By Christa Lesté-Lasserre

House-dwelling spiders avoid surfaces that certain aggressive ants have walked over, suggesting that there may be some sort of chemical the ants leave in their wake that could form the basis of an ecologically sound way to keep spiders out of people’s houses.

Alongside his regular spider sex pheromone research, Andreas Fischer at Simon Fraser University in Vancouver, Canada, has been seeking practical ways to maintain healthy ecosystems while keeping arachnophobic humans comfortable in their own homes.

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He says commercial pesticides “kill everything”, disrupting ecosystem balances. Meanwhile, Fischer’s previous testing revealed that popular “natural” spider repellants like lemon zest and mint oil have little to no effect on spiders.

Recently, Fischer realised other scientists were noting that where they found more ants, they found fewer spiders.

Wondering if ants naturally repelled spiders through chemical traces, Fischer and his colleagues gathered workers from three different species of ant and females from four species of common North American house spider from their university campus and nearby areas. In each experiment they let ants of a particular species run around on filter paper in one part of a glass cage for 12 hours. To keep the experiment fair, the scientists weighed the ants – which varied significantly in body size – and used an equivalent mass of ants of a given species in each experiment. This meant one ant species was represented by 43 ants but another by just 3 ants.

Then they removed the ants and put young female spiders, one at a time, into the cage and watched to see where they chose to settle after 24 hours.

Most of the black widows (Latrodectus hesperus), false widows (Steatoda grossa) and hobo spiders (Eratigena agrestis) avoided the filter paper previously traversed by European fire ants (Myrmica rubra), says Fischer. A fourth species, the cross spider (Araneus diadematus), showed a similar trend, but it wasn’t as strong.

There may be a simple reason why the spiders were reluctant to settle where they sensed ants might appear. European fire ants can swarm and kill spiders that invade their territory.

“From their perspective, the spiders are exploring, trying to find a new place to settle down and build a web,” says Fischer. “Building a nest is a big investment, and if they sense potential danger, they’ll think, ‘This is not the hill I want to die on,’ and go elsewhere.”

The spiders didn’t avoid filter paper that had been trampled by black garden ants (Lasius niger) or western carpenter ants (Camponotus modoc). That might be because European fire ants are particularly aggressive and spiders may have evolved to avoid them, says Fischer. However, he acknowledges that C. modoc – a large-bodied ant for which the team used just 3 individuals in their experiment – might have had a greater effect on spider behaviour if more individuals had been present.

The researchers don’t know yet what exactly the spiders are detecting on the filter paper – it could be some sort of ant pheromone or possibly a chemical in their faeces – but they hope to soon find out. Once they know the chemical that is repelling the spiders, they can start working on lab-made substitutes for use in the home, says Fischer.

While the findings are “exciting”, Fischer urges people to wait for the finished product rather than collecting ants as spider deterrents themselves.

“This is not an encouragement to use European fire ants as a pest remedy, because these ants have a very painful sting and are difficult to get rid of,” he says. “They would become a pest themselves that’s far worse than the spiders.”

Journal reference: Royal Society Open Science, DOI: 10.1098/rsos.210279

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