Widely Used Pesticides Are Causing Huge Spider Mite Outbreaks; Some Common Insecticides Alter Plants' Genetics, Making Them More Vulnerable to Tiny, Destructive Arachnids

By Main, Douglas | Newsweek, November 11, 2016 | Go to article overview

Widely Used Pesticides Are Causing Huge Spider Mite Outbreaks; Some Common Insecticides Alter Plants' Genetics, Making Them More Vulnerable to Tiny, Destructive Arachnids


Main, Douglas, Newsweek


Byline: Douglas Main

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. …

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