Clover Root Weevil Action Group

Clover root weevil is silently eating profitability under New Zealand pastures and farmers must have answers for this scourge. The NZ Clover Root Weevil Action Group has instigated research and published recommendations for the on-farm management of pasture clover in the presence of clover root weevil (CRW). A range of responses including nitrogen fertiliser applications are necessary. The findings and recommendations have relevance to farmers in regions where bio-control of CRW is not working or in years when it is less effective, and to regions where CRW is establishing for the first time.

Clover Root Weevil first appeared in Waikato and Bay of Plenty about 10 years ago.

Chronic infestations of CRW (Sitona lepidus) severely compromise nitrogen fixation in the root nodules of white clover, with subsequent effects on plant growth and the contributions to pasture and livestock performance.

On farms where clover is under stress, from winter pugging, for example, the first infestation of CRW can be catastrophic, giving the appearance that clover has been wiped out.

A review of the studies on the impact of CRW on white clover dry matter production has shown that losses over the medium term were generally in the range from 18-35%. If a “do nothing” approach is adopted, the weevil could potentially cost the pastoral economy around $400 million a year. Economic assessments at farm system level using decision support models showed that farm gross margins would fall by 10-15% with that do nothing approach.

The Irish parasitic wasp Microctonus (“little murderer”) aethiopoides, is being distributed throughout the country as a bio-control agent and can help to control CRW numbers. But its impact is affected by both environmental conditions and pasture management. In Northland, the parasitic wasp has not established, although in regions further south it has established and is spreading rapidly. However, even if the wasp is present, how effective it is at controlling the weevil appears to depend on how well pasture clover is managed.

The NZ Clover Root Weevil Action Group has fostered farm management trials alongside the bio-control programme. It obtained funding from MAF’s Sustainable Farming Fund and from AGMARDT, over the past four years and recently drew together the findings of commissioned research work.

The advice that the action group has received is that a combined approach is required to maintain healthy and productive clover in the presence of CRW.

Firstly, applying small amounts of nitrogen fertiliser after grazing in spring and autumn can help clover withstand CRW larval attack (on roots and nodules).

The right quantities of N will vary with the individual farm nutrient management plan.

A field trial was undertaken at three sites to compare pasture responses to four experimental rates of urea (0, 100, 200 and 400kg N/ha/year) under two contrasting grazing management regimes.

The main recommendations that can be drawn from this research are:

• Be aware that using high rates of N fertiliser may increase pasture vulnerability to drought.

• Even when using N fertiliser, clover content can be lifted by aiming for low post-grazing residuals in spring, autumn and winter. This prevents shading of white clover by grasses to maximise clover growth.

• Higher post-grazing residuals should be maintained in summer, to provide protection for the clover growing points from the summer elements.

• Although there was insufficient data to make a firm recommendation on the preferred N application rate in the presence of CRW, the 200kg/ha/year performed well at one site in terms of pasture production, N fixation and weed content.

The researchers also suggested a lower overall annual rate (e.g. 150kg/ha/year) applied in split dressings after grazing from mid-autumn to late spring might be economically and environmentally more preferable.

This may also increase overall clover content by shifting the competitive balance between grass and clover pasture components in clover’s favour during the summer.

Based on 2007 figures, provided pasture N response is in excess of 15kg DM/kg N, the 150kg/ha will be enough to restore farm gross margins (estimated to fall 10-15% after severe CRW attack).

Secondly, pasture renovation using a non-host plant such as brassicas or maize, removes pests such as CRW and nematodes so clover can be re-established into a “clean” site.

A paddock-scale field trial has shown that two-year-old pasture following a maize or turnip crop had more clover, fewer weeds and higher autumn production compared to pasture following grass-to-grass establishment.

The NZ Clover Root Weevil Action Group has developed a pamphlet summarising the advice above.

Further advice can also be found on www.cloverrootweevil.org.nz.

In regions of the South Island, like Nelson, Marlborough and Canterbury, where clover root weevil is establishing for the first time, farmers can use the management strategy while waiting for the bio-control option to establish.

DAVID WILSON:

David Wilson’s dairy farm was one of the trial sites for the Action Group SFF/AGMARDT-funded work. He has been putting on small applications of nitrogen after each grazing for about 10 years. Each application of 20 units of N and each paddock is grazed about 8 times during the year, so that means a total of 160kg N/ha/yr. David is pleasantly surprised with the level of clover in his pastures this autumn, but during last year’s drought it was a different story. About 20% of the farm is being undersown this year to help pastures recover from the drought. David is not a fan of pasture replacement, believing that the new cover doesn’t last. “If we had more clover (which we don’t because of CRW), then it would fill the gaps in the new sowing and lead to a better outcome from new grass,” he said.