Genetic diversity may be key to effective biocontrol, says an author of a paper investigating why a previously successful biocontrol agent has stopped working in parts of New Zealand.

And that shows that scientists need to understand as much as possible about the genetics of potential biocontrol agents and their target pests from the outset, says Prof Stephen Goldson, Programme Leader at the Bio-Protection Research Centre.

The scientists – including Dr Tom Harrop and Professor Peter Dearden from the Bio-Protection Research Centre and Genomics Aotearoa, and Dr Jeanne Jacobs from AgResearch, Lincoln – have shed light on why Argentine stem weevils have developed resistance to the parasitoid wasp Microtonus hyperodae, introduced in the 1990s to control them. Argentine stem weevils are a major pasture pest on New Zealand farms.

Using DNA analysis and genome sequencing tools that were not available until very recently, the scientists measured the genetic variation in weevils collected from 10 locations around New Zealand. This revealed that the weevils are much more genetically diverse than previously thought.

“Many scientists had presumed that Argentine stem weevils were probably introduced to New Zealand just once in the early 20^th Century,” says Prof Goldson. “But the genetic variation discovered suggests they were probably introduced several times, and in much larger numbers than thought.”

This genetic variation has allowed the weevils, which reproduce sexually, to mix their genes and become resistant to M. hyperodae, which reproduces asexually and so cannot easily evolve further.

Three years ago, the same scientists presented compelling evidence the weevils were developing resistance to the parasitoid. This was a world first – there had never been a documented case of an agricultural pest out-evolving its introduced predator before. But the BPRC ecologists didn’t know exactly how or why this had happened so quickly.

Ongoing study suggests a lack of diversity in New Zealand’s pastures led to the ecological pressure that caused the resistance.

“These discoveries were only possible because current tools let us measure genetic variation across the entire weevil genome,” says Dr Harrop, who is the lead author. “Previous work, studying only a handful of sites in the genome, had suggested that weevil populations had low genetic diversity.”

Prof Goldson says the findings show how useful it is for scientists to understand the genetics of pest species and biocontrol agents.