In 2005, New York City officials discovered Asian long-horned beetles in Central Park elms. To combat these pernicious pests, which can destroy entire forests, park personnel sprayed insecticides known as neonicotinoids on tens of thousands of trees infested by that beetle and another invasive pest, known as the emerald ash borer.
The treatment worked, but the spraying had an unforeseen effect: It led to an explosion of spider mites. These tiny arachnids, which weave small webs and puncture holes in plants to feed, sickened the trees, many of which began to drop their leaves.
This dilemma was the beginning of a long scientific quest for Texas A&M University agricultural entomologist Ada Szczepaniec. Why, she wondered, would neonicotinoid pesticides such as clothianidin and imidacloprid—which can kill a wide variety of insects—cause a boom in spider mites?
Szczepaniec began to seek the answer, in part because neonicotinoid pesticides, which were introduced on a large scale in the 1990s, are now nearly ubiquitous. She says that while they are considered to be safer than older insecticides, concerns about their unintended consequences have generally been downplayed, especially in the United States—though research shows the chemicals are relatively toxic to bees. For that reason, the European Union has banned several of them.
Her initial work led to a 2011 PLOS One study that showed elms treated with neonicotinoids—neonics for short—hosted smaller populations of creatures that attack spider mites. Her more important discovery: Mites that fed on treated elm leaves had 40 percent more offspring than those that fed on regular leaves. This suggested the insecticide was doing something unusual to the trees to make them more palatable to the mites.
Next, Szczepaniec turned her attention to agriculture, where she found similar results in corn, cotton and tomatoes. For all those crops, treated plants fostered larger populations of mites.
Her latest work, presented at the International Congress of Entomology in late September in Orlando, Florida, showed that when applied to soybeans, the neonic imidacloprid altered the activity of more than 600 genes involved in the production of cell walls and defense against pests. The activity of most of these genes was reduced. Szczepaniec suspects that reduced activity leads to more penetrable leaves and lower levels of hormones involved in pest resistance. No one knows that for sure, but it would explain why spider mites thrive in the presence of these pesticides.
Other researchers have made similar findings, showing that the use of neonics can lead to spider mite outbreaks in apple trees, elms and hemlock; ornamentals such as roses; and agricultural staples like soybeans. And a study in the Journal of Economic Entomology by Washington State University researchers found that spider mites laid more eggs when exposed to imidacloprid-treated bean plants.
Meanwhile, in the past few years, Szczepaniec and Penn State's John Tooker have noticed increasing numbers of spider mite outbreaks in corn and soybeans throughout the country, although they haven't quantified that. "The general findings are interesting because they highlight an unintended negative consequence of insecticides and because spider mites are an important plant pest with a very broad host range," says Gregg Howe, a plant researcher at Michigan State University who wasn't involved in the research.
However, two studies led by Ralf Nauen from Bayer's CropScience division, which manufactures neonicotinoids (including imidacloprid), came to a different conclusion. These two papers, in the Journal of Economic Entomology and Pest Management Science, found that imidacloprid reduced the fertility of some strains of spider mites. Further research will be needed to resolve the conflicting findings.
Spider mites can be controlled with a few pesticides and miticides, but these can be expensive and difficult to apply. Resistance to these chemicals also appears to be growing.
Tooker says that besides mites, neonicotinoid application can have other unwanted side effects. His work has shown that use of the chemicals has also led to outbreaks of crop-munching slugs by inadvertently poisoning the creatures' major predator, ground beetles.
These incidents suggest that neonicotinoids should be used less widely, Tooker and Szczepaniec say. They are most alarmed at the use of neonics to coat seeds, which is meant to prevent infestations that are unlikely to occur. And this practice is rampant: Neonics are added to about 95 percent of corn seeds and about half of soybeans.
The vast majority of these seed coatings run off in water, ending up in local waterways. Work by Christian Krupke from Purdue University has shown that more than 90 percent of these insecticide coatings isn't absorbed by the plants.
The U.S. Environmental Protection Agency concluded in a 2014 memo that "these seed treatments provide little or no overall benefits to soybean production in most situations." Tooker says the same goes for corn.
Jeff Donald, a spokesman for Bayer CropScience, disagrees. "Modern neonicotinoid seed treatments offer a number of important benefits, including increasing yield for farmers, reducing the amount of insecticide in the environment and minimizing potential exposure to nontarget organisms, when used according to the specific label directions," he says.
Both Szczepaniec and Tooker urge farmers to adopt integrated pest management, a set of policies that lay out when farmers should use insecticides in response to observed levels of pests in the field, rather than, for example, using them pre-emptively on the majority of corn seeds. Too often, Tooker says, these chemicals are used "to prevent a problem that isn't likely to occur." With unforeseen impacts like mite outbreaks, he says, "that's hard to justify."
