On a recent Extension ag crop team conference call, OSU Extension crop entomologists Kelley Tilmon and Andy Michael talked about some potential insect pests that growers should be scouting for in their fields, especially during dry, hot periods. Here are their comments and advice regarding spider mites.
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 to 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 mites of 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, 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.