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Irish wasp here to stay – but not in Northland

Irish wasp here to stay – but not in Northland

12 August 2008



Embargoed until noon Tuesday 12 August 2008


Farmers concerned about clover performance received good and bad news today at the 61st Annual Conference of the New Zealand Plant Protection Society being held at Paihia.

The AgResearch entomologist who leads AgResearch programme to control clover root weevil, Dr Pip Gerard, says the Irish wasp, introduced from Ireland by AgResearch to control the pest, has for the second successive season failed to establish in the Far North. Yet it has thrived in most other North Island districts despite the summer drought, she says.


The clover root weevil is a serious pasture pest. The weevil larvae attack clover roots and nodules throughout the year, which results in less clover available for animals, especially in spring and early summer. Farmers have observed that livestock have slower weight gains and lower milk yields on infested pastures, unless fertiliser levels are increased to compensate.

However, the Irish wasp should alleviate this problem. It strikes at the adult weevil, injecting one or more eggs into the abdomen. This makes female weevils sterile, thus breaking the weevil’s life cycle. The wasp larvae grow inside the weevil and kill it when the last larval stage 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.

Another AgResearch entomologist, Tina Eden, says that while the wasp can be distributed by parasitised adult weevils flying to new feeding sites, the number of weevils flying each summer is highly dependent on climate. “We were a bit worried that it had only spread 2 km at best from release sites last year, but following the warm temperatures last summer, we were pleased to pick up good levels of parasitism in autumn almost 20 km from the Hawke’s Bay experimental release site,” she observed.

To hasten the arrival of the Irish wasp on all farms where the weevil is a problem in the North Island, Dr Gerard’s team at Ruakura is undertaking a two-pronged distribution programme. Nursery sites, where the wasp can multiply and be collected for distribution, have been set up by large organisations such as the Northland and Taranaki Regional councils, and Landcorp Holdings Ltd. Hundreds of laboratory-reared samples of parasitised weevils were also distributed to farmers through the networks of  DairyNZ and Meat & Wool New Zealand last summer.

However, the widespread drought earlier this year has hampered progress. “We didn’t want to release wasps on farms that had no clover, so a large proportion of our releases were carried out in Northland, which had plenty of clover and weevils last summer. So it was a double whammy to discover that our Irish wasp won’t establish in the Far North,” rued Dr Gerard. “On the other hand we were delighted to find it did establish and persist at existing and new sites in the Waikato and Manawatu in spite of very hot dry conditions.”

Dr Gerard says it is not known at this stage why the Irish wasps seem to snub Northland, but her team will be carrying out research to determine the reasons. Meanwhile plans are well underway to distribute more wasps to farmers. DairyNZ, Meat & Wool New Zealand and the Foundation for Research Science and Technology have supported this biocontrol programme since its inception in 1997.


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