Industry-wide effort required to combat damaging weevil

AgResearch scientist Dr Scott Hardwick collecting clover root weevil from South Canterbury pasture using a blower-vac.

One million tiny insects are going to make a huge economic difference to the Southland region this autumn, thanks to the joint efforts of AgResearch, Beef + Lamb New Zealand, DairyNZ and Environment Southland.

The project, which has been releasing parasitised clover root weevils on Southland farms for some time, is being considerably accelerated.

“If we can make releases of clover root weevil infected with the Irish wasp parasite on up to 1,000 Southland farms before winter, then we will have done as much as we can possibly hope to this year,” says AgResearch pest specialist and scientist Colin Ferguson.

“To do this we aim to collect up to one million parasitised clover root weevil over the next few months.”

Last year’s mild winter has meant that clover root weevil has taken its small foothold on the farms of Southland to a widespread infestation that has had a huge impact on the production ability of the region’s pastures.

“Unfortunately there is little that farmers can do by changing their management practices to minimise the impact of clover root weevil and there are no current effective pesticides,” says Mr Ferguson.

“If the farm budget allows, farmers can use N fertiliser to boost pasture production and cover for the lack of clover, however there are no quick fixes and they shouldn’t adopt any unproven miracle cures.”

Based at AgResearch’s Invermay campus, Mr Ferguson has seen the damage wrought by the weevil on Southland farms first-hand.

“The clover root weevil has been present in Southland since 2010, but large numbers were present in only a few locations.” he says.

“That was until last year – what we didn’t need was the very mild winter.

“The mild conditions allowed a greater number of clover root weevil eggs to hatch and more of the larvae survived right through the winter. As a result the population of the weevil just exploded.

“Tremendous pressure is now being placed on pasture from the increased numbers of larvae that fed on the clover roots and their nodules last winter. That damage was followed by the large population of adult clover root weevil emerging in early summer and feeding on the clover leaf. Normal spring and summer grazing of clover has added to the stress on already struggling plants.”

“Affected farmers are either seeing clover that disappears very quickly once grazed or a complete absence of clover plants in their pasture. Without clover farmers can’t fatten lambs and those that are looking to overwinter dairy grazers can’t do that. It is having a huge impact on farm returns right now.”

There is a solution to this difficult situation and it comes in the form of a tiny parasitic wasp from Ireland.

The Irish wasp is a highly effective bio-control of clover root weevil. AgResearch scientists have seen reductions of greater than 90% of the clover root weevil population in monitored North Island farms where the wasp has been released.

AgResearch scientist Dr Scott Hardwick leads the Lincoln-based clover root weevil collection team.

“We are literally vacuuming up weevils from Canterbury farmland where the bio-control wasp is already hard at work,” he says.

“While most of these collected weevils are already parasitised, we boost the parasitism levels by adding a few Irish wasps to the packs of 100 we make up for farm release. These packs are then delivered to Southland for release. The result will significantly accelerate spread of the bio-control and the process of clover recovery will begin.

“Southland farmers can expect clover content to return to normal levels two to four years after the wasps’ arrival on their farms.”

Dr Hardwick advises farmers not to worry if their farm is not one of the selected release sites.

“In Canterbury and North Otago we have seen the Irish wasp spreading up to 30 km per year from release sites, so even if your farm is not one of selected release sites, the Irish wasp released on your neighbour’s farm will quickly arrive on your farm.”
Beef + Lamb New Zealand, DairyNZ, Environment Southland and AgResearch are working together to ensure that farms selected as release sites will have the maximum impact for the region.

Mr Ferguson advises farmers not to panic.

“While this is causing a lot of pain at the moment it will pass over time. Although things are going to be tough for the next few years farmers will get through this with the help of the bio-control. North Island farmers have already gone through this process and clover root weevil is not much of an issue now for any of them.

“If we had let things run their natural course the Irish wasp would have continued to spread down the South Island, following the clover root weevil, and eventually arrived everywhere in Southland. These actions by the industry this autumn will considerably speed up this process and make a huge economic difference to Southland.”

Southland farmers who suspect they may have clover root weevil – it can be identified by the distinctive U-shaped notches on clover leaves made by the adults – should attend one of the Beef + Lamb New Zealand clover root weevil field days being held over the next month. Four are scheduled. On March 17 one will be held in Waikoikoi starting at 10 am and in the afternoon another will run at Glenham starting at 3 pm. The following day, March 18, a workshop will be held in Winton starting at 10 and another will follow in Mossburn starting at 3 pm. Contact Beef + Lamb New Zealand Extension Manager, Paul McCauley for more information paul.mccauley@beeflambnz.com

Farmers can also contact their local Beef + Lamb New Zealand and DairyNZ consulting officers for more advice. DairyNZ also has a Farmfact on managing dairy pastures with clover root weevil which includes advice on how to support clover depleted pasture.

More information about the weevil and how to recognise it can be found at agpest.co.nz, the directory of New Zealand’s most damaging pests and weeds. Farmers can also sign up to receive management and control advice specific to their region through the AgPest Pest Alerts.

For further information or a high resolution of the above image please contact:
Alex Fear
Senior Communications Advisor – AgResearch
Phone: 07 834 6636 / 021 773 674
Email: alex.fear@agresearch.co.nz