The clover-killing clover root weevil has been found in four places on the Taieri Plain, meaning farmers might have to use more chemical fertiliser to encourage pasture growth.
Scientists were yesterday still trying to determine the extent of the outbreak, which was discovered by AgResearch Invermay principal scientist Barbara Barratt when weevils twice landed on her in recent weeks.
The weevils eat the roots of clover, killing the plant which is the main way nitrogen is naturally transferred to the soil.
To compensate for the loss of clover and to maintain pasture growth, affected farmers elsewhere in the country have had to apply more chemical nitrogen.
AgResearch Invermay scientist Colin Ferguson said for weevils to be flying indicated they were widely dispersed around the Taieri.
The weevil had been confirmed on properties at Wingatui, Mosgiel and Invermay, and pasture damage had been found at the Otago-Taieri A and P Society showgrounds, he said.
It had become widely established in the North Island since arriving in 1996, and in 2006 it was found in Canterbury, then Nelson, and a year ago at Clinton.
Mr Ferguson said the weevil was quite resilient and could have hitched a ride in a vehicle or on hay.
Mr Ferguson urged Taieri farmers to look for signs of the weevil - match head-size notches on the edge of clover leaves - in the hope that if a sufficiently dense population could be found, a parasitic wasp could be released to control the pest.
