The weevil had been found in Clinton and was established throughout much of the South Island, but Environment Southland biosecurity manager Richard Bowman said it would be established in Southland within a year.
Mr Bowman told an Environment Southland field day in Gore last week that southern farmers needed to start looking for tell-tale signs the weevil was present.
That included notches in clover leaves the size of a match head, and declining health in clover crops.
Once established, the pest could build to densities of up to 1400 larvae per sq m.
Clover damage started occurring at 300 larvae per sq m, which was a more typical infestation.
The weevil could result in loss of available plant nitrogen of up to 300kg per hectare per year.
AgResearch scientist Colin Ferguson said he hoped to establish whether there were any other populations in Clinton, following the initial discovery in February.
The weevil was first discovered in Auckland in 1996 and then spread to North Island farms before crossing to Marlborough and Nelson in 2006.
From there, it spread to Christchurch.
Mr Ferguson said the weevil was particularly adaptable and could have hitched a ride on a car, on livestock or in hay.
Mr Ferguson said in its native northern hemisphere domain, with natural predators and disease to slow its spread, weevil populations of 30 to 90 per sq m were common.
But they were much greater in New Zealand because of the abundance of clover and lack of competition.
Dairy farms were more susceptible because they tended to have higher vegetation cover, more clover and reliable moisture, either naturally or from irrigation.
North Island farmers had learnt to live with the weevil.
Mr Ferguson said one tactic to counter it was to grow a break crop - either maize, an annual ryegrass or winter crop - as it needed clover to survive.
This could give farmers a reprieve for a year or two, but if the weevil was in the adjoining paddock, it would soon repopulate a newly sown paddock.
North Island farmers were also regularly applying small amounts of nitrogen to compensate for the damaged clover and to keep pasture growing and in a healthy state.
Mr Ferguson said early detection was the key, as a natural biological control wasp could be introduced and although it would not stop the weevil spreading, it could certainly slow the process.