The Bio-Protection Research Centre’s (BPRC) investigations into why previously effective biological control against a ubiquitous, damaging pasture pest was failing is one such programme.
The Argentine stem weevil is a major pest of ryegrass, the backbone of New Zealand’s pastures. In the 1990s AgResearch began investigating biological control of the weevil, successfully identifying Microctonus hyperodae, a parasitoid wasp that lays an egg inside the weevil and kills it. As a result of this research, eight populations of the wasp were collected from across South America and released in NZ. They rapidly spread throughout the country, reducing the weevil’s impact.
All seemed well for 15 years, until the BPRC decided to advance the science to include rapidly improving genomic analyses and population modelling. Its first discovery was genuinely surprising: in just 14 generations since Microtonus was introduced, the Argentine stem weevil had developed resistance, rendering biological control ineffective in many parts of the country.
Further research showed the resistance resulted from the sexually-reproducing weevil’s superior ability to evolve, compared to the asexual wasp. The weevil had evolved to evade the wasp in areas where it had put the weevil under strong pressure. The non-evolving wasp could not keep up.
Answers keep emerging
Continued research answered other questions about pasture management, biological control of other pests, the viability of conservation biological control, and even when and how pest insects make it across the border.
At first, the wasp had been extremely effective in controlling the weevil, not least because it had escaped most of its own natural enemies, which were left behind in South America. Its very success forced the weevil to adapt.
Added to this, NZ’s pasture was, and is, almost entirely exotic. As well as comprising just a few plant species, it has very low insect biodiversity. With 20 million years of evolution separating this pasture and our indigenous ecosystems, very few indigenous natural enemies have moved into the pasture, raising questions about the usefulness of conservation biological control in such a setting.
There were also questions about when, and how often, the Argentine stem weevil had arrived here.
Often invading species have narrow gene pools resulting from ‘bottlenecks’ at their sites of introduction. But the weevil’s rapid evolution suggested it had enough genetic variation for resistance traits to become dominant.
The species was first described in NZ in 1927, but there is now good genomic evidence that it entered the country several times. For example, weevils from the Chatham Islands and Stewart Island are significantly smaller than those on the mainland.
Sequencing of the weevil and wasp genomes, combined with analysis of gene expression, now strongly indicate complex mechanisms leading to the weevil resistance. This raises questions about the genetic diversity of other invasive species and their ability to similarly evolve resistance to introduced biological control, including the parasitoids currently controlling the lucerne and clover root weevils.
Beyond the laboratory
What started as a BPRC programme to investigate genomics and population modelling, based on AgResearch’s initial success, has led to major advances in defining what makes biological control agents successful.
In collaboration with the University of Otago Biochemistry Department (Genomics Aotearoa), we can now determine which Microctonus strains and their associated traits have survived the original nationwide release experiment and which disappeared. We can further investigate which genes, genomic features, genome structures and microbial complements are associated with good outcomes. This will greatly advance our understanding across pest species of what is likely to offer effective and more precise biological control.
Lessons learned
Perhaps nowhere else in the world has there been a biological control experiment of such dimensions. We now understand:
-How low habitat biodiversity affects the risk of invasion and, conversely, the success of biological control;
•The threat of evolved pest resistance to biocontrol;
•How genetic diversity affects the probability of a pest species becoming resistant;
•How introduced ecosystems affect the ability of indigenous species to exert biological control;
•The probability that biological control of the lucerne and clover weevils could also fail.
What started as a programme of fundamental science has significantly advanced our understanding of our agricultural ecosystems and the risks involved in managing them. And so, it is clear to see what has been achieved and just what difference it has made – well beyond the laboratory and across NZ’s agricultural landscape.
Who am I? Professor Stephen Goldson is deputy director of the Bio-Protection Research Centre. The Bio-Protection Research Centre is a Centre of Research Excellence funded by the Government. It was established in 2003, with funding ending on June 31. It will then be replaced by a new Centre of Research Excellence, Bioprotection Aotearoa.
