Rash-Causing Moths Are Spreading in Maine Because of Climate Change

The outbreak of browntail moths in the state will likely grow worse as temperatures increase, researchers say

A browntail moth caterpillar on a plant
Browntail moth caterpillars have small hairs that can cause a poison ivy-like rash and difficulty breathing in humans. sandra standbridge

Warmer temperatures are likely causing an increase in invasive browntail moth populations in Maine, according to new research from the University of Maine and the Maine Department of Agriculture, Conservation and Forestry. 

The study, which was published last year in Environmental Ecology, compared predictive statistical models of how much foliage Maine’s moths had eaten annually in the past 23 years to climate data from the National Oceanic and Atmospheric Administration (NOAA). 

“Our study of variation in populations across infested areas in Maine and over this latest resurgence period suggests that climatic factors, particularly spring and summer temperatures, and spring precipitation are significant predictors of population outbreaks,” the study states. 

The caterpillars are a nuisance to residents in Maine because they have small hairs that break off and can result in a poison ivy-like skin rash and cause difficulty breathing. These hairs remain toxic for up to three years, per the Maine Department of Agriculture Conservation and Forestry, though they get washed into the soil and become less of a problem.

The caterpillars also eat leaves of hardwood trees and shrubs, which can eventually lead to the plants’ mortality.

Browntail moths were introduced accidentally in 1897 from Europe to Massachusetts, and quickly spread throughout the northeast. The population crashed in the 1920s, most likely because of natural causes, but since then, periodic outbreaks have occurred. In Maine, browntail moth numbers have been increasing since the 1960s, and the state has been in an outbreak phase since 2015. 

A white moth on the underside of a leaf
An adult female browntail moth Jan Samanek, Phytosanitary Administration, Bugwood.org

Last year, the infestation was the worst in the state’s history, reports Patrick Whittle for the Associated Press. A 2021 aerial survey showed almost 200,000 acres of damage from the insects.

This year, the Maine Forest Service predicts encounters with the moths and their hairs to be as bad or worse than 2021. The problem has gotten so bad that the Maine Forest Service named February "Browntail Moth Awareness Month." 

“Our current warming climate trends suggest that we will continue to be facing this menace at least in the near future if not longer,” Eleanor Groden, a professor of entomology at the University of Maine and the principal investigator on the study, says in a statement.

Warmer falls and winters allow the caterpillars to get fatter before they hibernate over the winter, Groden tells AP. 

“If they come out of those webs as hearty individuals, older individuals maturity wise, then they are better able to withstand that period and you get higher populations,” she tells the outlet. “And you get defoliation that spring, and populations are raising havoc for anyone who has them in their yards.”

Though climate warming favors the moths’ development, the ranges of other species could expand, leading to more competition. These variables make it difficult to predict long-term trends, write the authors. 

David Wagner, a professor of ecology and evolutionary biology at the University of Connecticut who was not involved in the study, tells the AP that climate change has already affected insect vectors like mosquitoes and ticks. 

“Climate change appears to be an important driver in this system,” he tells the publication. “So this outbreak can continue to increase, and it could come at great expense to land owners.”