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Common air pollutants like ozone and nitric oxide can change the way ants smell, prompting their nest-mates to attack them as if they were intruders.

Ants recognise their comrades by scent, and when they encounter an ant whose smell they don’t recognise, they respond aggressively, biting and sometimes killing the trespasser. But ozone, a greenhouse gas produced by cars and industrial activities, can break down the structure of alkenes, chemicals that make up part of the colony-specific scents.

at the Max Planck Institute for Chemical Ecology in Jena, Germany, and his colleagues knew from previous work that ozone-induced changes in alkenes can impair the way insects communicate with each other. They witnessed fruit flies mate with the wrong species and pollinators such as tobacco hawkmoths if their scent had been altered by ozone.

To test the impact on ants, Knaden and his colleagues set up artificial colonies of six ant species. They removed one individual ant from each and put it in a glass chamber filled with various concentrations of ozone, some of which matched levels measured in Jena in summer. When they put the ant back, the others attacked it.

“I did not expect it, I have to say,” says Knaden. “Because knowing that alkenes are such a minor part [of the ants’ scent], we knew that whatever we did with ozone would only change maybe 2 per cent or 5 per cent of the blend.”

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In the wild, this kind of behaviour could make a colony less efficient, he says, even if the ants are not killed, but designing experiments to capture these effects will be complicated.

at The Rockefeller University in New York, who wasn’t involved in the study, says alkenes are very important in nest-mate recognition, so the aggressive reactions didn’t shock him.

Alkenes are involved in other ant behaviours like trail following and communication between larvae and adults. The study found that, when exposed to ozone, adult clonal raider ants (Ooceraea biroi) can neglect their larvae, so these ozone-induced changes have the potential to disrupt more aspects of ant life – and the wider ecosystem too.

“If you took the ants out of most terrestrial ecosystems, they would probably collapse,” says Kronauer. This is because ants have crucial ecological roles. They disperse seeds, move soil and have mutually beneficial relationships with many organisms.

Insect populations are plummeting worldwide, and this study adds to a growing body of research that points to air pollutants as one of the factors behind the decline. Knaden says that even though the ozone pollution levels we are experiencing might not yet be harmful to humans, “we just should know that what we are doing has additional costs that we have maybe not thought about before.”