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Latest results in from the field show that a tiny Irish wasp released early last year is hitting adult clover root weevils hard in the North Island.
The AgResearch Biocontrol, Biosecurity and Bioprocessing team released the parasitic wasp from Ireland, known as Microctonus aethiopoides, in January and February 2006 at experimental release sites in Waikato, Hawke’s Bay and Manawatu.
The wasp, which is totally harmless to humans, injects the adult weevil with an egg that makes the female weevil sterile. This breaks the weevil life-cycle but the weevil stays alive and active while the wasp larvae grow inside. Eventually the larvae kill the weevil when they break out to pupate in the soil litter before turning into an adult wasp.
Dr Pip Gerard, leader of the Dairy InSight and Meat & Wool New Zealand-funded release programme, says that the wasp is ‘performing marvellously well’ – right up with the team’s most optimistic predictions.
“The four experimental sites we chose were all very different – ranging from the high altitude Hawke’s Bay site with snow in winter to the lowland Waikato site with mild, wet winters.
“We had some anxious times over summer when we thought the wasp had died out from lack of available hosts at two of the sites where the old generation of weevils from the autumn did not overlap with the new generation emerging in late spring. But now the wasp numbers have bounced back better than ever.”
She says whereas this time last year the team was getting 11-16% parasitism across all sites, now it has over 70% parasitism at its two best sites and 30% and 63% at its problem sites.
“We’ve found that the wasp can produce multiple progeny from a single weevil, and have several generations over summer which allows it to multiply very rapidly.
“Everything is tracking just how we would wish it,” says Dr Gerard. “The adult weevil populations appear to be going down and over two thirds of the remaining female weevils in autumn are sterile. That has to mean there are fewer clover root weevil larvae hatching in our pastures.”
Although clover root weevil infestations are usually detected by the characteristic feeding notches the adult weevils make on clover leaves, it is the hidden damage below ground by the larvae that gives the weevil its reputation of being one of New Zealand’s most serious pasture pests.
Newly hatched weevil larvae feed in the clover root nodules and the larger larvae on the roots and stolons. As adults are laying eggs from December through till September, there is continuous pressure on the nodules and roots. This reduces the amount of nitrogen fixation by clover and as the plant directs its resources to repairing root damage, it produces less foliage. Consequentially, there is less quality forage available for stock, especially in spring and early summer when dairy herds are in full production and farmers want to achieve desired live weight gains in young stock. Economic assessments at farm system level using farm decision support models such as StockPol® and UDDER have shown that typical weevil populations decrease farm gross margins by 10-15%, if a “do nothing” approach is adopted.
Buoyed with the success to date, the team has commenced the full scale North Island release programme. While the wasp is spreading by itself, the team is enlisting regional and pastoral community support to accelerate the distribution. Firstly, nursery sites are being established around the North Island where local people, who know the area and weevil hotspots, will be able to collect parasitized weevils and spread them round. Secondly, the team intends to distribute samples of parasitised weevils at key farmer events. Both distribution activities are likely to take place in January-April 2008 when conditions are best for establishment.
However, while delighted by progress so far, Dr Gerard emphasises that biocontrol is only one of several tools that can be used reduce weevil impact and lift clover performance.
“Ideally, biocontrol will be integrated with recommendations arising from other projects aimed at helping farmers establish and maintain high quality, productive and environmentally sustainable pastures,” she says.
Editor's Note: Original news release can be found here.
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