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 Spider Mites – Threshold Overview, Effects of Rain Minimize
Location: BlogsWisconsin Crop ManagerInsects and Mites    
Posted by: WCM Staff 7/28/2005 12:08 PM
Spider mites continue to be present on soybeans in Wisconsin. The main questions coming in by phone and at field days over the last week are “What is the threshold for two-spotted spider mite?” and “How will the recent strong storms affect mite populations?”.

Spider Mites – Threshold Overview, Effects of Rain



Eileen Cullen, Extension Entomologist UW Entomology Department

Spider mites continue to be present on soybeans in Wisconsin. The main questions coming in by phone and at field days over the last week are “What is the threshold for two-spotted spider mite?” and “How will the recent strong storms affect mite populations?”.
 
I gleaned the text that follows directly from the Handbook of Soybean Insect Pests published by the Entomological Society of America (1994). I also include text from two excellent July 18th articles written by soybean entomologist Matt O’Neal at IowaStateUniversity for the ISU Integrated Crop Management Newsletter.
 
Spider mite management in soybean involves careful consideration of environmental conditions and crop growth stage. In soybean, spider mite control is limited primarily to chemical application. Initial monitoring should concentrate along field margins where infestations are likely. Leaves in the middle and lower canopy should be examined for the presence of stippling. (white to yellow spots where chlorophyll has been removed from mite feeding).
 
Treatment of isolated “hot spots” in a field may prevent infestation of the entire field. However, usually when an infestation is recognized, mites already have dispersed throughout the field. Before spot treatments are applied, thorough monitoring of the entire field is recommended. If mite injury is evident in the field interior, the potential for economic populations throughout the field within 1-2 weeks should be recognized.
Yield expectations in mite-treated fields may require adjustment because of droughty condition associated with outbreaks. In 1988, many fields in several Midwestern states required multiple treatments for spider mite control. The rapid recovery of mites after treatment significantly adds to the cost and difficulty of controlling this pest.
 
Traditional recommendations suggested that spider mite populations decline after heavy rainfall, but current research has shown that rainfall only indirectly results in mite population decline. Cool temperatures and high relative humidity associated with rainfall promote growth, dispersal, and infection of fungal disease of the mite. As conditions remain favorable for disease, more mites are infected and natural control of the population is more likely. Conversely, if heavy rainfall is quickly followed by clearing skies and high temperature, the disease cycle is prematurely interrupted. Mite populations, regardless of the presence of disease, will likely increase during hot, dry conditions.
 
Before treating, check that living mites are present. Examine all areas of the field, especially downwind, to identify the extent of the infestation in apparently healthy plants. Because they are small and stay on the undersides of leaves, spider mites can be difficult to see. One way to spot them is to slap leaves against a white piece of paper and look for very small moving dots on the paper. A hand lens is useful to distinguish spider mites.
 
When normal weather patterns with locally dry conditions occur, treatment is recommended when plants in infested areas have substantial stippling or leaf yellowing and live mites. Treatment may be delayed if cooler temperatures and high humidity are expected; however, scattered thunderstorms and rain alone will not reduce mite infestations altogether. Closely monitor infested fields if treatments are delayed, and treat before mites cause browning and leaf drop. Stippled discoloration on 10% to 15% of the leaves on affected plants is mentioned as a treatment guideline after pods have set (see two good article links from recent issues of the University of Illinois Pest Management & Crop Development Bulletin). Spot treat if the infestation is localized, but check other areas for mites.
 
Under drought conditions, treatment is recommended if leaves in infested areas are stippled and live mites are present. Before treating, check the entire field (and adjacent fields) for mites. Under very dry conditions, mites usually will occur throughout the field and spot treatments are unlikely to prevent the infestation from spreading. If mites are found throughout the field (even in low numbers) in addition to the more heavily infested areas, treat the entire field. Closely monitor treated fields for reinfestations. Avoid unnecessary sprays, but treat before injury becomes severe and leaves drop.
 
Recent Regional Web Links on Twospotted SpiderMite
 
O’Neal, M. 2005. That soybean pest mite not be anaphid: Spider mites infest dry Iowa. ISU Integrated CropManagement Newsletter, July 18.
 
O’Neal, M. 2005. Hot, dry weather mite be a problem:Two-spotted spider mites infesting soybeans. ISUIntegrated Crop Management Newsletter, July 18.
 
Steffey, K. 2005. More about twospotted spider mites.University of Illinois, Pest Management & CropDevelopment Bulletin. No. 12 Article 3/June 10, 2005.
 
Gray, M. 2005. Twospotted Spider Mite Infestations inSoybeans Intensify as Drought Conditions Persist.University of Illinois, Pest Management & CropDevelopment Bulletin. No. 14 Article 4, July 1, 2005.
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