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Spongy moth a growing forest health threat, keeping managers on their toes

WISCONSIN—The spongy moth is here to stay, and it’s posing a growing threat to deciduous forests across the country. This invasive insect is keeping forest health managers in the Forest Service, states, non-governmental organizations, universities, and Animal and Plant Health Inspection Service busy while they try to keep up with, and respond to, its ever-shifting populations across much of the eastern United States.

Formerly known as the gypsy moth, the spongy moth was accidentally introduced to Massachusetts forests in 1869. “Since then, its population has expanded to more than 20 Eastern and Midwest states and has the potential to spread across the entire continental United States,” said Tom Coleman, the national Slow the Spread program manager working out of the Southern Research Station.

Uninfested forests in the southeastern U.S. are also at risk to the invasive insect. These include oak dominated forests in Arkansas, Tennessee, Georgia and Missouri.

Two caterpillars on a green leaf
In the caterpillar phase spongy moths feed on green leaves, with one caterpillar eating a square yard of foliage during its brief lifetime. USDA Forest Service photo by Tom Coleman.

Spongy moth caterpillars can survive on 300 species of trees and shrubs. Oaks, aspen and birches are some of the more affected trees, along with urban trees like crabapples and lindens. 

During their caterpillar phase, spongy moths harm trees by eating their leaves. Usually, a couple rounds of complete defoliation will kill a tree. The insect species only produces a single generation each year. Forest stands located in already-stressed oak dominated areas are at high risk of mortality.

“When we have outbreaks across the Northeast, it pushes populations elsewhere. We’re having huge outbreaks in Pennsylvania, Wisconsin, Michigan and New York. The population is moving into the STS transition area and even the West Coast. Any infestations beyond the STS program are managed by state and federal partners,” said Coleman.

Eastern Region Pesticide Use Coordinator John Kyhl said most spongy moth damage in the last couple years has happened in Pennsylvania and Michigan, with 2.2 million acres of defoliation.

There are four primary programs that state and federal forest health managers use to control the insect’s population size and spread:

  • Slow the Spread program
  • Suppression operations used to reduce the population size
  • Detection/eradication
  • Regulatory

The Forest Service provides funding, program management and coordination with states and other agencies for these spongy moth programs. The agency also manages the treatment contract for mating disruption applications, totaling about 300,000 acres per year.

“We support areas with established spongy moth outbreaks as well as other areas with growing problems,” said Kyhl. “This was an increasing year in a multi-year cycle.”

Forest health managers detect the insect primarily using traps. There are about 60,000 -65,000 spongy moth traps put out every year. “It’s a very data rich sort of work,” said Kyhl.

Map of the United States with red shaded areas
Locations of spongy moth infestations and the four primary management programs used by forest health managers to combat its damage and spread.

During the past year, forest health managers in the Eastern and Southern regions have used this combination of the four major program areas, and this tactic is expected to continue in 2023.

There are currently 11 states participating in the Slow the Spread program. About 20-25 states have spongy moth infestations and quarantines in place to prevent movement of infested products.

“We’re still focusing on the management side as we’re still seeing outbreaks in the Northeast. States have submitted suppression requests, and we continue to see higher spread rates in the STS program,” Coleman added.

The Forest Service primarily uses a bacterium application called Bacillus thuringiensis var. kurstaki, also called Btk, which is certified for organic applications, in spraying applications to reduce the insect’s populations. Another option in areas where Btk can’t be used is a spongy moth virus application, but this technique can be expensive and time consuming.

Weather patterns can affect the invasive insect’s population. Cool, wet spring weather promotes a fungus that kills the spongy moth and reduces its populations. “The fungus is probably the best population control we have,” said Coleman. The spongy moth virus also effectively kills high population densities.

“The budget for the STS program is about $7 million this year,” Coleman said. The state suppression programs take care of the states already infested. Last year the funding for that was about $2.5 million and funding varies from year to year. Participating state agencies also contribute to pay for the program.

“Without intervention, spongy moth populations can spread significantly,” said Coleman.

This story is part of a series and highlights one of the 14 common themes identified in the 2020 Eastern Region State Forest Action Plan summary report. The theme for January 2023 is Collaboration and Partnerships.


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