A few places in Iowa and surrounding states have already reported heavy populations of an insect pest -- the two-spotted spider mite -- in soybeans this summer. The first reports of spider mites in Iowa corn started the week of July 9 to 13.
As mentioned in an earlier ICM News article on the Iowa State University website, two-spotted spider mites are problems in droughty years. "We recommend scouting corn and soybean fields for mite infestations this year because they thrive in hot and dry conditions," says Erin Hodgson, Iowa State University Extension entomologist. "Two-spotted spider mites can aggregate at the field edges, especially if there are weeds surrounding the borders. Eventually this insect can disperse with the wind to develop a field-wide infestation. I encourage people to look at the edge rows first to see if mites can be found. If their presence is confirmed, then you should estimate the spider mite populations throughout the field by walking a 'Z' or 'W' pattern."
How to spot spider mite feeding symptoms on soybean and corn crops
What does spider mite feeding look like? What are the symptoms you should look for in fields? Photos accompanying this article show one of the places in Iowa where heavy populations of spider mites have been found this summer, with prolonged feeding on the soybean plants.
The first reports of spider mites in Iowa corn have just started to come in recently, as ISU Extension crop specialists began getting calls from farmers regarding a few fields in the state that had spider mites showing up for the first time last week.
Exact treatment thresholds for spider mites in corn and soybean do not exist, says ISU's Erin Hodgson. Instead, the decision to treat should take into consideration how long the field has been infested, mite density including eggs, mite location on the plant, moisture conditions and plant appearance. A general guideline for soybeans is to treat between the R1-R5 growth stage of the soybean plants (bloom through beginning seed set) when most plants have mites, and if you see heavy stipling and leaf discoloration apparent on lower leaves. Foliar insecticides are recommended in corn from R1 to R4 (silking through dough stage) when most plants have mites at or around the ear leaf and 15% to 20% percent leaf discoloration.
These products don't kill the eggs; you need to keep scouting fields after spraying
Organophosphate insecticides are the recommended type of insecticide chemistry to spray to manage spider mites (such as chlorpyrifos or dimethoate). Follow label directions and pay attention to application guidelines and preharvest intervals.
"Remember these products do not kill eggs, so you need to continue scouting after an application to evaluate product efficacy and residual control," says Hodgson. "To improve application coverage, consider increasing the water volume to make contact with spider mites. Border treatments may also be a cost effective option if heavy spider mite populations are restricted to edge rows."
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