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Mighty mites: arachnid soldiers deploy to protect a national treasure

Giuditta Beretta uses predatory mites to wage biowar on thrips, common pests that plague the beloved tulips of the Netherlands.
Chris Woolston is a freelance writer in Billings, Montana.

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Giuditta Beretta works in her lab of the greenhouse where she studies mites.

Giuditta Beretta is a PhD student in evolutionary and population biology at the University of Amsterdam in the Netherlands. Credit: Niels Jansen/Phenster for Nature

Even in a pandemic, my thrips come first. I worried more about these insects — common garden pests that feast on tulips, roses and other important crops here in the Netherlands — than I worried about myself. Here, I’m using a brush to gently herd hundreds of thrips (Echinothrips americanus) onto their new home, a bean plant. Later, I’ll try to kill them with predatory mites, a potential biological weapon that could be deployed in greenhouses. For now, I just want them to be healthy.

This climate-controlled chamber at the University of Amsterdam is a thrip paradise. It’s a constant 25 °C with 75% humidity. The purple light helps the bean plant to grow. A plain white light would probably suffice, but we decided we should do something nice for the plant after covering it in pests.

When the university partly shut down for nearly two months during the pandemic, starting in mid-March, I couldn’t do any of my mite experiments, but I was allowed to visit the thrips once a week. They thrived. They’re pretty hard to kill, as many gardeners know.

The predatory mites are trickier to keep alive. Instead of leaving them to fend for themselves for a week at a time, I took a bunch home in a plastic container. I fed them a mixture of even smaller mites and yeast, which they like.

We’re doing experiments with two types of thrip predator: plant mites (Amblyseius swirskii) and various species of soil mite. Plant mites are tiny, and it’s comical to see them try to wrap their legs around a thrip in an attack. The soil mites are about 2 millimetres long, nearly the size of the thrips themselves. We hope that they might be more effective against thrips, but it’s hard to get soil mites to climb up a plant. A combination might prove most effective.

Although I spend time caring for my thrips, I have no problem killing them. They are not nice insects, and they look ugly under the microscope. I used to work with caterpillars, and I definitely felt more guilty about them.

Nature 584, 490 (2020)

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