Spider mites showing up in soybean fields

U of I entomologist Mike Gray says farmers should scout soybean fields for signs of spider mites now.

Compiled by staff

Published: Jul 6, 2012

The hot and dry weather persists throughout much of Illinois, with the 7-to-10-day forecast showing daytime highs above 90°F and little rain (except widely scattered showers) expected for most of the state. Justifiably, producers' focus has turned from most pest-related concerns to the weather, yet the hot and dry conditions will likely exacerbate infestations of twospotted spider mites throughout many soybean fields in the coming weeks.

If you are seeing discolored leaves (yellowing or bronzing) along field margins, I encourage you to tap the leaves over a sheet of white paper to dislocate mites, if they are present, from the lower leaf surface. If you observe tiny specks moving on the paper, chances are very good you have mites in that field.

Continued hot and dry conditions will very likely cause the infestation to become more widespread throughout the field. If you find discolored leaves along field margins, detect mites, and determine that the midrange forecast (7 to 10 days) will bring more hot weather and no rain, you should consider a rescue treatment.

In 1988, the last year that twospotted spider mites caused widespread damage to the soybean crop (an estimated 6 million acres in Illinois received a treatment), the organophosphate insecticides chlorpyrifos and dimethoate were the primary insecticides of choice. Of the two, dimethoate provides some systemic activity. To be effective and provide longer residual activity, dimethoate must be absorbed by the leaf and translocated. Plants that are under severe drought stress are less able to absorb and translocate dimethoate as effectively. Chlorpyrifos (Lorsban 4E or generic products) is not systemic, and the residual activity may be reduced to as few as 3 to 5 days due to photodecomposition under intense heat and sunlight.

U of I entomologist Mike Gray contributed this report.

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