Study: Fungi With Broader Diets Have More Power to Kill Insects, Help Plants

Many fungi lead triple lives, whether killing insects, decomposing organic matter in the soil or transferring nitrogen to plant roots.

A new study offers a surprisingly simple answer for how they do all this: the ability to use many different food sources, known as metabolic flexibility. 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 Wednesday 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." 

The researchers analyzed eight M. robertsii strains because the fungi are already used worldwide as a natural biological control agent against insect pests and are increasingly being explored for their ability to promote crop growth. 

St. Leger and entomology postdoctoral associate Huiyu Sheng Ph.D. ’24 found that the strains split into two distinct groups. The fungal strains that branched off at least 6 million years ago behaved like “sleepers,” killing insects slowly and multiplying inside the host in order to produce huge numbers of spores and survive until they encounter another host. Strains that diverged more recently were "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. The creepers can grow on a wider menu of sugars, amino acids and organic acids, and consistently proved better at both infecting insects and colonizing plant roots.

The research has practical implications for agriculture. 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.