Two-spotted spider mites are feeding on soybeans in southern and central Michigan. Spider mites are difficult to control with crop protection chemicals, so proper timing, product selection and sprayer setup and operation are critical. Soybean producers should scout their fields and use the information contained in this article and a publication written by MSU’s Christina DiFonzo, titled Scouting and Managing Spider Mite in Soybean, to improve mite control. You can also go to http://www.msuent.com/ and read Spider Mite Warning – especially southern and central Michigan on the homepage under “Hot Topics.”

If mite populations and crop injury warrant a chemical treatment, mite control will be improved by equipping and operating your sprayer correctly. Follow the recommendations outlined below. A more detailed explanation of equipping and operating sprayers for insecticide applications is available in the MSU Extension article Equipping and operating sprayers to control insects and diseases in soybean.

Spray volume

In general, higher spray volumes provide better canopy penetration and leaf coverage improving mite control. Follow the product label for specific recommendations. For example, Lorsban should be applied with at least 15 gallons of water per acre and Dimethoate should be applied with at least 25 gallons of water per acre.

Droplet size

Producing the correct droplet size is critical as the product must penetrate the crop canopy and thoroughly cover the leaf surface. Large droplets penetrate the crop canopy well, but don’t provide adequate coverage. Small droplets lack the energy to penetrate the canopy, but provide excellent coverage. Droplets having volume median diameters ranging from 200 to 350 microns will provide the required canopy penetration and leaf coverage.

Nozzle type

Nozzles that produce either a single, flat fan pattern or a twin flat fan pattern are recommended. The twin fan nozzles will perform well in smaller crop canopies, but they do not perform as well as single, flat fan nozzles in large and dense crop canopies.

Nozzle pressure

In general, higher nozzle pressures provide better penetration and coverage as long as the droplets remain in the optimum size range (200 to 350 microns). Nozzle pressures of around 40 psi are adequate.

Ground speed

Maintaining groundspeeds at 10 miles per hour or less will improve canopy penetration as more of the droplets’ energy is directed downward into the canopy at slower ground speeds.

Boom height

Setting the spray boom at the proper height is important as it affects spray pattern uniformity and the velocity of the droplets entering the canopy. Nozzle companies state their boom height recommendations as the distance between the “target” and the boom. The target should be the midway point between the lowest leaves and the top of the canopy when spraying for spider mites. Follow the nozzle manufacturer’s recommendations for determining the height of the boom above the target.

The tricky part is to select a flat fan nozzle or twin, flat fan nozzle that produces the optimum droplet size under the spray volume, ground speed and pressure recommendations listed above. Nozzle manufactures publish charts and Dr. Robert Grisso, agricultural engineer at Virginia Tech University, created an Excel spreadsheet that makes this decision easier.