Spiders are just not themselves after being exposed to low doses of pesticides found in apple orchards. The personality changes discovered by researchers at McGill University suggest some pesticides may have insidious effects that can't be detected by ordinary toxicity tests.

The study looked at the effect of phosmet, a common orchard pesticide, on bronze jumping spiders. The creatures, which are about half a centimetre long, are considered beneficial because they prey on caterpillars, aphids or other pests that damage fruits and flowers, says Raphaël Royauté, who conducted the study for his PhD research at McGill with associate professor Chris Buddle and Charles Vincent of Agriculture and Agri-Food Canada.

Unlike many familiar spiders that spin a web, then sit and wait for insects to become trapped, jumping spiders hunt by pouncing on their prey.

"They're kind of like the big cats of spiders," added Royauté, now a postdoctoral researcher at North Dakota State University.

He said he worries the personality changes observed in the spiders could impact their ability to hunt, make decisions about their environment and whether to stay or go, or find the right mate.

"We don't understand what exactly the consequences are," he said. But he said personality differences are known to affect how animals find mates and who they mate with.

Beneficial insects could be affected

Royauté said the pesticide could also have similar effects on other beneficial orchard-dwellers, such as bees.

"Social organisms such as bees also have personality differences," he added. "If all of those species are affected by the insecticide, it puts the ecosystem out of balance."

Once many individuals are affected, that might have an impact on the entire population, he suggested.

Because of that, he thinks scientists who conduct pesticide toxicity testing should start paying attention to effects on individual insects, instead of just the average effect on the group.

Royauté's and his colleagues' experiment involved testing how adventurous 176 bronze jumping spiders were at exploring a square box and how quickly they tended to attack fruit flies in a petri dish. All spiders were tested twice. Half were exposed to very low doses of phosmet — similar to those they would be exposed to in an orchard — between the two tests, and half were not.

The "average" personality of the entire group didn't change after pesticide exposure.

"On average, you would say there's no effect," Royauté said.

But a statistical analysis showed that individual spiders who weren't exposed to the pesticide behaved the same both times. Those that were exposed to the pesticide behaved differently — much more like one another, instead of showing individual personality differences.  

"If you look at the individual level, you see very pronounced changes."

The study was published in the journal Functional Ecology earlier this month.

It was funded by the Natural Sciences and Engineering Council of Canada and by Agriculture and Agri-Food Canada.

McGill University researchers have used other tests to measure spiders' personalities. For example, when shown their reflection in a mirror, some spiders attack, while others back away.