Invasive insect fall armyworm on the march, but scientists fight back with an oozing virus and an egg-attacking wasp

By Jennifer Nichols

A virus that oozes out through a caterpillar's skin before exterminating it is being investigated as a way to combat an invasive insect that is devastating some Australian crops.

Since fall armyworm was first found in the Torres Strait in January 2020, it has spread to every state and territory except South Australia.

Their moths can travel up to 400 kilometres a night.

Researchers are looking at chemical-free options to attack the difficult-to-control bug, including a species-specific virus that oozes out of a caterpillar's skin before the larva disintegrates.

Melina Miles says fall armyworm has resulted in significant challenges for crop growers.(Supplied: Qld Department of Agriculture and Fisheries)

Dr Melina Miles, who leads the Queensland government's field crops entomology team in Toowoomba, said so far most farmers were using chemicals to attack the insect.

But she expected that use to drop because winter yields were still good despite damage from the caterpillars.

"Through the work that we've been doing at depth, we now know that there are a whole suite of natural enemies, parasitoids and predators, that are attacking fall armyworms," she said.

"You can have some confidence that there's some natural mortality going to occur so it's not just left to you, as the grower, to control it with insecticides."

Natural enemies

Non-chemical options, including spraying biopesticide onto the leaves of affected plants, could be used in conjunction with beneficial insects to reduce numbers.

Tiny adult Trichogramma pretiosum wasps on the tip of a fine paint brush.(Supplied: Melina Miles)

Last year, authorities approved an emergency permit for the use of Fawligen, an organically-certified biopesticide that contains a caterpillar virus that only kills fall armyworm.

"As the larva are eating, they'll take up these little virus particles and they are only activated in a very alkaline insect gut," Dr Miles said.

"It penetrates the gut and starts to replicate and the larva stops feeding. Then the virus eventually starts to ooze out through the skin. Eventually it gets to the point where the larva disintegrates, releasing all these virus particles into the crop.

"We just need to do a little more work to understand how growers might best deploy it."

Dr Miles says within 4-8 days of fall armyworm being infected with Fawligen, it turns into a sack of virus that eventually explodes.(Supplied: Melina Miles)

Four modern insecticides have had varying success against the pest, but fall armyworm have developed a high resistance to synthetic pyrethroids and moderate resistance to carbamate insecticides.

Third generation dairy farmer Don Davies was looking for a chemical free solution when he ordered tiny Trichogramma wasp eggs.

Regenerative dairy farmer Don Davies paid to have Trichogramma egg parasitoid wasps released on his property and was pleased with the results.(Supplied: Don Davies)

He spread them throughout his young corn crop to battle a fall armyworm invasion at east Cooyar, on the Darling Downs.

"They took a few weeks to work, but within a month or so we were getting pretty well on top of them," Mr Davies said.

The tiny Trichogramma pretiosum parasitoid wasp targets the eggs of fall armyworm.(Supplied: Melina Miles)

For the past thirty years he has avoided using fertilisers by using multi-species cropping on land his family has farmed for more than 100 years.

Brassicas including turnips, combined with clover and herbs, improve the soil and provide habitat and nectar to beneficial insects.

"We were counting up to 200 beneficials per metre in the corn crop," he said.

"There was up to seven different lady beetles, and there were all these other natural predators there as well as these Trichogramma wasps that seemed to get right on top of the fall armyworm."

"Some corn stalks were quite devastated with it, but they recovered quite well."

The caterpillars favour maize, sweet corn, sorghum, capsicum and C4 pastures, which are more adapted to warm or hot seasonal conditions, when fall armyworm are most active.

Beneficial insects including ladybirds helped Don Davies control fall armyworm.(Supplied: Don Davies)

Breeding beneficial bugs

Paul Jones from Bugs for Bugs breeds predatory species of insects and mites for mass release on farms.

Ladybirds, lacewings, pirate bugs, and the tiny parasitoid Trichogramma wasps have proven biological allies against fall armyworm.

Paul Jones breeds beneficial insects and sends Trichogramma wasp eggs out in compartments in this sheet of cardboard.(Jennifer Nichols)

"Growers have been using a lot of chemicals to control the pest but because it's so prolific and aggressive, and the resistance factor is such a big issue, it's very difficult to control," Mr Jones said.

"We're working on a premise of the more diverse and the higher the density of beneficials, the better."

He said bugs were not cheap.

"They are probably equivalent to the expensive chemicals as there are a lot of labour inputs in producing bugs, but the outcome is far more profitable to the grower if he doesn't have to spray and he's coming out with a crop which is not damaged with the pest," he said.

Dairy farmer Don Davies found fall armyworm after noticing damage on his maize crop.(Supplied: Don Davies)

Dr Miles said the challenge with insecticides was reaching the fall armyworm on the plants.

"Whereas for parasitoids, and predators, that's much less of a challenge. They can get down into little nooks and crannies, and under the leaves, and so on, much more easily than a grower could get droplets of insecticide," she said.

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