By Kelley Timon, State Specialist for Field Crop Entomology; Andy Michel, Entomologist
With continued dry weather, spider mites are one of the main pests to remain vigilant about in field crops. They will often show up in field borders first as they move in from other habitats — for example if nearby ditches have been mowed. Spider mites are difficult to see. Look for injury signs: yellow spotting or stippling on the upper side of leaves. In soybean, this damage usually begins in the lower canopy and progresses upward as the mite population increases. Heavily infested leaves may also have light webbing similar to spider webs.
There are no number-based thresholds available for mites, in part because counting them is not practical in a scouting context. During drought, populations can increase rapidly so scouting every 4-5 days is recommended during drought conditions. Walk a broad pattern in the field and examine at least two plants in each of 20 locations. Use the following scale developed by the University of Minnesota to evaluate spider mite damage in soybean, with treatment recommended at level 3. There are relatively few products available for the treatment of two-spotted spider mites and some pyrethroid insecticides may actually “flare” spider mite populations, making them worse.
0. No spider mite injury observed.
1. Minor stippling on lower leaves, no premature yellowing observed.
2. Stippling common on lower leaves, small areas on scattered plants with yellowing.
3. Heavy stippling on lower leaves with some stippling progressing into middle canopy. Mites present in middle canopy with scattered colonies in upper canopy. Lower leaf yellowing common and some lower leaf loss. (Spray Threshold)
4. Lower leaf yellowing readily apparent. Leaf drop common. Stippling, webbing and mites common in middle canopy. Mites and minor stippling present in upper canopy. (Economic Loss)
5. Lower leaf loss common, yellowing or browning moving up plant into middle canopy, stippling and distortion of upper leaves common. Mites present in high levels in middle and lower canopy
Common choices for spider mite control in soybeans are products containing chlorpyrifos, dimethoate and bifenthrin though other miticides exist. A product newly available for twospotted spider mite control is Agri-Mek (Syngenta), whose label was recently expanded to include soybean and sweet corn. We have not evaluated the efficacy of this product.
Grasshoppers, Japanese Beetles and Other Soybean Defoliators
Dry weather encourages grasshoppers. As with spider mites, infestations often begin along field borders. Grasshoppers are much easier to kill when they are small, so timely treatment is helpful. A general defoliation threshold can be used for all leaf-feeding insects in soybean regardless of species. These insects include Japanese beetles, grasshoppers, green cloverworms, and various other caterpillars. If soybeans are in pre-bloom, they can tolerate up to 40% defoliation before treatment is advised, and 15% defoliation from bloom to pod-fill. These percentages refer to whole-plant defoliation, not just a few leaves.
A word to the wise about treating for defoliators: A good reason to avoid pulling the trigger on a spray too soon, apart from simple economics, is that many pyrethroid products will actually make spider mite problems worse. We often see the most intense spider mite flare-ups in fields that have been treated with a broad-spectrum pyrethroid, particularly when environmental conditions favor them anyway.
Western Bean Cutworm
We have had a fair number of trap catches over the past week, which suggests that we are entering a period of intense flight. With the heat expected to turn up this week, we would predict that peak flight should occur by late next week. Over the next few weeks, corn should be inspected every 4-5 days for the presence of egg masses, especially corn that has not yet tasseled. Check 10 corn plants in 10 different locations — if greater than 8% of those have egg masses, treatment may be necessary. Controlling western bean cutworm is more difficult after the larvae enter the ear, so good egg scouting is critical to prevent ear damage.