Published on 07/12/2022
Namely, thousands of grass grub larvae thriving in the light, well-drained, flipped pasture soils around Cape Foulwind.
Now they’re planning how best to monitor and limit further spread of this costly insect pest on farms in the one part of New Zealand – coastal Westland - which historically have been free of it.
AgResearch senior scientist Dr Mark Hurst says a big challenge will be the close similarity between grass grub and manuka beetle larvae, which are difficult to differentiate.
In other words, grass grub may already affect more farms on the Coast than anyone realises.
“Who knows how far they’ve spread?” And because they are ‘new’ to the region, farmers there cannot rely on naturally occurring soil pathogens that help control the larvae in other parts of New Zealand.
The Cape Foulwind site featured more than 400 larvae per square metre when it was assessed last year.
These are assumed to have spread from a population discovered at Westport airport and golf course 10 years ago, possibly imported by accident in topsoil from Canterbury.
Mark Hurst says visual inspection of more than 1000 Cape Foulwind larvae found no evidence of disease, indicating grass grub will flourish on suitable soils on the West Coast without control.
“Farmers don’t really understand the implcations of this yet. It poses a significant economic risk.”
Gene sequencing shows the Foulwind larvae are most closely related to a larva found in Canterbury, supporting the case for accidental introduction from Canterbury to Westport.
It’s now important to develop tools for rapid differentiation of manuka beetle and grass grub larvae, to help determine how far grass grub has spread, he says.
“We also need to determine the levels of natural grass grub pathogens – if present – in West Coast soils, and work with local farmers to help identify pasture pest populations on their farms, and decide how best to control these.”
The Cape Foulwind farm was originally being assessed as a test site for a developmental biopesticide called AGR96X, which has already shown promising efficacy against both grass grub and manuka beetle larvae in other trials.
AGR96X is a strain of Serratia proteamaculans that Mark Hurst isolated in 1996 from a diseased grass grub larva found in Canterbury.
Unlike the grass-grub-specific Serratia entomophilia, which causes amber disease but takes between one and three months to kill its host, AGR96X kills grass grubs or manuka beetles within five to 12 days of ingestion making AGR96X more akin to an insecticide.
AgResearch has partnered with Midlands in Canterbury to develop AGR96X.