Crop & Pest Report


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Spider Mites in Soybeans in NC ND (08/17/17)

Spider mites are starting to be found in some soybean fields, mainly field edges, in north central ND.

Spider Mites in Soybeans in NC ND

Spider mites are starting to be found in some soybean fields, mainly field edges, in north central ND. Reports have come in from Glenburn, Minot, Underwood and Washburn. The hot dry conditions that this area experienced earlier in the summer stressed soybean, making them more susceptible to mite infestations. Here’s a review of spider mite scouting and thresholds that was printed in an earlier version of the Crop & Pest Report #13.

Mites are small and magnification (10x hand lens) is required to see them. A quick sampling procedure to determine whether mites are present is to hold a piece of white paper below the leaves, then slap or shake the leaves to dislodge the mites. Or, pull plants and examine the underside of the leaves from the bottom of plants upwards. The mites appear as tiny dust specks; however, they will move after being knocked off the leaf. 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. Mite webbing may be present on plants as mite numbers balloon on webs to disperse within and between fields. Watch the NDSU Extension YouTube video for more information on scouting for Spider Mites in Soybeans.

Mite Threshold: Deciding whether to treat is difficult. Sample plants at least 100 feet into the field and walk in a “U” pattern, sampling two plants per location at 20 locations. A general action threshold is to treat when heavy stippling on lower leaves occurs with some stippling progressing into middle canopy. Also, when mites are present in middle canopy with scattered colonies in upper canopy and lower leaf yellowing is common. (Source: University of Minnesota, Ostlie & Potter)

Insecticides for soybean aphids and spider mites:knodel.1. If spider mites are a problem along with soybean aphids, the only pyrethroid, IRAC 3A, that will work is bifenthrin(Tundra, Sniper, Brigade, Fanfare, Bifenture, etc.) in soybeans. While other pyrethroids, such as lambda-cyhalothrin (Warrior, Silencer, etc.) will control aphids, they will cause spider mites to flare up and then you may well have to spray again with bifenthrin or an organophosphate (OP) insecticide, IRAC 1B.

Two active ingredients of OP insecticides for control of soybean aphids and spider mites are chlorpyrifos and dimethoate. Dimethoate is weak on soybean aphid control, and has a shorter residual than bifenthrin. We think it’s realistic to expect about a 7 to 10-day residual from bifenthrin (if temperatures are hot, residual duration may be decreased), a 4 to 7 day residual from chlorpyrifos, and a 3 to 5 day residual from dimethoate. The efficacy of an insecticide can be improved significantly with sufficient coverage, >18 GPA of water by ground and 3-5 GPA by air and application at high pressure to penetrate foliage.

Other modes of action for spider mite control only include: IRAC 6 abamectin (Agri-Mek SC) and a miticide IRAC 10B etoxazole (Zeal SC). There also are several combination products with two modes of action that will control soybean aphids and spider mites: Tundra Supreme (bifenthrin + chlorpyrifos), Match-Up (chlorpyrifos + bifenthrin), Brigadier (bifenthrin + imidacloprid), Cobalt Advanced (chlorpyrifos + lambda-cyhalothrin), and Hero (bifenthrin + zeta-cypermethrin). Some combination products contain active ingredients that aggravate mites, such as lambda-cyhalothrin, imidacloprid and zeta-cypermethrin.

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 product 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, IRAC 3A) for the first application, use a non-pyrethroid product, such as dimethoate or chlorpyrifos (OP, IRAC 1B), for the second application. Look for different IRAC (Insecticide Resistance Action Committee) numbers on labels to indicate different Mode of Action labelling of insecticides. We want to prevent the development of insecticide resistance that has been observed in Minnesota for soybean aphids and spider mites.

Note: Always read and follow the insecticide label. Products are mentioned for illustrative purposes only. Their inclusion does not mean endorsement and their absence does not imply disapproval.

Janet J. Knodel

Extension Entomologist

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