Fighting back against the ‘evil weevil’

Seven years after its release in New Zealand a little wasp imported from Ireland is reducing the clover root weevil pest by around 90%in monitored areas.

The prediction that clover root weevil would eventually colonise nearly all of New Zealand pastures is being realised. However, the wasp’s level of parasitism is much higher in New Zealand than in its native Ireland and therefore it is a much more effective control agent here.

But the wasp is not a “silver bullet”, say AgResearch scientists. The wasp works best where there is plenty of clover, and an on-going supply of clover root weevils present. This cycle is disrupted by droughts.

By the time clover root weevil was first identified in New Zealand in 1996 it had become widely established on farms around Waikato, Bay of Plenty and Auckland.

The New Zealand pastoral system is dominated by ryegrass and white clover, which are highly productive and easy to manage. Clover is a critical component of the pasture sward. It is highly nutritious which leads to higher animal productivity. In addition, it fixes atmospheric nitrogen which helps drive vigorous ryegrass growth.

The discovery of the clover root weevil was therefore widely regarded as a serious threat to the New Zealand pastoral industry as clover had no resistance to this new herbivore. Young larvae, mature larvae and adult weevils each attack different parts of the clover plant. In 2005 the New Zealand Institute of Economic Research estimated that, if nothing were done to control the weevil, then it would cost the pastoral sector over 200 million dollars a year, with farm margins cut by 10 to 15%.

A sector-wide task force considered three control options – breeding resistant clover, farm management practices and biocontrol. In addition, farmers formed a Clover Root Weevil Action Group which provided a strong farmer-based support group for practical on-farm research and extension.

A team of AgResearch scientists was assigned to look for a biocontrol agent. One of the team members, entomologist Dr Pip Gerard, says that after an extensive search a parasitic wasp from Ireland was identified as a likely prospect. The tiny wasp, which feeds on nectar and doesn’t sting humans, lays its eggs in the adult clover root weevil where they hatch and grow before emerging from their host “just like in the film Alien”.

“Significantly all clover root weevils are females so only one individual is required to start a population,” says Dr Gerard.

After permission was gained from ERMA to bring the wasp into New Zealand it was released for the first time as a biocontrol agent for the clover root weevil in 2006. By that time, clover root weevil had spread throughout the North Island, and had just been detected for the first time in the South Island after AgResearch found two localised infestations in the Nelson region, plus a few specimens in pasture near Christchurch.

“We have an excellent partnership with DairyNZ consulting officers which meant we could distribute wasps to farmers throughout the North Island over a reasonably short time,” said Dr Gerard.

“Beef + Lamb New Zealand regional managers also provided great support organising their groups of farmers to release the biocontrol agents.”

In the South Island, AgResearch immediately released the biocontrol agent in the Nelson infestations in the hope the wasps would spread along with clover root weevil. This proved to be the case and, by 2010, both the weevil and the wasp had spread themselves southwards to Kaikoura. However, additional, separate widely separated clover root weevil infestations were found in Canterbury and Otago in 2008-09, and in Southland in 2010, which meant additional wasp releases had to be made.

To date, AgResearch, with support from DairyNZ through the dairy farmers levy and Environment Southland, has made a total of 91 wasp releases in Canterbury, Otago and Southland. In these areas the clover root weevil has appeared as isolated, but sometimes very damaging, populations which the wasp would have taken several years to reach by itself.

The AgResearch team also set up a number of monitoring sites throughout the country to track both the spread of the weevil geographically and their population concentrations.

The results were outstanding. The weevil populations started to drop within a year.

“We’ve found that the wasp is following the weevil with its natural spread, so as the weevil spreads so does its biocontrol agent,” says Dr Gerard.

“That’s great news.”

Human-assisted dispersal of the weevil has also been apparent in the South Island with new populations found 250 kilometres apart. This led to the further release of wasps in a number of locations throughout the South Island.

“These evil weevils are very good fliers and hitch-hikers. They can easily get onto hay and vehicles so they can move considerable distances,” says Dr Gerard.

Where the wasp has established, the level of weevil infestation has been kept below the damage threshold where previously it was causing greater financial loss to farmers. The wasp is part of a dynamic system – weevil levels may surge but if the wasp is present it will naturally respond. With the abundant clover in recent years, there was a resurgence of weevils, but the Irish wasp has caught up with high parasitism levels this winter.

Farmers with clover root weevil are also advised to adopt some farm management practices to maximise clover and biocontrol benefits on their properties. These include the application of small amounts of nitrogen fertiliser after grazing in spring and autumn, and intensive grazing in spring to allow the clover to thrive. If re-establishing pasture, a fallow of 2-3 months or including non-host crop such as brassica or maize in the rotation will greatly reduce the burden of clover pests and pathogens in the soil. These tips are available through Beef + Lamb New Zealand, DairyNZ and at AgPest.co.nz.

The scientists involved have been widely recognised for their success in identifying such a highly successful biocontrol agent and demonstrating the results of quality science working with farmers.