Study shows pesticides make some ants more aggressive

Research carried out at Victoria University of Wellington shows that exposure to some commonly used pesticides makes an invasive ant species more aggressive and more likely to survive conflict with a native ant species.

At the same time, the native species becomes less aggressive after being exposed to the pesticide.

Led by PhD student Rafael Barbieri, Professor Phil Lester and Associate Professor Ken Ryan from Victoria University's School of Biological Science, the research is the first in the world to demonstrate the impact that neonicotinoid pesticides can have on dynamics in insect communities and invasion success.

Neonicotinoids are the world's most widely deployed insecticides and have been associated with the decline of honey bees and other pollinators. The chemical is often soaked into seeds and spreads throughout the plant as it grows, affecting insects that eat it. The European Commission recently imposed restrictions on the use of neonicotinoid pesticides for two years.

The study examined the invasive Argentine ant, which is widely established in New Zealand, and the native Southern ant. The two species have similar habitat and food preferences.

"The Argentine ant is already known as an extremely aggressive invader, all around the world," says Professor Lester. "Here you have the Ghengis Khan of the ant world becoming even more aggressive after exposure to these pesticides.

"The success of such invasive species is often linked to their highly aggressive behaviour and to their ability to displace native communities and manipulate food sources."

Mr Barbieri says the level of increased aggression displayed by the Argentine ants as they faced off over food supply with their native competitors was significant.

"The native ant was clearly less able to hold its own when it was contaminated by sub-lethal amounts of the pesticide."

"In habitats where the local species has previously been exposed to neonicotinodis, invading Argentine ants are likely to have significantly higher chances of monopolising food sources and higher survival rates."

Overall, say the researchers, their results provide evidence that sub-lethal exposure to these pesticides can have a major effect on the dynamics of ant communities.

The Victoria University research was today published in the international journal Proceedings of the Royal Society B.