A battle of good versus weevil

You might catch more flies with honey, but you can dispatch more weevils with fungi. Researchers at the University of Southern Queensland, funded by the Australian Centre for International Agricultural Research, are finding ways to harness nature’s chemical weapons to fight the sweet potato weevil (Cylas formicarius), an innocuous-looking bug that can devastate entire crops.

Some 95% of sweet potatoes (Ipomoea batatas) are grown in developing countries, where they are the fifth most important food crop. Not only rich in vitamins, they’re also chock full of carbohydrates. A farmer growing sweet potatoes can produce more edible energy per hectare per day than they could with rice or cassava.

Sweet potatoes are hardy, too, happily growing in dry soil with little fertiliser or irrigation. But they’re not tough enough to withstand a sweet potato weevil onslaught. Adult weevils lay eggs in the stems and roots. After hatching, the grubs grow into adults which gnaw their way out, riddling the sweet potatoes with holes and rendering them inedible.

By the time a crop is infested, it’s usually too late to do anything about
it. Manufactured insecticides only kill the adult weevils, not the larvae already ensconced. In countries such as Papua New Guinea, where sweet potato is the primary food source, farmers depend solely on “cultural control”, such as crop rotation and sanitation, to stop the weevil’s spread. But the natural world has its own arsenal: the fungus Metarhizium anisopliae.

Found in soils around the world, M. anisopliae is entomopathogenic, meaning it only infects insects. Simply coming into contact with fungal spores is enough for infection to take place. The species burrows into the unfortunate insect, reproduces inside its body, and bursts out again, killing the host – if it’s not dead already. And M. anisopliae counts sweet potato weevils in its range of hosts.

So researchers such as Bree Wilson at the University of Southern Queensland in Toowoomba are finding ways to use it to the sweet potato’s advantage.

One possibility is a “lure and kill” approach: male weevils, attracted to baits
laced with commercially manufactured sweet potato weevil female sex pheromone and M. anisopliae spores, could pick up the lethal fungus and transfer it to females before dying.

But when Wilson and her colleagues added particularly virulent M. anisopilae strains in their baits, which would kill more insects faster, the weevils steered clear. The researchers suspect those strains secrete certain volatile compounds that sweet potato weevils can detect and know to avoid.

“While we haven’t identified the volatiles responsible for avoidance in our
isolates, this is next on our cards,” Wilson says.

She adds this will be helped by a new and “very fancy olfactometer” which will sniff out repelling volatiles. And when they figure out which genes are responsible for the weevil-deterring effects, they’ll use CRISPR-Cas9 gene editing technology to snip them out of the fungus genome.

The new, repellent-free version of the fungus will then be tested in glasshouses and in the field to see if it retains its lethality. Another option, she says, is to manufacture and spray the repelling volatiles around a crop to produce a smelly weevil-proof barrier.

Sweetpotato weevils aren’t the only pest in Wilson’s sights. Her biggest
challenge will be the root knot nematode (Meloidogyne species) – roundworms that infect roots, weakening or killing the plant.

The main defences against root knot nematodes are expensive and nasty,
so she hopes to explore how Pasteuria species of bacteria – which stop the worm reproducing – could help.

“I’m looking forward to working with the Australian and Papua New Guinean growers to test from of this research to offer genuine alternatives to control these pests,” she says.