In the evolutionary arms race between the Argentine stem weevil and the parasitoid wasp, the weevil has for the moment got the upper hand – at the cost of about $200 million a year to agriculture.

The story of the search for a biocontrol which could rein in the weevil which eats ryegrass, and the subsequent fight back by the South American invader, goes back to the 1990s and has attracted worldwide attention.

Scientists believe it the first time that a pest has out-evolved a parasite, the only example among all of the biocontrols used globally.

Research entomologist Professor Stephen Goldson, who works for AgResearch and the Bio-Protection Research Centre, worries other pests might also gain the upper hand over the parasites introduced to control them.

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Besides the stem weevil – which threatens New Zealand’s most important pastoral grass – there are the clover stem weevil and the lucerne weevil, which also cause economic damage of more than $200m a year.

Like many introduced pests to New Zealand without natural predators, the stem weevil populations took off out of control as soon as they arrived. No-one knows exactly how it came in, but it is part of the constant pressure caused by unwanted new arrivals says Goldson.

By 1993, scientists were finding 700 weevils per square metre compared to just one per sq metre in their native South America.

​Goldson therefore embarked on a South American odyssey to try and find a parasite to keep the weevil in check. He visited Brazil, Uruguay, Argentina, and Chile twice and sent back to AgResearch’s labs a huge number of weevils from which emerged some potential biocontrols, including a tiny parasitoid wasp called Microctonus hyperodae.

Initially, it looked as though the parasite was the solution because it killed 90 per cent of the weevils. Laying eggs in the weevil’s body, it leaves its larvae to eat the host from within.

But several years later only 15-40 per cent of the weevils were dying. By reproducing sexually, the insect pest had, in science-speak, developed an “evasive response” to the parasitoid.

Arguably the weevil’s genes could have changed, causing this kind of resistance response. However, the parasitoids that Goldson brought in are asexual;  all they do is produce identical daughters and have very little scope to evolve to counteract the weevil’s changes.

Actually, as it happens, the parasitoid wasp apparently can reproduce sexually.

“We reared a million females and indeed found four males but they were, shall we say, dysfunctional.  However, if we could find some more, we could bring them over and breed more effective parasitoid strains from them.  It could be the start of an evolutionary arms race on an equal footing with the pests, but it is a long shot; such is the nature of science,” Goldson says.

In general New Zealand appears unique among countries for being vulnerable to insect pests. Goldson has a theory for why.

“Our pasture ecosystems are hugely simplified compared to the ecosystems where these pests came from. The New Zealand pastures are pretty much empty. We’ve grabbed clover, ryegrass and a handful of other grasses, but that’s absolutely nothing compared to where these pests came from.

“The native natural enemy species in our forests don’t go into our pastures, they’re not going to move into a small transplant of European pastures.”

He points to the fact the stem weevil has been found in Europe a handful of times but has never established there, presumably because the ecosystems are full and there are their own natural enemies everywhere.

Goldson believes the clover root weevil could also possibly outwit its wasp biocontrols agent. It operates in a similar way to the Argentine stem weevil parasitoid.

One of the lessons he has learned is that scientists need the opportunity to learn more about the biocontrols and how and why they work, rather than in the past when after their introduction there was little support for follow-up study.

There are several ways forward to getting on top of the pests.

Some “heroic individual” may want to go back to South America and find functioning males, bring them to New Zealand and selectively breed to produce strains that will overcome the stem weevil’s resistance.

But as soon as he raises the notion, Goldson dismisses it.

“Someone could spend their entire working life trying to find these males in the vast land area of South America,” he jokes.

A smart idea led by Otago University that has not been tried before to Goldson’s knowledge is to turn the females into males. While on the face of it the method sounds like genetic engineering, it is not.

“They’ve got the genetic machinery to be males, so the question is how to regulate hitherto silent genes and so forth to make them into males.”

Goldson’s Otago University colleague, geneticist Professor Peter  Dearden, is working with tiny molecules and similar to turn females into males and potentially start a breeding programme. While it is early days, the programme is showing promise.

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