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Spider Mite Activity in the Region

Reports of spider mites in soybean fields are trickling in from the NW MN and other areas. Personal observations have found mites on field margins in areas where recent showers have missed or dropped negligible precipitation. Some of these northern fields are advancing quickly through the R5 growth stage. Areas where rains have occurred are less likely to have mites building on field margins.

Scouting and management

A magnifying lens can be useful for seeing this small-sized pest. Risk of infestation is greatest under drought conditions. Since infestations are typically first noticed in field edges, initial scouting can focus on these areas; particularly edges near alfalfa, grassy ditches and other perennial vegetation.


Beginning at the edge of the field, examine the undersides of leaves to determine if twospotted spider mites and/or webbing are present. If present, move farther into the field and examine two plants at each of twenty locations spread throughout the field.

Treatment thresholds

Once twospotted spider mites are found, use the following scale developed by Potter and Ostlie (1988) to determine if an insecticide application is needed:

  • No spider mites or injury observed.
  • Minor stippling on lower leaves, no premature yellowing observed.
  • Stippling common on lower leaves, small areas on scattered plants with yellowing.
  • Heavy stippling on lower leaves with some stippling progressing into middle canopy. Mites present in middle canopy with scattered colonies in upper canopy. Lower leaf yellowing common and some lower leaf loss. (Spray threshold)
  • Lower leaf yellowing readily apparent. Leaf drop common. Stippling, webbing and mites common in middle canopy. Mites and minor stippling present in upper canopy. (Economic loss)
  • Lower leaf loss common, yellowing or browning moving up plant into middle canopy, stippling and distortion of upper leaves common. Mites present in high levels in middle and lower canopy.
This rating scale will help you assess mite infestation and time insecticide application to protect the upper two-thirds of the plant canopy. Because populations can increase rapidly, scouting should be repeated every four to five days.


Several insecticides (e.g., chlorpyrifos, dimethoate and bifenthrin) also act as miticides and are labeled for controlling twospotted spider mites. However, beware that some insecticides have been known to worsen infestations by causing the populations to increase ("flare"). As with any pesticide use, follow directions on the product label.

Chlorpyrifos (organophosphate - Group 1B) has been used successfully to manage both aphids and mites with a single insecticide. However, there have been mites found several seasons ago that were rated as resistant to chlorpyrifos. Keep this in mind if a treatment is made where mites are a concern. Follow up with an evaluation of effectiveness. Avoid applications if not warranted.

Bifenthrin (pyrethroid - Group 3A) is another alternative that had controlled both aphids and mites. With pyrethroid resistance in aphids in the region, a decision to use it should be considered carefully and follow up with an evaluation for effectiveness.

Go here for a more complete summary of Spider Mite Management Guidelines 
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