monitoring sites, we observed some caterpillars beginning to pupate (the process of becoming an adult moth), however, most of them were still active and feeding. The silky cocoons surrounding the pupae contain the last cast skin of the caterpillar and are full of toxic hairs. People in areas with browntail moth populations should take precautions to minimize exposure to the caterpillars’ hairs.
Browntail moth caterpillars on and inside the beginnings of a pupal cocoon, Turner, ME.
Many people become exposed to the hairs through encounters with the pupal cocoons as they can pupate on host tree foliage, buildings, vehicles, trailers, equipment, or other ideal locations. To prevent spreading browntail to new areas, check these areas for caterpillars and cocoons that may be trying to hitch a ride. Use protective measures to avoid skin contact with the toxic hairs when carefully removing caterpillars or cocoons. Eliminate caterpillars and cocoons by moving them to a plastic bag or container, freeze them or soak them in a bucket of soapy water before disposing of them in the trash.
Multiple browntail moth caterpillars constructing a pupal packet, Dresden, ME.
Pupation means we will soon see browntail moth flights. Nearly every year, we receive the first reports of adult moths sometime during the last week of June or first week of July. Usually, the peak of browntail moth adult activity is somewhere in mid-July and the last flights are toward the end of July and early August. Although some moths stick close to where they developed, others can travel long distances. To reduce the attractiveness of your landscape to these travelers, limit outdoor lighting in July. If you can’t eliminate lighting, consider switching to yellow-spectrum lights. Preliminary research by the Mech Lab at the University of Maine indicates that these yellow-colored lights are less attractive to browntail moth. Preliminary results from the lab also indicate that higher density browntail caterpillar populations develop near lights. Limit the lights to have a better spring next year.
Browntail moth caterpillars constructing pupal cocoons in a crabapple tree, Skowhegan, ME.
We still have not observed the fungal mortality on the mainland that we saw on Eagle Island State Historic Site. We are still hopeful though and like all good things it just takes a little time. In previous years we saw much of the pathogen associated mortality occur right before or during pupation so there is still time. It is possible that the fungus requires both cool wet conditions for infection but also dry periods in between these weather events so that spores may be carried on the wind.
The Maine Forest Service would like your help in locating diseased caterpillars. Infected caterpillars will have a puffy, swollen appearance and are covered in whitish/yellowish dust which is spores of the fungus. As caterpillars succumb to the fungus, they will hold tight to the branches on which they die, which helps spread the browntail caterpillar-killing spores. Reports of ONLY DISEASED browntail caterpillars can be sent to foresthealth@maine.gov.
Will you have Browntail Moths flying in your area?
Maine Forest Service conducts surveys for browntail moth from small planes and from moving trucks. These are broad-scale surveys that do not completely cover the impacted area. You can get a broad idea of where browntail moth is in Maine from our interactive map, updated with 2023 winter web surveys. To understand what browntail moth is up to near you, take a look at host plants for signs of caterpillar activity, and in this month of June, pupal cocoon formation.
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