This is the furthest south the clover root weevil has been found since its discovery in the Waikato in 1996 and in the South Island 10 years later.
AgResearch Invermay entomologist Colin Ferguson stumbled across the weevil recently while visiting a Clinton sheep farm after taking pasture samples out of general interest, but also to check for signs of the weevil's presence.
The weevil was a native of Europe and could be expected to handle southern winters.
Mr Ferguson said a wasp imported from Ireland had successfully attacked the weevil in the North Island, but it needed a large population to establish and survive.
Mr Ferguson said the weevil was likely to be on farms neighbouring the one where he made his discovery.
"The discovery was on a sheep farm, but there are many dairy farms in the area and dairy farms tend to better support clover root weevil populations."
The pest was so tiny it was difficult to spot, but the cost to the industry could be huge ($300 million to $500 million) through lost production and the need to compensate (with fertiliser) the reduced nitrogen-fixing by damaged clover plants.
Early signs of infestation were uniform, match-head size notches out of clover leaves, but as the population grew, Mr Ferguson said, the clover content in pasture reduced and eventually the pasture took on a yellow colour due to a lack of nitrogen in the soil and plants.
The weevil most likely hitched a ride south on a vehicle, farm equipment or on stock, as he did not think it would have come south under its own steam.
It was well established in the North Island, Nelson and Marlborough and there were large populations in parts of Canterbury.
It was inevitable it would find its way south, he said.
Mr Ferguson was not certain how the weevil would react in the South, but in the North Island it had decimated clover crops, requiring nitrogen fertiliser application to maintain production and for resowing.