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The Tetranychus urticae spider mite is less than half a millimetre long, yet it causes significant damage to food crops and ornamental plants. From home gardens to large-scale agricultural production, and even inside greenhouses, fighting it is an 
uphill battle. That’s because T. urticae, also known as the “Two-Spotted” spider mite, has evolutionary advantages that humans can only dream about.
“T. urticae’s ability to adapt is reflected in its ability to feed on more than eleven hundred plant species,” says Dr. Vojislava Grbic, an associate professor in Western University’s Department of Biology. “When the mite feeds, the plant tries to defend itself. But the mites have evolved a phenomenal toolbox to combat these defenses.”
Plants defend themselves with toxins they produce when attacked, but T. urticae has enzymes to break down those toxins. It is able to adjust the enzymes with a flexibility to survive a large spectrum of toxins from many different plants. Moreover, the same detoxification toolbox lets T. urticae break down man-made pesticides too.
“There are several acaricides [man-made mite pesticides],” says Dr. Grbic. “However, mites are record breakers in establishing resistance. They do it in two to six years.” Moreover, in some instances, the mites gain an ability to control the plant, triggering it to shut down its own defenses.
Spider mites reproduce quickly – 30 generations a year in year-round growing environments – which means they adapt to a new plant and its toxins very quickly. In human terms, 30 generations could take several centuries, and while we gain immunity and pass on some of that to our children through breast milk, we lose immunity over generations if we’re not repeatedly exposed to the pathogen. But T. urticae keeps its adapted state.
“T. urticae raised on a tomato plant will retain their tomato ‘tools’ and pass them on to their descendants, even if moved to another plant” says Dr. Grbic. Generations later, if some of those descendants are moved back to a tomato plant, they already have the immunity as well as the ability to shut down the tomato’s defense. And since the mites can adapt so quickly to natural and man-made compounds, there aren’t a lot of solutions to the problem right now.
“Farmers and home gardeners who use chemical products may see short-term success, but it won’t last. Furthermore, the chemicals usually kill off the mites’ natural predators without harming T. urticae, making the mite problem even worse.” Dr. Grbic adds.
T. urticae is a worldwide problem and there are some predatory mites such as Phytoseiulus persimilis that are being deliberately bred and introduced. However, they require a warm, humid climate and Ontario only has that for part of the year. They need to be re-purchased each season, making them costly for large-scale open-air agriculture.
“In Ontario, we could suffer substantially. With global warming, we’re seeing that T. urticae populations are increasing,” says Dr. Grbic. “In particular, drier growing seasons put plants under more stress, while allowing mite populations to increase. Monoculture makes it easier for mites to adapt to and devastate their hosts.” She adds, “We lack knowledge and control to support sustainable production.”
In 2011, a consortium from Canada, Spain, Belgium, France and the United States sequenced T. urticae’s genome in a project led by Western University’s Grbic team. That sequencing is a significant advance.
“We have created research tools to allow us to ask questions at a new level about relationships between plants and herbivores,” says Dr. Grbic. “But we are just starting to understand how spider mites’ genetics work.” She adds, “Still, this research will lead us toward a better understanding of the mechanism, and that does get us closer to a solution.”
Dr. Voijslava Grbic’s research is part of an international collaborative initiative (GAP-M, Genomics in Agricultural Pest Management) that is funded by Genome Canada and Ontario Genomics Institute, and by Ontario Ministry of Research and Innovation.
