Super-hungry beetles kill off pest

Dr Mike Cripps (left) shows landowners the result of an insect used to control California thistles. Photo / Laurel Stowell

Farmers are hoping beetle larvae with voracious appetites will curb a $685 million loss from New Zealand agriculture.

About 40 farmers gathered at Fraser Laird's farm in Fordell, Whanganui, afternoon for a field day about green thistle beetle, a European insect introduced to the country in 2007 to control California thistles.

The field day was run by Agresearch and Beef + Lamb NZ.

Farmers got to see a population of beetles and larvae inside a cage. They had pretty much cleaned up the Californian thistles and were starting on a Scotch thistle.

Green thistle beetle larvae finish up the California thistles and start on a Scotch thistle.

Afterward there was a food and drink, and a draw to win one of four beetle populations to take home.

Californian or creeping thistle is a problem all over New Zealand. Farmers from Taihape, Fordell and Marokopa in the King Country all said they were affected. Tim Matthews, from the Whangaehu Valley, had seen thistles as tall as himself in Raetihi.

AgResearch scientist Graeme Bourdot said 97per cent of sheep and beef farms had the creeping thistle, and for some it covered 12 per cent of pasture area. Infestations were worst on bare, droughty or overgrazed land.

The thistles spread mostly through shoots from their creeping underground roots - most of their seed is eaten by birds or rodents.

The underground roots only live for a year, and the amount they grow depends on the amount of leaves the plants are able to grow in that season.

More leaves make for more underground roots, and more thistles next season.

Therefore defoliation is a good way to reduce numbers. Mowing in December and February is best - especially if it can be done during rain, when pathogens like fungi get into the cut stems and kill whole plants.

Spraying is another way to defoliate, and best done when the plant has lots of leaves and is actively growing.

A dense pasture of grass and clover will also suppress the thistle.

AgResearch scientist Mike Cripps said green thistle beetles were introduced to New Zealand after other failed attempts at biocontrol.

He discovered their larvae will eat all kinds of thistles, after providing them with a "thistle buffet".

They are safe to release here because New Zealand has no native thistles they can wipe out.

The beetles are now in across most of the country and having good defoliating effects in the Wairarapa.

It could be years before their populations build up enough to make a big difference.

The beetles usually live only a year. Adults winter over under trees or debris on the ground, with females coming out in spring and each laying hundreds of eggs. The larvae eat thistles, then pupate and become adults.

They have no New Zealand predators and breed up rapidly.