Pesticide Alters Personalities of Helpful Spiders

Nature provides free pest control, from bats and birds to snakes and spiders. These predators can help protect agricultural crops, but we often try to supplement their services with our own synthetic pesticides. And as a new study suggests, one common insecticide might affect pest-killing spiders' ability to do their job.

The chemical in question is Phosmet, a broad-spectrum insecticide that's used in fields and orchards across North America. It's highly toxic to a wide range of insects — including honeybees, unfortunately — but it was thought to be relatively safe for spiders. As the study's authors report, however, it can have an insidious effect on at least one key species of jumping spider that normally protects crops.

"Bronze jumping spiders play an important role in orchards and fields, especially at the beginning of the agricultural season, by eating many of the pests like the oblique-banded leafroller, a moth that attacks young plants and fruit," lead author and North Dakota State University researcher Raphaël Royauté says in a statement.

"Farmers spray insecticides on the plants to get rid of these same pests, and it was thought that it had little significant effect on the spiders' behaviors. But we now know that this isn't the case."

Yes, spiders have personalities

Previous research has shown that spiders — like humans and many other animals — have distinct personalities, resulting in different decisions made by "bold" and "shy" individuals. This can affect their ability to catch prey or their interest in exploring new territories, both of which are key to their survival and their success in limiting pests.

"Most individuals have an individual signature in their behaviors, what scientists call 'personality types,'" Royauté says. "Some individuals are willing to take risks when predators are present, explore new territories faster, or capture prey more quickly."

Yet the effects of insecticides on spider personalities are poorly understood, he adds. "We know that drinking alcohol can make us act in weird ways, by removing some social inhibitions for example. So one of the primary questions of my research became: can insecticides cause similar personality shifts in individual spiders?"

To shed more light on this, the study's authors focused on how spiders behaved before and after sublethal doses of Phosmet. They found that, in general, the spiders' behavior grew less predictable, with individuals deviating from their personality types once they were exposed. This could be because some individual spiders are more sensitive to the insecticide than others, the researchers say.

Male and female spiders also showed different responses to the toxin. Males were able to continue capturing prey about as well as they had before, but their personality types seemed to fade away when exploring their environment. Females, on the other hand, showed a much stronger effect in their hunting behavior.

"Inactive females were quicker to capture prey in the absence of insecticide exposure, a tendency no longer expressed in the treated group," the researchers write in the journal Functional Ecology. "Males did not show evidence for such an activity-prey capture syndrome, even in the control group, but showed a decrease in correlation strength among all activity traits. Taken together, our results suggest that insecticide-exposed individuals showed a strong departure from their personality tendencies."

Our spider sense is tingling

Phosmet is mainly used on apple trees to control codling moths, according to a fact sheet by the Oregon State University Extension Service, but it's also used on various other crops to fight aphids, suckers, mites and fruit flies.

While Phosmet was the focus of this study, the researchers say the real lesson of their findings isn't about a single pesticide. It's about how we evaluate the safety of all pesticides for non-target wildlife, especially beneficial, pest-controlling predators. The spiders' personality shifts weren't evident when researchers averaged the behavior of a whole population, but they were significant on an individual level.

"By looking at the way that insecticides affect individual spider behaviors, rather than averaging out the effects on the spider population as a whole, as is traditionally done in scientific research, we are able to see some significant effects that we might have otherwise missed," says co-author and McGill University ecologist Chris Buddle.

"It means we can measure the effects of insecticides before any effects on the spider population as a whole are detected, and in this case, it's raising some red flags."