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Extension > Agriculture > Crops > Soybean Production > Insects & mites > Twospotted spider mites in Minnesota soybean

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Twospotted spider mites in Minnesota soybean

Robert Koch, Extension entomologist and Suzanne Burkness, Research assistant
2016

Name

Twospotted spider mite adults and eggs

Photo: David Cappaert, Michigan State University, Bugwood.org

Adult mites traveling on webbing

Photo: David Cappaert, Michigan State University, Bugwood.org

Figure 1. Twospotted spider mite adults and eggs (top) and adult mites traveling across webbing (bottom).

Twospotted spider mite (Tetranychus urticae)

Identification

Adults (Figure 1)

Eggs

Eggs are clear, round, and shiny. They are difficult to see and are most often on webbing.

Nymphs

Natural history

Twospotted spider mites overwinter as eggs on plant debris. Egg to adult development takes 1-2 weeks depending on temperature, with individuals developing through larval and nymphal stages before becoming adults. Multiple generations occur per year. Under high temperatures (>90°F) and dry conditions, twospotted spider mite infestations can increase rapidly over short periods of time.

Twospotted spider mites establish colonies on the undersides of leaves and produce webbing over infested leaf and stem surfaces (Figure 1). This webbing is what gives them the "spider" part of the name. Infestations generally begin low in the canopy and move upward as the infestations progress.

Impacts

Photo: Whitney Cranshaw, Colorado State University, Bugwood.org

Figure 2. Soybean leaf yellowing caused by spider mite feeding.

Twospotted spider mites feed on soybean leaves and cause injury by sucking contents out of leaf cells. The plant cells injured by mite feeding appear as white or yellow spots (stippling) on leaves and are usually most abundant on the undersides of leaves (Figure 2).

In severe infestations, infested leaves will turn yellow to tan, or sometimes bronze-colored, and may drop off plants. Infestations can reduce yield.

Damage is often first noticed on field borders as mites disperse into fields from surrounding vegetation. This movement is often greater in the direction of prevailing winds, because the mites release silken threads which allow them to be carried on the wind ("ballooning").

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.

Scouting

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:

  1. No spider mites or injury observed.
  2. Minor stippling on lower leaves, no premature yellowing observed.
  3. Stippling common on lower leaves, small areas on scattered plants with yellowing.
  4. 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)
  5. 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)
  6. 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 2/3 of the plant canopy. Because populations can increase rapidly, scouting should be repeated every 4-5 days.

Treatment

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"). More information on scouting and treatment selection can be found in the Extension publication, "Managing twospotted spider mites on soybeans." As with any pesticide use, follow directions on the product label.

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