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Soybean Aphids & Spider Mites Increasing (07/13/17)

Soybean aphids and spider mites are starting to increase in soybean fields.

Soybean Aphids & Spider Mites Increasing

Soybean aphids and spider mites are knodel.4starting to increase in soybean fields. Most soybean aphids are still non-economic and averaging about <10 aphids per plant. However, there are a few hot spot in fields with higher numbers. Spider mites are more spotty and mainly being found on field edges. Hot, dry weather will favor mite development; however, soybean aphids prefer more moderate temperatures in the lows 80s. Scouting is important now as soybeans are near R1 (beginning bloom).

In addition, they are still some thistle caterpillars being reported, but most caterpillars have matured and near the end of their feeding period (defoliation of foliage). They will be forming a chrysalis and then developing into the painted lady butterfly. Scouting is critical for all 3 insect pests of soybean, especially soybean aphid and spider mites.

A review of thresholds and which insecticides are appropriate to use for soybean aphid and spider mites.

Soybean aphid economic threshold:  Average of >250 aphids/plant and when populations are actively increasing in 80% of field form R1 through R5 (beginning seed). The benefit of any insecticide application is reduced after soybeans reach the R6 (full seed) growth stage.

 Spider mites:  Mites are small and mknodel.5 6agnification (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 leaves then slap them to dislodge the mites. Or, pulling plants and examining 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 mites 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 different locations. A general action threshold is to treat when heavy stippling on lower leaves with some stippling progressing into middle canopy; mites present in middle canopy with scattered colonies in upper canopy; and lower leaf yellowing common. (Source: University of Minnesota, Ostlie & Potter)

 

Insecticides for soybean aphids and spider mites:  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 in soybeans. 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 it is hot, residual 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 miticides 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 soybean aphid and 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 label 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:  Mention of a product does not constitute an endorsement by NDSU Extension Service or the author.

 

Janet J. Knodel

Extension Entomologist

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