Weather or weevils? It pays to check

Is it the weather or is it weevils? That’s the question farmers should be asking if poor pasture growth is threatening on-farm productivity.

Clover root weevil is being reported across the country and especially in the Lower South Island where its prevalence is particularly high this summer. Nodules on clover roots fix nitrogen from the atmosphere and provide a 'free' form of nitrogen fertiliser. Weevils feeding on them disturb the nitrogen fixing with subsequent damage to foliage and pasture quality.

"Many farmers may be putting slower pasture or animal growth rates down to lack of sunshine and overcast weather given the mixed summer we have had. However clover root weevil may also be an issue on their properties and is often a hidden cause of poor pasture productivity," says Ballance Agri-Nutrients Research and Development Manager Warwick Catto.

A U-shaped notching in clover leaves signals infestation and spells trouble. This is especially so with the loss of clover significantly reducing potential lamb growth rates.

"Dairy farmers have also reported substantial loss of productivity due to clover root weevil. An infestation can reduce nitrogen fixation by clover by 50% to 100%. Currently in the Lower South Island famers have reported up to 100% knockout of clover due to infestation."

Applying nitrogen fertiliser can compensate for weevil damage, though Mr Catto says the amount needed will vary with soil type, fertility and climate.

"Small and frequent applications of nitrogen fertiliser applied year round will assist to improve clover growth and plant survival compensating for the loss of normal nitrogen supply that would come from the plants own nodules.

Mr Catto cautions that while the nitrogen applications will go some way to minimising the impact on productivity, it will not eliminate the pest or its impact.

"Farmers should talk to their fertiliser consultant to get the best plan of attack for their particular farm and conditions."

A biocontrol has been released by AgResearch in the form of a tiny parasitic wasp first released in 2006. AgResearch is monitoring the spread of the weevil in the South Island and making additional releases of the biocontrol agent as required.