Last week, AgResearch Grasslands plant breeder Chris Mercer, found specimens on the roadside of State Highway One near Marton, and at a site in Palmerston North.

The discovery has come as no surprise to Mr Mercer, as this pasture pest has been spreading north and south from its initial discovery site in the Bay of Plenty in the mid 1990’s. 

Where it is firmly established, the clover root weevil reduces clover content and nitrogen fixation.  “On one Waikato farm where sward composition has been monitored, clover levels fell from 30 percent to about 10 percent”, Mr Mercer said.

The weevil larvae feed underground on the nitrogen-fixing nodules and roots of white clover, reducing production and persistence.  The adults live on the surface and feed on clover leaves causing “U”-shaped notches on the leaf edges. 

These notches are quite distinct from slug damage - slugs usually leave behind some leaf membrane creating a window, but adult weevils eat cleanly through the whole thickness of the leaf.  The adult weevil feeds at night when the leaf is folded up, so many notches are in opposite pairs. 

Clover is subject to many stresses that affect the ability of the plant to survive.  “Clover root weevil, especially when new to a region, can produce the type of stress that becomes the proverbial straw that breaks the camel’s back,” he said. 

The Waikato Clover Management group, convened by dairy farmer Lorraine Bilby, has identified some strategies for farmers to use to reduce the stress on clover. 

In their information leaflet distributed widely by Ballance Agri-nutrients to farmers in regions affected by clover root weevil, the group suggests that in the spring, farmers should avoid rank pasture which would shade and weaken clover.  Farmers should consider boosting nitrogen over this period to take advantage of the better growing conditions. However, it is important to graze or cut the pasture before the nitrogen-boosted grasses start to shade out the clovers. 

In the summer, pasture managers should keep sufficient cover on pastures to protect the clover stolons from direct sunlight.  Clover root weevil populations do not build up in dry summers, which can lead to clover resurgence in the autumn. 

Autumn management advice again emphasises care with grazing to ensure that clover is not out competed.  Options in winter are more limited but avoid pugging at all costs. 

BACKGROUND

Clover root weevil research – what’s happening

In addition to developing this pamphlet, the group is seeking funding to identify management practices to maximise pasture production in the presence of clover root weevil.  “Farmers need immediate information on how to cope with this new pest”  said Helen Moodie, Northern Region convenor of the NZ Landcare Trust.  “Currently there is no work being carried out on what farmers can do to maximise production in the presence of clover root weevil. The Waikato Clover Management group aims to help farmers develop such strategies in the short-term before the long-term control options are available from the Crown Research Institutes work.”

   
With support from the NZ Landcare Trust,  Waikato Clover Management has been instrumental in getting clover re-establishment trials underway. They have yet to resource their main trial – identifying recommendations for the use of nitrogen fertiliser to maximise clover growth.

Longer-term control studies by researchers are centred on breeding red and white clovers tolerant of the weevil, and on biological control.  AgResearch Ruakura plant breeder, Jim Crush, is screening a wide range of clovers, and has found in on-farm trials, new generation creeping red clovers are showing better persistence under dairy grazing than traditional red clovers and are less attractive to the clover root weevil than white clover. 

However, Dr Crush’s conclusion is that well-adapted, vigorous white clover cultivars are able to cope better with the additional stress imposed by the weevil.  “Good pasture management and attention to fertiliser requirements can help farmers survive the root weevil,”  Dr Crush said at a recent Australasian Conference on pasture insects, held in Christchurch. 

At the same conference, researchers learned about the biological control efforts being undertaken.  A major overseas search for parasites saw weevil researcher Dr Steve Goldson collecting insects in the USA and the European Union- particularly in Ireland.  One strain is now being evaluated in AgResearch laboratories for its potential as a control agent. 

“It’s early days yet,” Dr Goldson emphasised at the conference, “but we are hopeful that an introduced parasite could contribute towards controlling the clover root weevil. The goal is an integrated pest management package consisting of better clovers, better management of clovers and the biocontrol agent.”

While collecting parasites in Europe, Dr Goldson also collected diseased weevils.  Spores from these have been multiplied up by insect pathologist Dr Travis Glare in AgResearch’s Lincoln laboratories and shown to be effective in controlling weevil larvae.  “The work is now extending into full field trials with novel formulations and application methods, with the aim of providing a prototype biopesticide within the next two years,” Dr Glare said. 

For further information, or a copy of the pamphlet, please contact Chris Mercer, AgResearch Grasslands, phone 06-351-8017.