Warning on Clover Root Weevil

(450 words)

Improving growing conditions for white clover can help reduce the impact of the clover root weevil, scientists say.

Han Eerens, of AgResearch’s plant breeding and genomics group at Ruakura, says farmers can help reduce weevil damage by changing pasture management.

"Early indications are that using a less aggressive grass like tall fescue instead of ryegrass, grazing hard in spring and more lax over summer, with a faster than usual rotation and higher post-grazing residues, increases the amount of clover grown," Dr Eerens says.

"Look after the clover and reduce the production pressure on the system, and clover root weevil is much less of a problem than when the whole farming operation balances on a knife-edge," he says.

AgResearch’s work into clover root weevil is jointly funded by WoolPro, Meat New Zealand, the New Zealand Dairy Board, the Game Industry Board and Foundation for Research, Science and Technology.

WoolPro research manager Ken Geenty says the weevil, found in March 1996 near Tauranga, has spread to an area from Whangarei to around 30km south of Te Kuiti. Further spread is inevitable, he says.

Damage of white clover plants by clover root weevil is widespread already this summer and autumn. Warm and moist conditions in early summer meant exceptional clover growth and very high egg production by the clover root weevil adults.

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Ruakura entomologist Bruce Willoughby reports weevil populations are higher than ever and there’s been enough rain to produce two generations this year. Numbers per square metre in December in the Waikato were 1515, four and five times the level in the two previous years.

Young larvae can each destroy nine to 12 clover root nodules as they develop, greatly reducing the legume’s ability to fix atmospheric nitrogen.

More mature larvae damage the roots and stolons, and adult weevils eat the leaves. This combines to greatly impair the productivity and persistence of the white clover.

Dr Eerens is also identifying white clover plants which are resistant or tolerant to the weevil. "At one of the trial sites most of the transplanted white clover plants were destroyed by the clover root weevil. But multiple plants of a few lines survived, and were productive, even though their roots were covered in weevil larvae."

These surviving plants are currently being crossed and the progeny will be further tested, he says.

Dr Geenty says other research includes:

• A large biological control effort, with an AgResearch programme, Operation Sucker, sourcing weevil samples from sites throughout Europe including the Mediterranean and Arctic Circle
• DNA analysis of clover root weevil
• Microbial control possibilities including the promising fungus Beauveria bassiana, which occurs naturally in New Zealand. Several products using this fungus are already used around the world.
• Pasture renovation options to re-establish clover.

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Farm editors

For more information about the control of Clover Root Weevil, contact: WoolPro research manager Ken Geenty, tel 06 356 8611.