Irish wasp hammers Hawke’s Bay weevils

Wednesday 15 August 2007

Delegates to the 60th Annual Conference of the New Zealand Plant Protection Society Conference in Napier were told today (Wednesday 15 August) of the ‘astounding’ success of an introduced biocontrol for the Clover Root Weevil.

Nicknamed the ‘Irish wasp’ but scientifically known as Microctonus aethiopoides, the Irish native has been introduced to control the clover root weevil which is a severe pest for white clover.

The weevil larvae attack clover underground, resulting in reduced nitrogen fixation and clover growth, especially in spring and early summer. Farmers have observed that livestock have slower weight gains and lower milk yields on infested pastures unless nitrogen fertiliser levels are increased to compensate.

The Irish wasp strikes at the adult weevil, injecting one or more eggs into the abdomen. This makes female weevils sterile, thus breaking the weevil life cycle. The wasp larva grows inside the weevil and kills it, when during the last larval stage it bursts out of the weevil’s body. The larva then pupates in the pasture litter, before emerging as a wasp to start the next generation.

The Irish wasp was released in January 2006 by AgResearch entomologists on a farm at Patoka, and programme leader Dr Pip Gerard announced today at the conference that the wasp was thriving there.

“It is doing astoundingly well at Patoka,” she said. “In just over a year it appears to have knocked the autumn weevil population down by over 75% and 86% of the remaining female weevils are sterile. It is these autumn weevils that we want to control as they lay the larval generation that knocks clover production the hardest, especially in spring. “

While the Irish wasp is doing best at the Hawke’s Bay site, the team is over the moon that it is also doing well at the other three experimental release sites on different types of farming country in the North Island.

“We were very concerned that the Irish wasp wouldn’t persist at sites where there are hardly any adult weevils available as hosts in early spring,” said Dr Gerard. “But obviously the wasps are better at finding weevils than we are and parasitism is running at over 60% this winter even at our worst site. We are now very confident that the Irish wasp will provide useful control of clover root weevil throughout New Zealand.”

Buoyed by this confidence, the AgResearch entomologists have released the Irish wasp in Nelson, Northland, Taranaki and Wairarapa and are planning a big push to get it widely distributed next summer and autumn. Independent consultant Mike Slay, who monitors the Hawke’s Bay site for Dr Gerard’s team at Ruakura, says that the Hawke’s Bay, the Wakarara, Mangleton, Takapau west and east and Elsthorpe districts should be targeted first.

“Most sheep/beef farmers probably don’t relate notching damage by clover root weevil adults in autumn to poor clover growth and animal live weight gains in spring and early summer. The weevil has been working very hard last year along the ranges and 5-10 k out from the foothills. I’ve found pockets with 400-500 larvae/m,2” he said.

The research supporting the parasitoid release is currently funded by Dairy InSight, Meat & Wool New Zealand and the Foundation for Research Science and Technology.

ENDS

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