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Monday, August 3, 2020  
Printable Page US Ag News   Return to Menu - Page 1 2 3 4 5 6
The Scavengers of Drought     07/31 09:40

   Spider Mite Infestations on the Rise in Droughty Areas

   Twospotted spider mite populations have been building during the long, hot 
month of July. 

Emily Unglesbee
DTN Staff Reporter

   ROCKVILLE, Md. (DTN) -- 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 

   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 

   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 

   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 

   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: 
y-areas, and from Iowa State here: 

   Emily Unglesbee can be reached at Emily.unglesbee@dtn.com.

   Follow her on Twitter @Emily_Unglesbee.

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