Scout for Spider Mites in Soybeans, Dry Beans and Corn (07/21/21)
The first reports of spider mites being found on field edges of soybeans came in this week from West Fargo, ND and Crookston, MN. Spider mites are small (1/50 of an inch long) and magnification is usually required to see them. A 10x hand lens is helpful in seeing spider mites in the field.
Spider mite infestations typically are first noted near field edges, so start scouting at field edges to see if spider mites are present. A quick sampling procedure to determine whether mites are present is to hold a piece of white paper below leaves, then beat the leaves to dislodge the mites. Spider mites appear as tiny dust specks and they move slowly after being knocked off the leaf. Another method is to pull plants and examine the undersides of the leaves for mites and webbing.
Spider mites start feeding on the bottom leaves of plants and then move to higher leaves. Feeding damage by spider 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 full seed (R6) stages since these crop stages are the most important stages for soybean yield.
If fields are above the action threshold for spider mites, then an insecticide or a miticide treatment may be necessary. In Table 1 (next page), most of the insecticides and miticides available for control of spider mites in dry bean, soybean and corn are listed in addition to the IRAC Group, low and high rates, cost of products, and the pre-harvest interval (PHI).
The only pyrethroid insecticide that will control spider mites is bifenthrin (Tundra, Sniper, Brigade, Fanfare, Bifenture, and other generics) in dry bean, soybean and corn. For residual, we expect about a 7 to 10 day residual from bifenthrin (if it is hot, residual will be decreased). While other pyrethroids, such as lambda-cyhalothrin (Warrior, Silencer, and generics) will control other insects like grasshoppers and soybean aphids, they will cause spider mites to flare up and increase in numbers. Then, you will need to re-spray with a different mode of action, such as an organophosphate (OP) insecticide or miticides.
The OP insecticide chlorpyrifos (Lorsban and generics) will control spider mites in soybean and corn. However, chlorpyrifos is NOT registered for foliar application in dry bean. It is only registered as a preplant broadcast or at-plant T-band for control of seed corn maggot in dry beans. Dimethoate will control spider mites in dry beans and soybeans. Expect a 4 to 7 day residual from chlorpyrifos, and a 3 to 5 day residual from dimethoate.
Several insecticide premixes (2 active ingredients mixed together) available for spider mite control include Hero, Tundra Supreme, Match-Up, Skyraider and Minecto Pro. See Table 1 for details.
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 (40 PSI) to penetrate foliage. For insecticide resistance management of mites, do not apply the same class of insecticide (or mode of action) more than twice and alternate the mode of action between applications to prevent buildup of resistant mite strains.
It is extremely important to re-scout and monitor for recurring spider mite populations after treatment, since the insecticides do not kill the egg stage of spider mites. 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 mode of action to prevent the development of pesticide resistance. For example, if you use bifenthrin (pyrethroid) for the first application, use a non-pyrethroid product, such as dimethoate or chlorpyrifos (OP), or a miticide for the second application.
Miticides also are available for spider mite control and some products control all life stages, such as Zeal SC in soybean and corn.
Always read and understand the label. The label is the law.
Research Specialist, Extension Entomology