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Twospotted spider mite (Tetranychus urticae) outbreaks in soybeans can occur in hot and dry weather conditions. Scouting fields can help detect outbreaks early and facilitate timely and effective control measures.
There are currently no economic thresholds established for spider mites3. However, an insecticide application is often recommended when stipling is commonly observed in the lower canopy and is extending into the middle canopy, and when mites are present in the middle canopy with a few colonies in the upper canopy3. Treatments should be made before mites cause much leaf bronzing and leaf drop. If spider mites are found early, it may be feasible to just make an insecticide application to the outside border of the field, making enough passes to extend beyond any noticeable damage from the spider mites1.
Chlorpyrifos (the active ingredient in Lorsban®-4E) and dimethoate are two widely recommended insecticides available for control of spider mites. It is important to have adequate water volume and spray pressure since mites are on the undersides of leaves1. Dimethoate can be systemic in soybean plants; however, systemic activity is reduced in drought-stressed plants4. Chlorpyrifos does not have systemic activity4. These products will not control eggs and have short residuals, so mites can begin rebuilding their population in a few days2. Although more than one application may be necessary to maintain control under certain conditions, it is important to follow all label restrictions. The Lorsban 4E label states that if large amounts of eggs are present, the treated area should be scouted in 3 to 5 days and if newly hatched nymphs are present, an application of a non-chlorpyrifos product should be made. Additionally, only 3 applications of chlorpyrifos are allowed annually, and there must be at least 14 days between applications. The reapplication interval for dimethoate is 7 days. The preharvest intervals are 28 and 21 days for chlorpyrifos and dimethoate respectively. There are synthetic pyrethroids labeled for spider mites, but generally they are not recommended due to the lack of field data for their control of twospotted spider mites6. Additionally, their application may control the beneficial insects, increasing the potential for growth of the spider mite population.
Rain can help reduce spider mite populations, but expectations should be set appropriately. Rain washing mites off of the plants is less beneficial than the other effects rainfall can have on plant and fungal growth2. Rainfall, preferably repeated rainfall in respectable amounts, can improve plant health, allowing for the plants to better tolerate injury from spider mites. The reproductive rate of spider mites can be reduced due to rain diluting the nutrient content of the soybean plant available for spider mites and/or cooler temperatures that often accompany rain. With longer periods of rain and/or high humidity that encourages long lasting heavy dews, there could be increased growth of the fungus Neozygites floridana to help suppress the spider mite population. After long periods of drought the fungal population may take time to rebuild due to low inoculum levels. The bottom line would be that if a field warrants an insecticide application, and there is a chance of rain the forecast, it is not recommended to cancel the insecticide application to wait for the rain2. If the application is made and rain comes, the soybeans will likely be better able to take advantage of the precipitation.
Sources: 1 R. Hammond, A. M, and J. Eisley. 2009. Twospotted spider mite on soybean. The Ohio State University Extension Fact Sheet FC-ENT-0024-09. http://ohioline.osu.edu (verified 7/1/2012). 2 K. Ostlie and B. Potter. 2009. Managing two-spotted spider mites on soybeans in minnesota. University of Minnesota Extension http://www.soybeans.umn.edu (verified 7/1/2012). 3 Plant Health Initiative. www.planthealth.info (verified 7/1/2012) 4 Gray, M. 2005. Twospotted spider mite infestations in soybeans intensity as drought conditions persist. University of Illinois Extension. The Bulletin. Issue 15. Article 4. http://bulletin.ipm.illinois.edu/ (verified 7/1/2012). 5 Varenhorst, A. et. al. 2012. Early confirmation of twospotted spider mite. Iowa State University Extension. Integrated Crop Management. http://www.extension.iastate.edu (verified 7/1/2012) 6 Krupke, C. and J. Obermeyer. 2012. No surprise, spider mites responding to dry conditions and stressed soybean. Purdue Extension. Pest & Crop Newsletter. Issue 12. http://extension.entm.purdue.edu (verified 7/2/2012) Individual results may vary, and performance may vary from location to location and from year to year. This result may not be an indicator of results you may obtain as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible. ALWAYS READ AND FOLLOW PESTICIDE LABEL DIRECTIONS. Leaf Design℠ is a servicemark of Monsanto Company. All other trademarks are the property of their respective owners. ©2012 Monsanto Company. 07242012EJP