Irish wasp more than match for clover root weevil

Parasitiod (Irish wasp) laying egg in a clover root weevil

The Irish wasp is back to battle the clover root weevil this summer, after a slump in numbers.

Like many farmers in the northern North Island, Mike McKie from Opunaki has been benefiting from plentiful clover in recent years, but is now alarmed at the reappearance of the distinctive feeding notches of the adult clover root weevil and the number of larvae in the soil.

“We’ve not needed to apply nitrogen and been carrying 320 cows over the past two years, but are now down to 255 and facing higher inputs,” he says.

However, AgResearch entomologist Dr Pip Gerard, the leader of the DairyNZ-funded Pasture Pest Biocontrol project, assures farmers that the tiny Irish wasp, the biocontrol agent introduced to control the weevil, is up to the task and no more releases are required in the North Island.

“It all comes back to summer rainfall,” says Dr Gerard.

“The severe 2008 drought hit not only the pastures, but also the clover pests and pathogens, and what weevils were left got mopped up by the wasp. With above normal summer rainfall in 2010/11 and especially 2011/12, and few pests and pathogens, clover has boomed. Unfortunately, wet summers and plentiful clover are also perfect conditions for clover root weevil and being the egg-laying machines that they are, populations have increased very rapidly from low levels.

“It got ahead of the tiny Irish wasp in 2011, but the results of our survey this winter show the wasp’s bounced back with a third of farms in Waikato, Bay of Plenty and Taranaki having excellent biocontrol (greater than 90% parasitism) and an equal number having good control (60-90% parasitism).”

Dr Gerard is very pleased about the performance of the wasps. “Nevertheless, I still have some concerns,” she says.

“The Irish wasp is still struggling north of Auckland, and may never be highly effective there because conditions are so different from its Northern Ireland homeland.”

“In addition, the team has found the closely-related biocontrol agent for the Argentine stem weevil is still well below effective levels, averaging only 19% parasitism this winter in the North Island. To support the biocontrol they strongly recommend that farmers use treated seed when renovating or undersowing pastures and always sowing ryegrasses with endophyte.”

In the South Island, the clover root weevil is still spreading, with the Irish wasp close behind.

Surveys on the West Coast showed the weevil is present near Reefton and Westport, but absent from Greymouth through to Franz Josef Glacier. The Irish wasp has spread with clover root weevil down the Buller River valley from Nelson as far as Reefton. On the east coast, the Irish wasp has continued to spread south and has met up with Irish wasp populations spreading east from a release site in the Culverden basin. Further south, a number of successful releases of the Irish wasp have been made in response to new clover root weevil populations detected in Canterbury, South Otago and also Southland.

Of the approximately 3.5 million hectares of improved pasture in the South Island, about 37% is now infested by the weevil, and with the help of release activities and natural dispersal, the Irish wasp is present over 50% of this infested area.

Farmers wishing to retain clovers and optimise the efficacy of the biocontrol agent in their pastures are advised to take a “little and often” approach to nitrogen applications, not to let pastures become rank and ensure sufficient cover in summer to protect the vulnerable clover stolons from the sun.

Map showing spread of clover root weevil and Irish wasp

Map showing spread of clover root weevil and Irish wasp

Further information on clover root weevil, including what to look for, can be found on AgPest(, and at, including an up-to-date map of distribution of the weevil and Irish wasp and AgResearch contacts: Advising AgResearch at the earliest possible opportunity that clover root weevil is suspected in a new South Island location will help scientists to ensure the biocontrol agent is quickly dispersed to the area.

Dr Pip Gerard, AgResearch Ruakura
T: 07 838 5103

Alex Fear, Senior Communications Advisor, AgResearch
T: 07 834 6636