Watch out for an opportunistic pest that thrives in the summer heat: the twospotted spider mite.

In normal, well-watered years, insect predators and certain fungi prey on these tiny mites, keeping their levels low. But when hot, dry weather sets in for weeks on end, the predators fade, the fungi recede and the mites have their way.

That’s why entomologists from Nebraska, Minnesota, Iowa and Ohio are advising growers to watch for signs of spider mite infestations this summer, as they steal precious moisture and nutrients from drought-stressed corn and soybean fields.

“Every year, you will have some mites, but the weather conditions lined up for an outbreak this year in some places,” said Bob Koch, Extension entomologist at the University of Minnesota. A hot, dry start to the summer allowed populations to build, and now even recent rains haven’t been able to wash away the stubborn pests, he noted.

“Timely rains have alleviated moisture stress on the plants in some areas, but the mites are still there,” he warned.


The twospotted spider mite couldn’t have handpicked a better July.

They thrive in heat. When temperatures climb past 80 degrees, the mites become more amorous than ever. According to University of Connecticut entomologists, their reproduction and development hit optimum levels at temperatures between 85 and 95 degrees. They also like dry weather and moisture-stressed plants.

Because they reproduce quickly, populations can build rapidly in the right conditions, beyond the point where a single rainfall can knock them back, Koch said.

The tiny, pin-sized mites attack plant cells individually, emptying their contents and causing pale specks on the leaf surface. The little pinpricks of death are permanent and can add up quickly.

“They’re destroying the cells and causing irreversible damage that creates stippling on the plants,” explained Koch. “The other sign is the webbing they make.”

True to their name, the spider mite produces wispy white threads that can build on infested leaves. The strands also serve as transportation, sometimes called “ballooning,” Koch said.

“They release a thread of this silk webbing, and it gets picked up by the wind and carries them away,” he said.


“I recommend drought-stressed fields should be scouted, starting at the perimeter,” said Iowa State University Extension entomologist Erin Hodgson. “Finding mites is pretty typical this time of year. But if they are seeing signs of injury — discoloration, stunting, webbing — or noting mites in the field interior, I would consider a rescue treatment.”

Infestations often start on the field edges, where mites move in from vegetation in ditches and pastures. The mites first feast on the lower canopy of corn and soybean fields, before moving up in the plant.

Because they are so tiny — entomologists recommend shaking a leaf over a piece of paper to even spot them — scouting and treatment thresholds rely on their plant damage levels.

University of Minnesota entomologist Ken Ostlie and IPM specialist Bruce Potter recommend growers use a scale from 0 (no injury) to 5 (heavy stippling all the way to the top of the plant) when rating damage, and consider treatment when your field reaches 3 (heavy stippling on lower leaves, with scattered colonies in the upper canopy). See the full scale here:….

No border sprays for this pest, the Minnesota entomologists warned. “By the time infestations are detected on field edges, mites are likely well into the field,” they wrote in a university article.

Most pyrethroids do not work against the mites, and may actually cause their populations to flare, cautioned Ohio State University entomologists Andy Michel and Kelley Tilmon.

“Lorsban [chlorpyrifos] and generics have been popular choices against mites but may be less available now,” they wrote in a university pest alert. “Check the field five days after application for resurgence because these products do not kill mite eggs.” (See a DTN article on the status of chlorpyrifos here:…).

In Minnesota, some mite populations may be resistant to chlorpyrifos (Lorsban) and possibly cross-resistant to dimethoate, the Minnesota scientists added.

Some miticides are available that will also target egg populations, all the entomologists noted. See a full list of products in the University of Minnesota article here:….

See more from the Ohio entomologists here:…, and from Iowa State here:….

Emily Unglesbee can be reached at

Follow her on Twitter @Emily_Unglesbee.

Source: Emily Unglesbee, DTN