Jack Simpson

The Argentine stem weevil is a pasture pest that has caused significant damage to crops since it was introduced to New Zealand in the 1920s. In an effort to control the weevil, a wasp was introduced from South America as a form of biocontrol.

Whilst initially a success, the effectiveness of the wasp to limit weevil numbers began to decline within 7 years of its introduction to the country. A recently published paper looking at the past 20 years of data now tries to explain why this change has occurred.

What are biological controls?

Biological control is a technique where a new species is introduced into an area in the hopes of controlling or eradicating the numbers of another species that is causing environmental and/or agricultural damage.

While Australians are painfully familiar with the disastrous introduction of the cane toad to control the cane beetle in the 1930s, there have also been a number of extremely successful instances of biocontrol since then, including the introduction of a moth from South America which virtually eradicated the prickly pear plant which had become a serious problem for the agricultural industry.

Biocontrols can present an alternative to increased use of pesticides, which is more expensive, can negatively impact the environment, and may become ineffective if the pest evolves resistance. As the species introduced to control a pest is also able to evolve along with the pest, they can participate in a form of natural ‘arms race’.

How did the weevil evolve resistance?

Initially it was believed that it was possible that the weevil had been able to evolve more rapidly than wasp due to the higher levels of diversity the species had in New Zealand compared to the recently introduced Argentine wasp.

However, the researchers believe that the two primary factors behind the resistance of the weevils is that of low enemy and plant biodiversity. This lack of natural enemies (predators and parasites) aside from the wasp means that there were no other selective pressures for the weevil to adapt to other than wasp in New Zealand.

Experimentation also revealed that low plant diversity was also be a factor, as there appeared to be a relationship between the ability of the weevil to evade the wasp and the plant-host it was living on. Weevils on the ryegrass Lolium perenne (a high-intensity plant grown nationally) fared far better than their counterparts on the lesser-grown pasture grass Lolium multiflorum against the wasp.

This research provides a fascinating insight into one of the first cases of a pest evolving resistance to a biological control. The lessons we can learn from this unique case will help us hopefully limit or prevent the potential for other cases like this to occur around the world.