by Carolyn Bernhardt in collaboration with Sayesha Khanna
April 14, 2025
Since 2005, scientists have been studying an uninvited guest in Minnesota’s forests—the spongy moth (Lymantria dispar dispar). Feasting on hundreds of tree species, the cumulative effect of the hungry caterpillars weakens forests and makes trees more vulnerable to disease and other threats. But tackling this invasive pest isn’t as simple as spraying pesticides. Scientists are working to crack the code on when the eggs of these moths hatch and how to control them without harming Minnesota’s native butterflies and moths.
Currently, the Minnesota Department of Agriculture (MDA) uses aerial sprays of Bacillus thuringiensis kurstaki (Btk), a natural pesticide that targets only feeding caterpillars, to keep this invasive insect’s numbers in check. But, given how quickly Btk breaks down, timing these treatments correctly presents challenges. Existing models for tracking populations of spongy moth were designed for other states and may not always perform well in Minnesota’s environmental conditions, especially in the colder northern regions where caterpillar emergence can be delayed. In addition, there are knowledge gaps in whether Btk affects caterpillars of native moths and butterflies.
Emerging research funded by the Minnesota Invasive Terrestrial Pests and Plants Center and conducted in partnership with MDA, the Department of Natural Resources, and the US Department of Agriculture aims to address these issues. Leading the research are Sayesha Khanna, a second-year PhD student, and her advisor, Brian Aukema, PhD, a professor in the Department of Entomology in the College of Food, Agricultural and Natural Resource Sciences. Khanna is developing a localized egg hatch phenology model that accounts for Minnesota’s unique climate, making this work critical for improving local pest control strategies.
Trapping, analyzing, and strategizing
“One of the biggest challenges is timing aerial sprays of Btk to coincide with the most vulnerable stage of spongy moth caterpillars,” says Khanna. She and the team are studying spongy moths both in the lab and in the field to figure out exactly how temperature influences when they hatch.
“We designed and built our own automated black-light traps powered by car batteries,” says Khanna. She adds that this brought a creative element to the research, with Aukema lending his expertise in electrical systems to developing the prototype.
The team is also tracking native moth and butterfly populations to see how they respond to Btk treatments. “The phenology model I am developing will help the MDA optimize Btk treatments by forecasting spongy moth egg hatch timing based on Minnesota’s climate,” Khanna adds.
She says weather has been the biggest challenge in this research, though. “I rely on spongy moth egg masses for my studies. In 2023, I found plenty in the wild, but last year was different.” She suspects that heavy precipitation in 2024 likely triggered an outbreak of a fungus that kills spongy moth caterpillars, leaving fewer eggs to collect. “While this is great news for forest health in Minnesota, it presented a significant hurdle for my research.” Fortunately, she says landowners have been amenable to and supportive of her research efforts, making it much easier than she expected to gain access to private lands.
Where are the moths?
According to Khanna, preliminary findings from 2024 summer’s research have already revealed some surprising patterns and ongoing questions about spongy moth control.
The team focused on assessing the impact of Btk on caterpillar populations across 16 sites—half of which were treated with Btk and half they left untreated as controls. “We collected larvae before and after spraying and found no significant differences in the amount of butterfly and moth caterpillars between treated and control sites,” says Khanna. “We are continuing to study biodiversity in these areas in 2025. One possible explanation is that the larvae were transitioning into adults during our sampling period.” This, she says, raises new questions about where those caterpillars may have gone.
The team’s findings will not only help make treatments more precise, they’ll also provide much-needed data to help land managers and agencies like MDA address public questions and concerns. The team also expanded their study to include black-light trapping to monitor adult moth populations, which will help them explore the full impact of Btk on the moth lifecycle.
Smarter solutions for a persistent pest
Khanna says Aukema’s mentorship has been instrumental to the project. The abundant literature available on spongy moth has also been an asset, as well as the ability to conduct research close to home. “The field sites are just three hours away,” she says.
Some limitations and challenges linger, however. “Finding egg masses to work with in the quarantine laboratory remains a big challenge,” says Khanna. Female spongy moths cover their egg masses with hair from their abdomens, which can camouflage the egg masses. Field conditions can also be unpredictable due to weather. And, Khanna adds, “In our surveys of moths collected using black-light trapping in both Btk-treated and control regions, our traps captured hundreds and hundreds of specimens. It is logistically difficult to process and identify the tiniest specimens due to expertise and time constraints, but we are doing our best.”
Khanna is confident that her localized approach will improve the effectiveness of pest management, making a Minnesota-specific phenology model a valuable tool for protecting forests and managing invasive species. She and her team remain committed to addressing public concerns around what happens to Minnesota’s native moths and butterflies. “The data that I’ll collect will be critical for public education campaigns, helping build trust in pest management programs by demonstrating the results,” she says. “Ultimately, it’s about striking a balance—protecting forests from invasive pests while preserving the biodiversity that makes Minnesota’s landscapes so unique.”
More information
- Proposed spongy moth management canceled in 12 areas, Minnesota Department of Agriculture press release, April 2025
- Information about Bacillus Thuringiensis, Minnesota Department of Health
- Improving management of spongy moth in Minnesota, research project
- Spongy moth information, MN Department of Natural Resources
- Spongy moth insect info and history, MN Department of Agriculture
- Spongy moth quarantine, MN Department of Agriculture
Research from the Minnesota Invasive Terrestrial Plants and Pests Center is supported by the Environment and Natural Resources Trust Fund, as recommended by the Legislative-Citizen Commission on Minnesota Resources.
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