The surprising give-and-take between healthy crops and insect-killing fungi
IIn the first few centimetres of topsoil, a battle for nutrients is raging that’s bred a surprising alliance between plants and Metarhizium — a common but deadly “entomopathogenic” (insecticidal) fungus that grows at their roots. Because all 12 varieties of Metarhizium kill, mummify and feed on soil-dwelling, potentially destructive insects, agricultural companies mass-produce it as a bioinsecticide for use on crops. In 2012, however, researchers at Brock University in St. Catharines, Ont., discovered it also naturally fertilizes the plants above by feeding them nitrogen from insects’ bodies. “That ecological aspect of Metarhizium was ignored for a long time,” says Larissa Barelli, part of the Brock team studying the fungus. “We needed to know more about how it interacts with plant systems.” And their work is still showing how much there is to learn: in 2017, they proved that the relationship is symbiotic — the plants give back. Here’s how.
Canadian scientists are unlocking how an insect-killing fungus could be used to grow healthier crops
Metarhizium has a worldwide distribution and broad tastes. The variety tested by the Brock team, M. robertsii, alone infects more than 200 insect species, including moth larvae and beetle grubs (but is harmless to humans, bees and other animals). “If it’s soil-dwelling, it’s pretty much a target,” says Barelli.
Plants can’t absorb nitrogen in its abundant atmospheric form, so it must somehow be “fixed” (bound to other chemicals). Metarhizium sucks usable nitrogen ( N) from poisoned insects and pumps it into the roots. In return, the plants send down photosynthates — sugary, high-energy carbon compounds ( C).
DOES THIS LOOK INFECTED?
Microscopic Metarhizium spores in the topsoil germinate when they make contact with an insect. They inject themselves through the bug’s outer membranes and start growing rapidly, stealing nutrients and releasing toxins. The host dies in about five days and is then cocooned in spores and mycelia (masses of threadlike, nutrient-moving hyphae) that extend outward to form networks connecting the fungus and plant roots.
Metarhizium strains have been tweaked to create more virulent, fasterworking biopesticides often since the early 1900s. Brock’s research is instead looking at how the fungus’s natural role in the carbon and nitrogen cycles could also be harnessed — and at how widespread commercial fertilizers might be disrupting this beneficial plant-fungus relationship.