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Spider mites in soybean

Along with soybean aphids UMN Extension scouts are scouting soybean fields looking for spider mites, gall midge injury and larvae and disease. This growing season has been a little bit drier than the 30-year average for parts of northwest Minnesota (Figure 1). This along with the hot temperatures today and forecast for the next several days could lead to drought stressed soybeans in some fields.
Departure of accumulated precipitation (in inches) from mean for May 15-August 2, 2019. Source: Midwest Regional Climate Center CLIMATE.

University of Minnesota Extension entomologist Dr. Bob Koch tells us why drought conditions can favor two-spotted spider mite infestations,

"Spider mite populations are held in balance by natural enemies, weather and host quality. Drought triggers spider mite outbreaks in soybean and corn by upsetting this balance in four ways.
  1. It accelerates spider mite movement from surrounding permanent vegetation and alfalfa as it dries down or is cut for hay. Under drought conditions, cutting sparks a mass movement into adjacent soybean and corn.
  2. It improves the food quality of soybean.
  3. It diminishes or stops fungal disease activity that attacks mites, such as Neozygites. Disease outbreaks are fostered by cool, highly humid conditions that favor spore formation and mite infection. Hot, dry weather stops these diseases.
  4. It speeds spider mite reproduction, so predatory insects and mites can't keep up."
While the maps of data collected through last week Friday did not indicate that there were spider mite infestations that had moved beyond field edges (Figure 2) there was at least one field in northwest Minnesota where they could be found along the field edge (Figure 3).
Figure 2. Spider mite infestations were not observed inside scouted fields. Map: NDSU IPM.
Figure 3. A spider mite infestation was found in one field in NW MN last week. Map: NDSU IPM.

Continuing to scout your fields for insects and waiting until thresholds are reached is the best way to both make sure that insecticide applications make biological and economic sense and preserve the effectiveness of insecticides for when they are truly needed to manage economically damaging infestations.

For more information about the two-spotted spider mite biology, scouting and management visit the UMN Extension Managing Spider Mite on Soybean webpage.
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