by Carolyn Bernhardt
October 8, 2025
Every summer, Japanese beetles (Popillia japonica) chew their way through Minnesota gardens, golf courses, and fruit trees. These shimmering pests skeletonize leaves, leaving only delicate veins behind. Their grubs feast on turfgrass roots, damaging lawns and fairways. Nationwide, Japanese beetle damage and control costs exceed $450 million each year.
Although the insects have been present in Minnesota since 1968, their numbers exploded in the early 2000s. Mild winters and hot, humid summers helped populations soar. And because Japanese beetles feed on more than 300 plant species and have few natural enemies, management is challenging. Hand-picking beetles, setting traps, and spraying insecticides can help, but these tactics are labor-intensive, often ineffective, and harmful to pollinators.
A University of Minnesota research team is pursuing a different strategy: using the plants themselves to fight back.
A clue hiding in the leaves
Matthew Gullickson, PhD, a postdoctoral associate in the Hegeman Plant Metabolomics Lab, who is helping to lead the research, says the idea for this project grew from a simple observation. Between 2017 and 2020, Japanese beetles swarmed some birch trees at the Horticultural Research Center at the Minnesota Landscape Arboretum—but a few neighboring birches stayed untouched. “Based on those results, Dr. Katrina Freund Saxhaug found beetle feeding was correlated with the plant compounds betulinic acid and betulin — the compound that produces the white bark on the birch trees,” he says. “But it was in really low doses on the leaves.”
Arboretum researchers wanted to know whether those compounds actually repel beetles or merely appear alongside resistance. “Just because something is correlated doesn’t mean it’s the cause of resistance,” Gullickson explains. So, he collaborated with the Horticultural Research Center to take a deeper dive into birch leaf chemistry, hoping to discover natural compounds that deter feeding and could someday be turned into a “green insecticide or green deterrent" so landscape managers and nurseries have a less toxic chemical suite they can use for managing Japanese beetle infestations.
The project launched in January 2024 with support from the Minnesota Invasive Terrestrial Plants and Pests Center (MITPPC). Over the course of two summer field seasons, the team ran feeding assays, offering Japanese beetles different birch cuttings in both no-choice and two-choice tests. “We saw there was a highly resistant species at the Arb, it’s just labeled as UMN Birch accession #29. It was highly resistant, but when we went out to the field, it was almost dead from a different insect pest,” Gullickson says. That discovery raised new questions: Was the tree’s beetle resistance genetic, or triggered by other stresses such as infestation by a native insect called the bronze birch borer?
To gain more control, the researchers built their own birch nursery. “I wish we had started with a nursery right away rather than pivoting this year,” says Gullickson. “I feel like we got much stronger, clearer data this year because we were able to control for environmental and genetic effects so much easier.”
The experiments also tested plant “elicitors” — chemicals that prompt trees to produce their own defenses. “We saw a very strong anti-herbivory response when we applied the jasmonic acid to the birch trees,” says Gullickson. “To date, there isn’t anything about this on birch trees, and we were able to see there was a pretty significant effect with the jasmonic acid, and that’s a pretty exciting, novel finding.”
Compounds and colors that hold promise
Betulin and betulinic acid are compounds of interest to the research team. “They were some of the first plant extracts identified in 1788,” says Gullickson. “They have some saponin, or soap-like characteristics.” According to Gullickson, these qualities could either be unpalatable to Japanese beetles or even negatively impact their internal biology and metabolism.
Other surprises emerged in this research, too. Birch #29, which has unusually dark maroon leaves, seemed less appetizing to beetles, suggesting that leaf pigments, such as anthocyanins, might play a role in beetle feeding preference.
The ultimate goals are specific recommendations for homeowners about resistant birch varieties and eventually, a foliar spray derived from host-plant compounds—something nursery managers or home gardeners could apply to vulnerable plants. But the researchers stress caution. For one thing, Gullickson says, plant molecules can have significant effects on organisms that users are not targeting. "While [plant molecules] are often used for purposes like medicine or pest control, we need to understand that they aren't a 'silver bullet.' They might be more broad-spectrum than expected, potentially affecting beneficial species as well."
Because Japanese beetle populations fluctuate with the weather, the team also had to adapt to smaller beetle numbers during the past two summers. “Japanese beetles just haven’t been that bad locally over the past two years,” says Gullickson. “We have had just enough beetles to do our assays and not a lot extra.”
Despite these challenges, he says the project benefits from collaboration. “I think having a pretty interdisciplinary research team is a strength of this project. We can bounce ideas off each other, and I think there is excellent knowledge transfer within our group, and outward into our fields.” The team also involves undergraduate and graduate students, giving them hands-on research experience.
Science, not silver bullets
For gardeners eager for a natural fix, Gullickson emphasizes that rigorous science—not wishful thinking—as the main guide to the work. “To me, ethical science is defined by objectivity: clear methods, unbiased data collection, appropriate controls, rigorous statistics, and peer review to validate results and claims. As scientists, I believe our responsibility is to continually reevaluate our ideas as new data emerge. That process—updating our understanding rather than hanging on to fixed ‘facts’—is what makes science powerful.”
The Japanese beetle may never disappear from Minnesota, but by uncovering the natural chemistry of resistant plants, this project aims to give growers, landscapers, and homeowners a safer, more sustainable line of defense—one leaf at a time.
More information
- Harnessing host plant resistance for Japanese beetle control measures, research project
- Japanese beetles and birch trees information sheet
- Japanese beetle information, Minnesota Department of Agriculture
- Japanese beetles in yards and gardens, UMN Extension
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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