Thistle populations in the Bay of Plenty have exploded and are causing a headache for farmers.
Federated Farmers' Bay of Plenty provincial president Rick Powdrell said he had thistles to varying degrees on most of his property because of last year's drought.
"When you have a drought the pastures get grazed heavily because farmers haven't got feed to maintain cover on their paddocks.
"In an extreme drought the pasture dies and opens up. We live in the Bay of Plenty, which is weed heaven, so all the weeds germinate and establish," he said.
The weed also caused problems for stock and hindered grass production.
"Lambs, in particular, can get a scabby mouth eating thistles and around them. They get sores."
Wool sent to scourers with thistles in it was also severely downgraded and would fetch lower prices, he said.
Farmers combat thistles by spraying or using weed wipers and grubbers.
However, some were fighting back using biological methods.
In 1980, Mr Powdrell and Bay of Plenty Regional Council introduced a biological weevil on his farm.
The weevil laid eggs on the thistle head so its grubs could dig their way down and eat all the seeds but it tended to exit mid-January when it got too hot, he said.
Thistles, particularly the nodding variety, have a long seed life and can stay in the ground until conditions are ripe to germinate.
Another weapon is the gall fly that works later in the season and complements the weevil.
Bay of Plenty Regional Council land management officer Andrew Blayney said it had fielded a number of enquiries about the increase of thistles in the Western Bay, particularly Californian thistles, Scotch thistles and nodding thistles. It worked with landowners to carry out pest control.
"Biological control is the use of one organism (not always just a 'bug' or insect; sometimes a fungi or some other kind of organism) to control another."
After the drought last year, pasture vigour was severely reduced and because the thistles had been at low densities for some time so were biological agents, he said.
"The good news is with so many thistles to eat this year, we would expect the biological control agent populations to increase very quickly and be able to help farmers control the thistles."
However, chemical control was a good option if thistles were a significant problem, he said. "There is often an explosion of pasture weeds after a dry summer - the pasture becomes a bit 'threadbare', allowing space for the weeds to germinate.
"Farmers sometimes respond with heavy grazing. However, that risks exacerbating the problem by trampling weed seeds into the ground and thinning the pasture further. Where possible, light grazing is the best option so as dense a sward of grass is maintained as possible."