Wasp may be losing its pasture pest battle

TIM CRONSHAW
Last updated 06:00 29/06/2014
The parasitic wasp sizing up an Argentine stem weevil may have lost some of its biocontrol powers.
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LOSING ITS STING?: The parasitic wasp sizing up an Argentine stem weevil may have lost some of its biocontrol powers.

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Scientists are finding signs that a successful biocontrol agent might be losing its battle against a pest feeding on farming pastures.

A South American wasp could be a victim of its own success against the Argentine stem weevil with initial evidence pointing to its numbers declining and the host gaining the upper hand.

Winter numbers were found in a published study to be down by up to 50 per cent compared with 15 to 20 years ago when it reached its peak after being released in the early 1990s.

Scientists are looking at reasons why it was less efficient with one theory being that the weevil had developed resistance to it after about 15 years of selection pressure in a pasture environment dominated by few grass species apart from mainly ryegrass and clover.

National team leader Professor Stephen Goldson, who works at Lincoln University and AgResearch, said research into the "biocontrol instability" was a work in progress, but serious investigation was needed as the initial understanding was that weevil activity was increasing.

"Based on current data and analysis it really seems there is a reduction in activity and that has been matched by reports of increased damage of stem weevils in ryegrasses in particular."

He said scientists remained cautious about over-emphasising the wasp's decline because they were looking at a big eco-system with host and parasite numbers constantly moving up and down depending on the season.

The wasp puts an egg inside the weevil's body, instantly sterilising it, with the host acting as an incubator for its "alien" larvae. This larvae kills the weevil and forms a cocoon and another wasp. Three generations are produced over summer and the parasite and host go into hibernation in winter to crank up numbers in spring.

Scientists believe the wasp's inability to produce males - it only creates clones of identical daughters of the mother - could be partly behind its possible downfall.

Goldson said there was a potential "perfect storm" going on between the weevil and the wasp.

"First of all the parasite can't evolve because it's stuck being like its mother. The weevil can evolve because it has a sexual reproduction cycle, and if enough pressure is put on a population resistance can occur. Therefore, there is no evolutionary arms race going on and usually the parasite would evolve to keep up with changes to the host, but it can't and the other component of this storm, and it's somewhat speculative, but the eco-system we have on our pastures is remarkably simple."

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Pastures based mainly around ryegrass and clover allow pests such as the weevil to grow to large numbers because they have no enemies. Until now parasite numbers have also been able to explode and this is why biocontrol agents have been so successful in New Zealand pastures.

Goldson said a strong line of thinking was that weevil survivors were struggling to hide in the simple pastures which usually slows down the rate of resistance.

In the mature eco-systems of many other countries, pests could escape selection pressure and crossbreed with ones that have evolved to slow down resistance, he said.

He said the investigation was being taken seriously because clover root weevil also had a biocontrol wasp.

No white clover plants have yet been found to be resistant against the clover root weevil and farmers are dependent on an Irish wasp for its control, estimated to be worth $444 million a year for farmers in reduced pasture damage.

Old data collected nationally from the 1990s helped the national team track wasp numbers waning a year ago. The data was kept but research funding went elsewhere because of its success.

Research into what is going on between the stem weevil and the parasite is being carried out by the Bio-Protection Research Centre at Lincoln University and AgResearch with support from the Foundation of Arable Research and DairyNZ.

Among theories are that there has been a genetic shift in the weevil and this is being investigated by a team of molecular biologists.

Goldson said scientists had yet to look at options which could include the unlikely chance of finding other strains of the parasite in South America or developing new pasture management such as using more plant species.

The research is attracting international scientific attention because there is only one recorded case of a host developing resistance and that was in Canada involving the larch sawfly and a wasp.

- The Press

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