“It’s probably worse this year than it has been for some time,” said Malcolm Deverson, project manager for the Clutha Agricultural Development Board, which hosted a pub lunch to discuss the issue at Clydevale recently.

A couple of dry summers had really set up ideal conditions for grass grub infestations, Mr Deverson said. It was more of an issue in lower rainfall areas where soils were still dry, rather than in areas with wetter soils, he said.

In South Otago, grass grub damage had been reported in the Clydevale, Clinton, Waiwera South, Warepa, Owaka and Milton districts.

Wider afield, there have been reports of damage to irrigated pastures in drier parts of Otago, in Canterbury and central Southland.

AgResearch Invermay scientist Colin Ferguson said it was common for grass grubs to cause serious pasture damage three years after resowing, but some farmers were now reporting problems year after year.

The population explosion is believed to be caused by an imbalance between grass grub numbers and pathogens that regulate their populations.

Usually the two co-exist, maintaining a moderate population of grass grubs to pathogens. However, during drought conditions or when pastures are resown, the pathogens and grubs die out but grass grub numbers recover faster and their population explodes.

At its worst, there could be up to 20 grubs in a single spadeful of an affected paddock.

Mr Ferguson said grass grubs were native to New Zealand and there was no way of eradicating them. The best defence for farmers was to maintain the natural balance between grass grubs and their pathogens.

Farmers were advised to keep pastures healthy by not overgrazing and maintaining adequate nitrogen levels, because healthy pastures would tolerate more damage than unhealthy pastures,

Direct drilling was also believed to cause less damage to the grass grub and pathogen balance and thus less serious pasture damage.

The insecticide Diazanon offered a “chemical fix” but a number of farmers at the Clydevale workshop were not keen on using a broad spectrum insecticide because of inevitable damage to earthworm populations and soil micro-organisms.

Mr Deverson said farmers were advised to check pastures in January and February each year to monitor larvae numbers.

“It’s just another worry for farmers that they don’t need,” Mr Deverson said. “But a bit of planning and action at the right time might just avoid the really serious outbreaks from continuing.”