Key points
- Fall Armyworm (FAW) is a serious insect pest in food and fibre crops, with different populations carrying varying genes for insecticide resistance.
- Further pest movement and gene flow could increase global resistance, making alternative control methods, such as biopesticides, even more critical.
- Though still in its early stages, this work opens various exciting avenues for future FAW biocontrol research in Australia.
It’s always a bit of a lucky dip when you go to a library – especially if the library is a collection of fungi and bacteria!
Reaching into a freezer to pull out a sample of freeze-dried fungal spores stored at minus 80 degrees was the start of a search for an Australian fungi that could kill this serious insect pest.
Some of the fungi were collected before our researcher was born. Many were based on taxonomic classifications that had changed. And it wasn’t always clear how they’d been identified – whether by the collector or by the fungi librarian.
Moth in the frame
FAW is an invasive moth, native to tropical parts of the Americas, which has been confirmed in Africa, Europe, Asia, and the Pacific. It was reported here in early 2020 and has rapidly established populations across Australia.
It has been detected on over 350 plant species. And it’s capable of completing its lifecycle on key food crops such as corn, wheat, sorghum, and vegetables.
Our researcher Dr Wee Tek Tay has been researching how FAW spread around the world.
“Different FAW populations are resistant to different insecticides, and further pest movement and gene flow could lead to increased insecticide resistance globally,” Tek says.
No spore-dinary fungi
Beatrice Apirajkamol (Bea) is a PhD student working on bio-pest management strategies. Bea identified 11 types of fungi from our fungal collection as potential biocontrol agents: six types of Beauveria and five types of Metarhizium.
“Our fungi collection has around 900 Australian endemic and regional fungi which infect insects. So, I thought there was a good chance that at least 1 out of the 900 would infect Fall Armyworm,” Bea said.
The known host species for these local fungi include the Emperor Gum Moth (Opodiphthera eucalypti), and a major invasive agricultural pest moth widely known as the Cotton Bollworm (Helicoverpa armigera).
“This fungus, Beauveria bassiana, has been used overseas as a commercial biocontrol solution for other insect pests,” Bea said.
Spore than meets the eye
Out of our 11 fungal candidates, Bea found two that were highly effective. Usually with fungi, the spores infect the insect host, spread through the host’s body and eventually kill it. But the Fall Armyworm caterpillars were dying within 24 hours, which was a lot faster than expected.
Under the microscope, Bea couldn’t see any germination of the spores in that first 24 hours.
“I thought maybe the spores were stopping the caterpillars from breathing,” Bea said.
“So, I zoomed in on their breathing holes: the spiracles. But it turned out that the spores weren’t covering them.”
Rise like a helix
The fungi were killing the insects in a different way. This was most likely by producing some kind of toxic compound before the spores germinated.
“We followed up with whole genome sequencing to identify candidate genes,” Bea said.
It turned out the fungi were carrying insecticide-producing genes.
These genes produce compounds similar to Beauvericin, Oosporein and Bassianolide, which are biopesticide compounds known to be produced by Beauveria bassiana.
Leafing a lasting impression
This modus operandi may have been unexpected, but in the biological world it makes sense.
“It’s easier for the fungus to kill the caterpillar first and then consume it. It means the fungi doesn’t have to fight the insect’s immune system,” she said.
Bea said it was fascinating to find out how the fungi worked.
“Fungi might be small, but they can produce something that can kill an insect or an animal much bigger than they are,” Bea said.
Casting a cell
Bea also looked at the effect of bacteria on FAW. Bacillus thuringiensis (Bt) toxins are already used in insect pest control, although FAW populations in the Americas are resistant to some Bt toxins and proteins, including the ones used in Bt cotton plants.
She found three strains from our Bt bacteria collection which caused 100 per cent mortality in FAW, and three other strains which caused more than 70 per cent mortality. One of these seemed to involve unknown toxins.
What’s in spore?
Understanding these compounds from the fungi and the bacteria could open new avenues for controlling invasive pest species. We need to find out how these bacterial proteins are causing mortality in FAW. And we need to understand the impact of these fungal and bacterial agents on other species, including their host specificity and their toxicity to other species.
“Fungal-based insecticide products that are already available generally work by parasitising the caterpillar,” Bea said.
“What we’ve found is a different mechanism where fungi make these bioactive compounds to first kill their insect host before consuming it.”
“This is early-stage research, but it opens various exciting avenues for future Fall Armyworm biocontrol research in Australia,” Tek said.
Bea is supported by an International Macquarie University Research Excellence Scholarship (iMQRES) and co-supervised by Tom Walsh and Wee Tek Tay (CSIRO) and Phil Taylor and Bishwo Mainali (Macquarie University).
CSIRO has long standing biopesticde research exploring endemic fungi and bacteria. Incorporating genomics in the search of novel biopesticides can offer a targeted solution to mitigate resistance in insect pests. DAFF-CSIRO Catalysing Australia’s Biosecurity initiative is supporting projects to develop biopesticides.