Surveys: once-feared browntail moth population continues to collapse

Recent ground surveys in southern Maine indicate the state’s once-feared browntail moth population continues to collapse.

The report is good news for both local residents and tourists who frequent Maine’s Casco Bay area during the summer. It also confirms earlier observations that the browntail population appeared to be in decline.

The browntail is not about to become extinct, said Charlene Donahue, a Maine Forest Service entomologist. “The population is down by orders of magnitude, though. Where overwintering webs were once measured in the thousands, they are now measured in the dozens.”

People who come in contact with the browntail can suffer poison ivy-like rashes and occasional respiratory problems. Browntails can still be found in high numbers in parts of Brunswick and abutting sections of Harpswell.  People in these areas should expect to still be impacted by browntail hairs, said Donahue.

Coastal parts of Freeport and the midsection of Harpswell have moderate levels of webs where people should take care in the spring. Yarmouth and Cumberland have low populations overall but there are patches of trees where multiple webs can still be found. Further afield, in Sagadahoc, Lincoln and Kennebec counties, there are few to no webs.  Webs are also smaller this year indicating fewer caterpillars are inside.

The browntail moth began to decline in some areas in 2004. Donahue said she first saw further indications of the state’s browntail’s population collapse last summer. It was noticeably absent in many areas where it was usually abundant. Prior to that, the browntail population was a stable if not growing problem in the Casco Bay area and had expanded inland.

State forest health professionals surveyed southern Maine forests in January and February, searching for and counting browntail moth webs in the trees. They found few browntail moth winter nests during the recent surveys where they used to find thousands, Donahue said. Many of the webs they saw held diseases and parasites. 

Recent cold, damp springs served to decimate the moth’s population by weakening its health and reducing its resistance to disease and parasites. A fungus is one of the primary causes for the decline of the browntails and wet weather helps spread the disease around. Several kinds of fly and wasp parasites prey on the browntail as well. None of these organisms affect people.

The invasive moth is feared for the toxicity of its caterpillars. The caterpillars become active during May and June each year. It is most toxic during the caterpillar phase, when it grows poisonous barbs all over its body. The caterpillars shed their skins five times as they grow.  These old skins remain toxic even after they are shed and the hairs from blow around on windy days.

Populations for both native and exotic insects can fluctuate widely from year to year, said Donahue. It is not uncommon for an insect population to grow into the millions during an outbreak, and then decline precipitously in following years when conditions are no longer as favorable. 

The browntail is an invasive insect originally from Europe.  It came to North America on a shipment from England in the late 1800’s.  It is found today only in the Casco Bay area of Maine and Cape Cod, Massachusetts.

Previous to its recent collapse the state’s browntail population counted in the millions and populated much of southern and midcoastal Maine.  When its population is abundant it is capable of defoliating large areas of forests.

Forests provide a variety of social, economic, and environmental benefits. Trees take in carbon dioxide from the air and release oxygen. Forests provide habitat for rare and endangered plants and animals. They also serve as places of recreation and sources for clean water, renewable energy, and wood products, such as timber, pulp for paper, and biomass energy fuel.
 
There are thousands of invasive species of insects and plants in America today that disrupt native ecosystems.  The USDA Forest Service Northeastern Area works with partner agencies such as the Maine Forest Service and stakeholders to mitigate the harmful impacts of invasive species.