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Scout for Spider Mites in Beans (08/16/18)

It is not surprising to see spider mites showing up in soybeans and dry beans with the hot dry weather.

Scout for Spider Mites in Beans

It is not surprising to see spider mites showing up in soybeans and dry beans with the hot dry weather. Mites are small and magnification is required to see them. A quick sampling procedure to determine whether mites are present is to hold a piece of white paper below leaves, then beat them to dislodge the mites. The mites appear as tiny dust specks; however, they will move after being knocked off the leaf. Another method is to pull plants and examine the undersides of the leaves for mites and webbing. Begin from the bottom of plants and move upwards into the canopy. Feeding damage by mites first appears as small yellow spots (stippling). As feeding activity increases, leaves become yellow, bronzed or brown, and eventually shed from the plant. Be sure to scout during full pod (R4) through beginning seed (R5) stages since these crop stages are the most important contributors to soybean yield. Mite infestations typically are first noted near field edges. Soybeans are susceptible to spider mite feeding injury up through R6 crop stage. Cooler temperature and any moisture (or rain) could help slow the mite problem.

Spider Mite Threshold: There is no specific threshold that has been developed for two-spotted spider mite in dry beans or soybeans. Treatment is advised when heavy stippling on lower leaves with some stippling progressing into middle canopy. Mites may be present in middle canopy with scattered colonies in upper canopy. Leaf yellowing is common on lower leaves.

Pest Management: Insecticides registered for spider mites management in dry bean and soybean are listed in the 2018 North Dakota Field Crop Insect Management Guide E1143.

The only pyrethroid that will control spider mites is the active ingredient bifenthrin (Tundra, Sniper, Brigade, Fanfare, Bifenture, etc.) in dry beans and soybeans. Other pyrethroids, such as lambda-cyhalothrin (Warrior, Silencer, etc.), they will cause spider mites to flare up and then you will need to spray again with an organophosphate (OP) insecticide.

Two active ingredients of OP insecticides for control of spider mites are chlorpyrifos and dimethoate. However, chlorpyrifos (Lorsban and generics) is NOT registered for foliar application in dry bean. However, chlorpyrifos is labeled for spider mites in soybeans. Chlorpyrifos resistant spider mites were detected in NW MN, so be sure to re-scout field after application to be sure it worked. Dimethoate will control spider mites in dry beans and soybeans, but has a shorter residual than bifenthrin. We think it’s realistic to expect about a 7 to 10 day residual from bifenthrin (if it is hot, residual may be decreased), a 4 to 7 day residual from chlorpyrifos, and a 3 to 5 day residual from dimethoate. It is extremely important to scout and monitor for recurring spider mite populations after spraying. Check your fields five days after treatment and again at regular intervals to make sure your insecticide is holding. If newly hatched spider mites are observed after 5 days, a second treatment may be necessary with a different insecticide mode of action. For example, if you use bifenthrin (pyrethroid) for the first application, use a non-pyrethroid product, such as dimethoate or chlorpyrifos (OP), for the second application. Agri-Mek SC (active ingredient abamectin) is also registered for spider mite control in soybeans and dry beans.

Complete coverage is important for spider mite control. Use high water volumes 15-20 GPA via ground and 3-5 GPA via air, and high pressure to penetrate foliage (40 psi).

Check out the NDSU Extension YouTube video on pest management of two-spotted spider mites in soybeans. Early detection of leaf injury symptoms, such as stippling and leaf discoloration, is important to prevent crop damage from spider mites and facilitate rescue treatments. Proper scouting techniques, action thresholds and pesticide selection for managing spider mites are discussed.

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Janet J. Knodel

Extension Entomologist

This site is supported in part by the Crop Protection and Pest Management Program [grant no. 2017-70006-27144/accession 1013592] from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed are those of the website author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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