Pesticide-Fueled Toxic Slugs Are a Nightmare for Farmers

It's long been known that neonicotinoids, the most widely used insecticides in the world, contribute to a host of environmental problems, including honey bee colony collapse disorder. But according to a study published Thursday in the Journal of Applied Ecology, neonicotinoids are harmful for another reason: they reduce crop yields by creating toxic slugs.

Yes, you read that right: toxic slugs are marauding around farms across the world, immune to neonicotinoids, but capable of paying the poison forward to their insect predators. When the predators die from eating these pesticide-laced mollusks, the result is even more crop-devouring toxic slugs—and thus, lower crop yields.

"It's safe to say that the beast that we've been dealing with, which is called the gray garden slug, is among the more damaging pests in the world because it affects so many types of crops on different continents," co-author John Tooker, an entomologist at Penn State University, told me over the phone.

"In our research with the slugs, it's quite clear that these [neonicotinoid] seed treatments aren't doing [growers] any favors," he continued. "That shouldn't be too surprising because slugs are mollusks. It's a whole different phylum than arthropods where insects reside, so something that kills an insect does not necessarily kill mollusks."

In this way, neonicotinoids simply substitute one apex pest for another. Tooker and his team observed the dynamic by letting slugs chow down on soybeans treated with various levels of neonicotinoids and fungicides (and some with neither). The slugs were then released with predatorial ground beetles, to test the rate at which the beetles were poisoned by the toxic slugs.

As it turned out, about 60 percent of the beetles were killed or impaired by the neonicotinoid-treated slugs. The team's trials in the field further revealed that this extra stress on slug predators reduced soybean densities by 19 percent, and crop yields by five percent.

"If you combine that with some of the environmental concerns that are being raised about these compounds, say the amounts found in water," said Tooker, "the costs will outweigh the benefits."

According to the team, growers would get better results by nurturing diversity within crop foodwebs, rather than pumping them up with insecticides. One way to achieve that end is to convert to no-till farming, which is pretty much exactly what it sounds like.

"A no-till crop field is one that never sees a plow," Tooker explained. "The only time the soil is really disturbed is at planting and at harvest. No-till is the perfect way to maintain a stable habitat where predator populations can build."

In addition to nixing plows, integrating cover crops can foster a healthy farm ecosystem. Typically planted after the cash crop, cover crops are not intended to be harvested, but to enrich and strengthen the soil over the winter.

"Rather than having a dirt field there in the winter," Tooker said, "you're actually providing some habitat for beneficial insect species that might be able to hang out, and then they'll be there ready to attack pests if they arrive in the spring. If you can really get cover crops to be widely adopted, I believe you can get the pest populations to go down by maintaining these populations of natural enemies."

Incidentally, we published an article yesterday about how killing off wolves and other large predators leads to more predatory attacks on livestock, due to social disruption within the pack. Tooker and his colleagues, meanwhile, have demonstrated that neonicotinoids produce a similar counterintuitive effect: instead of reducing pests and increasing crop yields, the compounds enable new pests to emerge that, in turn, decrease crop yields.

"These are managed ecosystems and to make them operate better you have to be cognizant of all the moving pieces," Tooker said. And that is true whether you are managing apex predators like wolves, or resilient pests like the gray garden slug.​