Potential for Two-Spotted Spider Mite Outbreaks Exists

Eileen Cullen, Extension Entomologist, UW Entomology Department

In the July 13^th issue of Wisconsin Crop Manager, I reported Two-Spotted Spider Mite (TSM) documented in Jefferson County by Tim Bender, UWEX Jefferson County Crops/Soils Agent; and recommended scouting for spider mites particularly under drought conditions in the northwestern and north central parts of the state. This article is to keep that reminder in the forefront.  To date, additional two-spotted spider mite detections have been made as follows …

July 21^st, Kevin Jarek, UW-Extension Outagamie County Agriculture Agent related a report from a July 20th evening meeting that at least one field in Outagamie had potential two-spotted spider mite populations. Kevin is following up with the agronomist who observed two-spotted spider mite in that field report.  Krista Hamilton, WI DATCP Pest Bulletin likewise alerted readers July 21st to the possibility of spider mite outbreaks in soybeans http://pestbulletin.wi.gov/. WI DATCP reports of moderate two-spotted spider mite infestations were received from growers in the south central district and low populations were observed in Juneau, Portage and Wood Co. fields; no economic populations were found. Although heavy rains received in the past few days in some areas may reduce the potential of spider mite problems, populations can build quickly under hot and dry conditions.  Moreover, several areas of Wisconsin continue to experience drought conditions, and two-spotted spider mite potential should not be overlooked. 

TSM adults are tiny (less than two-tenths of an inch), yellow-green, with eight legs. Adults are named for the dark pigmented spots on either side of their oval bodies. Eggs are round and white to light yellow and are found on soybean leaves (typically underside) along with adults, and webbing that may be noticeable on infested leaves. Because TSM are so small, they often go undetected until leaf stippling, webbing and eventual leaf bronzing and leaf drop under severe infestations. A hand lens is often necessary to clearly see TSM adults, nymphs and eggs on leaves.  You can also tap plants onto a white sheet of paper to dislodge TSM from the plant, if present you can see the black spots moving on the paper and look closer to identify.

More information can be found on TSM identification, plant damage symptoms and soybean scouting methods and treatment thresholds in the 2006 Wisconsin Fertilizer Aglime & Pest Management Conference Proceedings on-line, Spider Mites: A to Z (Author search, Cullen) http://www.soils.wisc.edu/extension/wfapmc/; and archived Wisconsin Crop Manager Vol. 12. No. 21, July 28, 2005 Spider Mites Threshold Overview, Effects of Rain http://ipcm.wisc.edu/wcm/.