Hot, dry weather and two-spotted spider mites just go together, says Mike Gray, University of Illinois Extension entomologist.

Reports of spotty infestations are surfacing in some areas of the state. Robert Bellm, Crop Systems Extension Educator at the Edwardsville Extension Center, reported Monday that twospotted spider mites were discovered in several southwestern Illinois counties, including Clinton, Jersey, Montgomery and Washington.

Bellm indicates that soybean plants along field edges were most severely infested, particularly where the ditch banks had been mowed or sprayed for weeds. This matches the typical scenario in which spider mites begin moving towards greener and more succulent plants, such as border row soybeans.

"As hot and dry conditions persist throughout the state, two-spotted spider mites could become an increasing problem," Gray adds. "Watch this potential development closely and be sure to scout fields, especially border rows for this pest."

The suggested economic threshold for two-spotted spider mites is to consider a rescue treatment when 20% to 25% of plants are discolored before pod set, or if 10% to 15% discoloration is observed after pod set. Border row treatments should also be considered in lieu of treating entire fields.

Harvest restrictions will need to be adhered to if rescue treatments become necessary. For Lorsban 4E, a treatment should not be made to soybeans within 28 days before harvest. Dimethoate has a 21-day harvest interval. These are the primary insecticides that have been used to limit spider mite injury in previous outbreak years, Gray explains.

"Spider mites have piercing and sucking mouthparts that they insert directly into leaf cells and remove fluid," Gray notes. "As mite injury intensifies, small discolored spots (yellow or white stippling) become apparent. Stipples may appear on both sides of leaves; however, it is most noticeable on the undersurface."

Damage to soybean plants is caused due to the reduction in chlorophyll within leaf tissues resulting in reduced photosynthetic efficiency. Severely-damaged soybean plants become bronzed and the leaves fall prematurely from the plants. During outbreak years, such as 1988, yield reductions of 40% to 60% were common in many Illinois fields.

All life stages of two-spotted spider mites may be found on soybean plants, including eggs, one six-legged larval stage, two eight-legged nymphal stages, and adults. The speed of development is temperature dependent and generations may be completed within a range of 4 to 14 days.

"Although mites cannot fly, they are quite efficient at dispersal by moving to leaf tips on crowded plants from which they are easily blown to other plants within a given field or to adjacent fields," Gray adds.

For more information, check out The Bulletin, an online publication written by U of I Extension specialists in crop science at ipm.illinois.edu/bulletin.