Browntail Moth Field Research & Trials

By Mary Ann Nahf, Harpswell Conservation Commission

Photo courtesy of Maine Forest Service

When you are out walking, look up at the barren trees to try to spot what looks like dead leaves clinging to the very tips of tree branches. These are the nests of the browntail moth, and they are easy to spot against a clear blue sky. Large oak trees are especially attractive to the moth.

There is still time to cut the nests you can reach before the caterpillars emerge. Removing just 10 nests can prevent as many as 4,000 new caterpillars from hatching. Destroy the cut nests by dunking them in soapy water overnight and dispose of them double-bagged in the trash. Any work done now will reduce your exposure from the toxic hairs later in the season.

Photo courtesy of Maine Forest Service

Browntail moth caterpillars will begin feeding as the weather warms and the leaves begin to emerge on the trees. They will be active from April through June. During this time they will molt four times, and each time their discarded skin contains toxic hairs. As you probably know, these hairs can cause an uncomfortable rash and sometimes can cause respiratory problems.

To reduce your contact with the hairs, we recommend you wear protective clothing and work in damp, windless weather, if possible. Hairs remain after the caterpillars are gone, and they can be toxic for three years.

As browntail moth infestations have steadily worsened in Harpswell over the past few years, citizens have reached out to the Town of Harpswell for help in dealing with this pest. Pesticide treatment options are limited because many of the infested areas are in the shoreland zone and treating with common pesticides can be harmful to the marine environment.

Browntail moth caterpillar showing the distinctive red dots on its back

Unfortunately, some of the safer controls that can be used near the water are either prohibitively expensive for many residents (stem cell injection of pesticides) or practically impossible (clipping in mid-winter) because so many of the nests are too high to reach safely. Treatment of a large oak tree by bucket truck with stem injection of a pesticide or extensive clipping can cost $200-300 per tree. Also, using a pesticide by stem injection can take a toll on the health of a tree. Widespread spraying had been tried in the past and resulted in killing off marine life, which is not a good option for our community.

To get a better understanding of the infestation and explore some options for Harpswell, the Selectmen met with Entomologist Elinor Groden, PhD from the University of Maine. The University submitted a research proposal to investigate safe control options.

Last month at the Harpswell Town Meeting, residents approved funding for the field research and trials. In late March, Dr. Groden began surveying nest densities throughout the town. Once locations are identified they will be monitored throughout spring and a field trial will take place in mid-May to evaluate the efficacy of at least three organically-certified biorational controls. On-site evaluations will continue over the summer months.

Dr. Groden is looking for sites to test in Harpswell. If you are plagued with browntail moth and would like to become a test site, you can find information on the Town website by clicking here. This site also gives much more information about the pest.

The project will conclude in February 2019 with monitoring for the presence and viability of winter webs near the treatment sites. A final report will be provided and a public presentation on managing browntail moth will be presented to the community.

April 2018