In the evolutionary arms race between the Argentine stem weevil and the parasitoid wasp, the weevil has for the moment got the upper hand - at the cost of about $200 million a year to agriculture. 

The story of the search for a biocontrol which could rein in the weevil which eats ryegrass, and the subsequent fight back by the South American invader, goes back to the 1990s and has attracted worldwide attention.

Scientists believe it the first time that a pest has out-evolved a parasite, the only example among all of the biocontrols used globally.

Research entomologist Professor Stephen Goldson, who works for AgResearch and the Bio-Protection Research Centre, worries other pests might also gain the upper hand over the parasites introduced to control them.

Besides the stem weevil - which threatens New Zealand's most important pastoral grass - there are the clover stem weevil and the lucerne weevil, which also cause economic damage of more than $200m a year.

Like many introduced pests to New Zealand without natural predators, the stem weevil populations took off out of control as soon as they arrived. No-one knows exactly how it came in, but it is part of the constant pressure caused by unwanted new arrivals says Goldson.

By 1993, scientists were finding 700 weevils per square metre compared to just one per sq metre in their native South America.   

Read more: https://www.stuff.co.nz/business/farming/112317761/argentine-stem-weevil-fights-back-costs-agriculture-200m-a-year