Drought-stressed crops are more susceptible to pest damage, and two-spotted spider mites are making the most of this year's hot, dry weather.
University of Minnesota Extension researched spider mite control during the last major outbreaks in 1988, 2007 and 2009; the recommendations below are based on that research.
For farmers and crop advisors not familiar with spider mites, the damage they cause may be mistaken for drought symptoms. Infestations usually begin on fields' edges, particularly adjacent to cut grass or alfalfa.
If you see a discoloration of the lower leaves on your soybeans or corn, you need to get in the field and scout for mites. To do so:
1: Start at the field edge where symptoms occur.
2: Examine leaves from the bottom upwards. Look at the underside of leaves. Note yellow spots (stippling), webbing and how far up the plant the damage has progressed.
3. Tap damaged leaves over a white sheet of paper and look for mites with a hand lens or magnifying glass. They are very small — half the size of a soybean aphid nymph.
4. If mite presence is verified, it's time to progress into the field. Move at least 100 ft. into the field before making your first stop. Walk a "U" pattern, checking at least two plants at each of 20 locations.
5. Check fields every 4-5 days if drought persists.
Treatment is recommended only if damage and mites are detected throughout the field. Edge treatments are not effective since mites are usually spreading throughout the field before any visual symptoms are noticed.
For soybean, a general guideline is that if stippling reaches mid-canopy leaves, a treatment is likely necessary. For corn, the goal is simply to keep damage from reaching the ear leaf; treat when the lower one-fourth to one-third of the canopy shows damage and mites can be seen in the middle third of the canopy.
Spider mites live on the undersides of leaves and are more concentrated lower on the plant. If you decide to spray, canopy penetration is critical. Do not skimp on water—for ground applications, use 20 or more gallons per acre; for aerial applications, use 3 to 5 gallons. Evaluate control three to five days after spraying.
If you've decided that an infestation warrants spraying, don't hold up a spray waiting for rain. Instead, if rain is unlikely to occur before the spray dries, go ahead with the spray. Reducing spider mite pressure will allow the crop to take full advantage of any moisture from the rain
If your field is infested with both spider mites and soybean aphids, base the treatment decision on the worst problem.
As drought intensifies, spider mites become the predominant problem and they are more difficult to control. Some aphid treatments can cause spider mite infestations to flare up, so check previously sprayed fields. For dual mite/aphid infestations, only chlorpyrifos and bifenthrin, and mixes containing these ingredients, have been shown to work. Follow all instructions for application.
For more information on two-spotted spider mites in soybean, including scouting advice, a threshold scale to use when scouting and recommended insecticides, see www.extension.umn.edu/go/1116.