Bay of Plenty Regional Council is employing the help of the Californian thistle stem miner to combat the spread of problematic thistles in the Lower Kaimai.

An adult Californian thistle stem miner.

Bay of Plenty pest plant officer Walter Stahel says the adult weevils were released this week.

'Californian thistle is an aggressive pest plant that's well established in the Bay of Plenty,” says Walter.

'Control of the thistle costs our farmers millions of dollars each year in control and lost production through sprays, tractor time and loss of pasture alone.”

He says it is the weevils' larvae which does most of the damage to the thistle.

'The damage is caused by the larvae mining in the stems and roots which either kills the plants or reduces the plants' ability to compete with other vegetation.”

Along with the newly introduced Californian thistle stem miner, the green thistle beetle was also introduced to the Lower Kaimai area this week.

The green thistle beetle was successfully released in the Taneatua area in the Eastern Bay of Plenty earlier this year.

Walter says the release of these insects was not a quick fix as their introduction would require ongoing and wider on-farm thistle control.

The Environmental Risk Management Authority (ERMA) has approved the release of the Californian thistle stem miner as a biological control agent.

Sites where the weevil is released will be monitored and when sufficient numbers have been established, the weevil will be redistributed to other parts of the Bay of Plenty.

Bay of Plenty Regional Council assists Landcare Research with funding for biological control programmes. This weevil joins a group of seven biocontrol agents to be released throughout the region in the past year.