Two-spotted Spider Mite detected in Jefferson County

Inspect Fields in Dry Counties

Eileen Cullen, Extension Entomologist, UW Entomology Department

On Friday, July 7^th, Tim Bender, (UW Extension Crops/Soils Agent, Jefferson County) reported Two-spotted spider mite (TSM) from his soybean rust sentinel scouting field location (southern WI). Thus far, Tim’s report is the only one I have received of TSM, and it is surprising since Jefferson County is not particularly dry as compared to the Northwestern and North Central portions of Wisconsin that are extremely dry.  TSM is associated with hot, dry weather and infestations can increase rapidly under such conditions. At this time, TSM have not reached economically damaging levels in Tim’s Jefferson County sentinel plot.  After a follow-up visit to the plot July 10^th, Tim reports TSM still present in the field, on 4 out of 20 plants closely inspected.  He found webbing on the underside of leaves as well as minor leaf stippling. He detected adults, immature nymphs and eggs. Additionally, Tim observed a possible fungal infection associated with the TSM. As part of close inspections Tim is conducting in the soybean rust sentinel plot, and based on his experiences during the 2005 growing season with TSM, Tim has a keen eye and detected TSM presence in the field.

At this time, we are not aware of detections elsewhere in Jefferson County, nor in the state. However, Tim’s report serves as a reminder to be vigilant, particularly in the very dry counties of Wisconsin. 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 other damage symptoms start to appear. Reference photos from the 2005 season are provided with this article.

At this time, I am reporting Tim’s find of TSM, and reminding readers to brush up on TSM identification and scouting methods.  A closer look at soybeans, particularly in dry counties is recommended. This will help avoid situations where producers/consultants are taken by surprise by TSM, should this mite pest become an issue later this month and into August. More detailed information can be found on TSM identification, plant damage symptoms and soybean scouting methods 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/.

Photos: Peter Sonnentag, UW Madison Entomology Department 2005