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Spider mites bugging area soybean fields

Posted Thursday, August 2, 2012, at 3:17 PM

As if the dry weather wasn't enough, spider mites are now having a feast with the soybean crop. At this point in time, spider mites are the insect of greatest concern that can be controlled. Soybeans, in general, look better than the corn in dealing with the dry, hot weather. They have the best chance of responding to any rain that we may receive in the future. If you have soybeans, they need to be scouted on a regular basis in order to control the spider mites in a timely manner. Depending on temperature, development takes 5 to 19 days. Temperatures greater than 90 degrees speed reproduction, while cooler temperatures slow it down. Populations can increase up to 70 fold in as little as 6 to 10 days. Spider mite populations are generally held in check by natural enemies, weather and host quality.

Drought triggers spider mite outbreaks in four ways.

1.Drought accelerates spider mite movement to soybean and corn from surrounding vegetation.
2.Drought stress improves the food quality of soybean for spider mite.
3.Drought diminishes or stops the activity of fungal disease that attack mites.
4.Hot temperatures speed spider mite reproduction so that predatory insects and mites can't keep up.

Soybeans need to be scouted for spider mites. Scout soybean fields early and often. Spider mites are usually noticed at the edge of the field. Examine plants at the edge of the field first, especially adjacent to roadsides, drainage ditches or alfalfa fields. Spider mites are tiny and are almost impossible to see without magnification. A common scouting method is to lay a sheet of white paper under a leaf and then tap the top. If tiny yellow specks drop onto the paper, you have spider mites. There are several insecticides that will control spider mites. When spraying, don't skimp on water, with insects hiding under the leaves it takes more water. Aerial applicators can get by with three gallons as the aircraft prop turns the leaves upside down in the flyover.

There is a belief that rainfall will stop spider mite infestations. Single rainfall events during a drought (unless drought breaking) are usually not sufficient to stop a rapidly growing spider mite infestation. Rainfall exerts its biggest impacts not by washing mites off leaves, but by slowing down infestations in several ways.

1.Alleviating drought stress lowers the food quality for spider mites and indirectly lowers reproductive rate. It also increases the plants ability to tolerate spider mite feeding.
2.Cooler temperatures will slow spider mite activity and reproduction.
3.Prolonged rainfall or elevated humidity may create the highly favorable environment for a fungal disease outbreak.

As Wayne Bailey, MU extension pest management specialist said, "One good rain will solve a lot of spider mite problems and other problems."



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As our local University of Missouri Agronomy Extension Specialist, Crook has been writing a column for the print edition Agriculture page for the past three years and we will now be sharing it on our web version. Crook has a bachelor and masters degree in agronomy from University of Missouri and received his doctorate in Agronomy from Kansas State University. He was in soybean variety development research for 22 years for various seed companies and has been Saline County's agronomy specialist for 10 years.
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