Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu
Despite the recent rains, continue sampling for two-spotted spider mite (TSM) in soybeans. In a previous article, I mentioned that rain doesn’t always cause TSM populations to crash. Based on what I have seen in soybean fields in southern DE and in an untreated check in one of my field trials, TSM numbers seem to be holding on. Below is a graph showing rain events and observed TSM populations in the untreated check from a TSM field trial conducted in Georgetown, DE.
Observed Influence of Precipitation on Two-Spotted Spider Mite Populations
Weather data obtained from the Delaware Environmental Observing System (DEOS): http://www.deos.udel.edu/data/agirrigation_retrieval.php
The good news is that with the recent rains, the plants are not stressed and appear to be putting on new growth. Time will only tell if the TSM populations will increase or crash but as we move into August, the cooler nights and high humidity have historically caused TSM to decrease. This will hopefully be to our advantage. Regardless, make sure you are scouting your fields so that you can closely monitor if populations are increasing or decreasing in your fields.
The threshold for Two-spotted spider mites during bloom to podfill is 20-30 mites per leaflet and 10% of plants with one third or more leaf area damaged. To scout for mites, examine the underside of 5 leaflets in 10 locations, noting the number of mites and eggs and amount of leaf damage.
Here is a link to our Soybean Insecticide Recommendations for chemical control options: https://cdn.extension.udel.edu/wp-content/uploads/2012/05/18063934/Insect-Control-in-Soybeans-2017-final.pdf
In 2016, two miticides were registered for use on soyeans; Zeal SC and Agri-Mek SC. These are the only labeled formulations of these products on soybean. Based on preliminary results, both of these products appear to be providing excellent control in my research plot. Please consult the label for rates, additional restrictions, and adjuvant requirements.
Zeal SC Supplemental Label for use on soybeans: http://www.cdms.net/ldat/ldCCK003.pdf
Agri-Mek SC Label: http://www.cdms.net/ldat/ld9NL020.pdf