I Hate Crying Wolf…… But Forewarned is Forearmed !!

Doug Johnson, Extension Entomologist

Twospotted spider mites (TSSM) are an occasional pest of soybeans and corn in Kentucky (KY). They are present every year but only important during hot, dry periods. Unfortunately, this is the type of weather we currently have and have had for several months. Additionally, states to the north of us, including Nebraska and Iowa, are already experiencing some problems. I have not received information about spider mites on KY field crops but have seen some large populations in fruit trees (different mite but same circumstances). I, therefore, think it wise for those with field crops interest to keep an eye peeled for this pest.

Twospotted spider mites on soybean leaf
Photo: Frank Peairs, Colorado State Univ.,
 Bugwood.org

TSSM are greenish-white with two dark spots on the back. Adults are about the size of salt grains. Juvenile TSSM have six legs, but adults have eight legs. Other species of spider mites also attack field crops.

Spider mites become active early in the spring and remain active throughout the summer. They live in thinly webbed colonies on the underside of leaves. Occasionally, you will see web strands between leaves. Hot, dry weather allows rapid population growth with each generation taking about five to seven days to complete.

Infestations tend to start on field margins near broadleaf weeds and/or the side from which the wind is blowing, as they may drift on a breeze. They may then spread out in ever enlarging spots if the weather allows.

In corn, damage presents as yellow stippling on the upper surface of the leaves. Heavy and / or prolonged infestations may cause premature drying resulting in loss of tissue, stalk breakage and kernel shrinkage.

In soybean, injury could resemble herbicide damage, foliar disease or nutrient deficiency. Feeding results in tiny yellow spots or stipples which may turn orange. Multiple damaged spots may merge in to larger areas of damage. With severe damage, leaves may turn brown and eventually fall off. Spider mites reduce yields by causing pod shattering, wrinkled seed and early maturity.

Scouting is best done in times of prolonged hot, dry (low humidity) weather. In soybeans the most important time is during the reproductive stages of R1-R5 and where a synthetic pyrethroid insecticide has been used. Shake plants over a piece of white paper and look for tiny white moving specks. A hand lens is very useful in determining if the specks are actually mites. In corn, scouting is much more difficult and less is known about making a control decision. The mites are no harder to find, simply follow the instructions for soybean but using corn leaves. One simply wishes to determine if the infestation is only on the outer edges or is across the field.

Large and widespread, mite infestations usually happen when hot, dry and low humidity conditions have occurred early in the season (like this year!). In these circumstances, scouting should start well before R1 (soybean) and in younger corn. Cooler temperatures and high humidity allow a natural fungus to control spider mites. Rainfall will help the plant tolerate the infestation but will not reduce the mite population. Application of synthetic pyrethroid insecticides and fungicides may worsen the infestation because these insecticides don’t work well on mites and the fungicide may hamper the natural occurring fungus that kills the mites.

If direct control is required, consider using an organophosphate insecticide like chlorpyrifos or dimethoate. If you must use a pyrethroid consider bifenthrin. An application is warranted when most plants are infested with spider mites and leaf speckling and discoloration are apparent.

Reference: Field Crop Insects. Iowa State Univ. CES. CSI 0014. Jan. 2012