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 sweetpotato 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
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
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
sweetpotato 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 sweetpotato’s advantage.
One possibility is a “lure and kill”
approach: male weevils, attracted to baits
laced with commercially manufactured
sweetpotato weevil female sex pheromone
and M. anisopliae spores, could pick up
the lethal fungus and transfer it to females
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 sweetpotato 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
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
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.
Belinda Smith is a science and technology journalist in Melbourne, Australia.
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