Twospotted Spider Mite Persists in Northwestern Wisconsin

Eileen Cullen, Extension Entomologist, UW Entomology Department

As featured in Wisconsin Crop Manager articles http://ipcm.wisc.edu/wcm/insects.html, July 13^th (Vol. 13, No. 19 Twospotted spider mite detected in Jefferson County) and July 20^th (Vol. 13, No. 20, Potential for twospotted spider mite outbreaks exists), twospotted spider mite (TSM) problems did develop, and are persisting, in Northwestern Wisconsin.

Severe drought conditions in Northwestern WI counties have led to the current situation in soybeans.  TSM damage was evident in field inspections during my recent visit August 2^nd to the UW Extension St. Croix/Pierce County Soybean Field Day near Baldwin, WI, and from the road driving through Dunn, Chippewa and Eau Claire Counties.

Calls continue to come in August 11^th-14^th from the Northwestern district (including Washburn Co.) seeking information on how to identify TSM damage symptoms, treatment thresholds, effects of rain and a natural enemy fungal pathogen, and whether or not treatment is economically justified given the current soybean stage (R4 to beginning R5) and extreme drought conditions.  In a couple of calls, consultants and producers are balancing costs: benefits of a miticide treatment with a visit from the crop insurance adjuster as some fields are so severely affected by drought.

In this article, I provide an overview of information referenced earlier this season in Wisconsin Crop Manager on TSM life cycle, relation to weather and natural mortality factors, as well as damage symptoms and conditions that must be present to consider a miticide treatment. Hopefully, this information will help producers and consultants factor the TSM problem into soil moisture levels and drought condition of the soybean crop as they assess the situation on a field by field basis on-farm.