Regional Spongy Moth Outbreak After Drought

Main Takeaways

Managers in the Northeast react to a longtime threat re-emerging under novel conditions due to climate and share adaptive management methods and reflections on the successes, challenges, and lessons learned for future management applications.

Climate Impact

Drier spring conditions due to climate change have altered the effectiveness of the fungus Entomophaga maimaiga as a biological control for Spongy Moth (Lymantria dispar) resulting in increasing spongy moth populations and tree mortality. 

Adaption Planning / Actions

Management used a mix of methods including monitoring, forest management, and public outreach to manage the habitat and wildlife impacts of Spongy Moth and less so on the insect pest itself.

Background

The Spongy Moth (Lymantria dispar) is an invasive forest pest first introduced to Massachusetts in the late 1800s. Since then, it has spread across the eastern U.S., cycling between quiet periods and major outbreaks that defoliate millions of acres of forest. Oaks are its preferred host, but caterpillars also feed on other hardwoods and even conifers when populations are high.

Over the years, managers have used chemical controls, biocontrols like Bacillus thuringiensis kurstaki (BtK), and viruses. But the most successful has been the fungus Entomophaga maimaiga, which can regulate moth populations even at low densities—though only under moist spring conditions.

Climate Impacts

In the mid-2010s, a period of spring drought reduced the effectiveness of E. maimaiga, allowing moth populations to surge. By 2017, 2.25 million acres were defoliated across the Northeast. Many trees survived one year of defoliation, but repeated stress from drought, pests, and defoliation led to widespread oak mortality. This outbreak was the largest since the 1980s.

Tree mortality has cascading effects: loss of acorn production for wildlife, altered forest regeneration, reduced water and soil quality, economic losses for landowners, and even diminished cultural and wellness benefits.

Flowchart depicting An events chain showing several contributing causes, including climate-related ones, and some wildlife and habitat impacts of widespread tree mortality caused by recent Spongy Moth outbreaks in the Northeast U.S
Figure 3. An events chain showing several contributing causes, including climate-related ones, and some wildlife and habitat impacts of widespread tree mortality caused by recent Spongy Moth outbreaks in the Northeast U.S. Credit: Figure designed by Liz Sisk, U.S. Geological Survey, National Climate Adaptation Science Center

 

A timeline showing the progression of a key climate condition (springtime drought), biological response (defoliation by Spongy Moth), and ecosystem impact (tree mortality) in the Spongy Moth outbreak in Connecticut. Drought data are sourced from the National integrated Drought Information System, and Spongy Moth defoliation and mortality data are from the Connecticut Department of Energy and Environmental Protection.
Figure 4. A timeline showing the progression of a key climate condition (springtime drought), biological response (defoliation by Spongy Moth), and ecosystem impact (tree mortality) in the Spongy Moth outbreak in Connecticut. Drought data are sourced from the National integrated Drought Information System, and Spongy Moth defoliation and mortality data are from the Connecticut Department of Energy and Environmental Protection. Credit: Figure designed by Liz Sisk, U.S. Geological Survey, National Climate Adaptation Science Center

 

Management Actions

State agencies responded with a mix of monitoring, forest management, and public outreach.

  • Monitoring: Egg mass surveys, aerial defoliation flights, and long-term plots helped track outbreaks, though scaling data to management decisions remains a challenge.
  • Forest Management: Salvage logging, thinning, and strategies guided by climate adaptation frameworks (like NIACS’s Resistance–Resilience–Transition model) helped improve forest health.
  • Outreach: States engaged landowners and the public through bulletins, news media, and extension programs. Collaboration across states also improved consistency and coordination.

Lessons Learned

The outbreak underscored how climate extremes, like drought, can weaken natural pest controls and drive forest disturbances. Managers faced challenges including limited resources, public opposition to chemical treatments, and shifting policies on forest use. Still, adaptive forestry practices that improve diversity and resilience are proving effective not just for Spongy Moth, but for broader climate adaptation.

Looking ahead, climate projections suggest drier springs may increase Spongy Moth risk, though changing tree composition could also reduce susceptibility. Proactive, climate-smart management—built on monitoring, adaptive forestry, and collaboration—will be key to managing future outbreaks.

Reference

Adapted from

Staudinger, M.D., A.V. Karmalkar, K. Terwilliger, K. Burgio, A. Lubeck, H. Higgins, T. Rice, T.L. Morelli, A. D'Amato. 2024. A regional synthesis of climate data to inform the 2025 State Wildlife Action Plans in the Northeast U.S. DOI Northeast Climate Adaptation Science Center Cooperator Report. 406 p. https://doi.org/10.21429/t352-9q86

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