UMD entomologists have discovered that a single underlying trait—metabolic breadth, or the range of nutrients a fungus can use—links its ability to kill insects, partner with plants and thrive in different ecological roles. Rather than trading one lifestyle for another, some fungi become better at all of them.
Many fungi lead triple lives—acting as deadly insect pathogens, decomposers in the soil and helpful partners living inside and transferring insect-derived nitrogen to plant roots. Scientists have long wondered what allows a single species to pull off these very different roles.
A new study offers a surprisingly simple answer: metabolic flexibility—the ability to use many different food sources. Working with the insect-killing fungus Metarhizium robertsii, University of Maryland entomologists found that strains capable of using a wider range of nutrients were both faster and deadlier at killing insects and more effective at colonizing plant roots. The findings were published July 1, 2026 , in the Proceedings of the National Academy of Sciences.
"We expected to see a trade-off—that becoming a better plant partner would come at the cost of being a good killer or vice versa," said the study's senior author Raymond St. Leger , a Distinguished University Professor of Entomology at UMD. "Instead, the two abilities rise and fall together, and what links them is the fungus's underlying nutritional flexibility."
Different strains, different lives
The researchers combined genome-based analysis of eight M. robertsii strains spanning the species' evolutionary tree with laboratory tests measuring virulence, plant-root colonization, toxin activity and growth on 95 different nutrients. They chose to study M. robertsii because it's already used worldwide as a natural biological control agent against insect pests and is increasingly being explored for its ability to promote crop growth.
St. Leger and entomology postdoctoral associate Huiyu Sheng (Ph.D. '24, entomology) found that the strains split into two distinct groups. The fungal strains that diverged early (at least 6 million years ago) behaved like "sleepers." They kill insects slowly but pour resources into multiplying inside the host and producing huge numbers of spores, allowing them to survive until they encounter another host. Fungal strains that diverged more recently behave like "creepers." They germinate quickly on both insect skin and plant roots, kill rapidly, often deploy paralyzing toxins and grow as creeping threads from insect cadavers onto nearby roots, rather than forming spores.
The key difference between these two fungal strategies was metabolic breadth—the range of nutrients each strain could feed on. Fungi that could grow on a wider menu of sugars, amino acids and organic acids consistently proved better at both infecting insects and colonizing plant roots.
New thinking, new applications
The new study's results reframe some insect-killing fungi as broadly "environmentally competent" organisms—whose ability to attack insects and partner with plants comes from the same nutritional toolkit. The team's findings provide a useful model for understanding how microbes evolve the capacity to switch ecological roles.
"Instead of forcing fungi to choose between being insect killers or plant partners, evolution appears to have favored strains that are simply better at making use of whatever resources they encounter," St. Leger said. "Their versatility begins with metabolism."
The research also has practical implications for agriculture and could help researchers select fungal strains tailored for different agricultural goals. Broadly metabolizing strains of fungi could provide rapid suppression of insect pests while colonizing crop roots and promoting plant growth in the field. In contrast, fungal strains that produce large numbers of spores may be better suited for longer-term pest control.
"What we've learned could help growers use fungal pathogens more effectively," St. Leger said.
