Soybean Insect Scouting Update

David Owens, Extension Entomologist, owensd@udel.edu and Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

Some fields are experiencing borderline economic defoliation. The rain this week means that soybeans are growing again and that is going to help them. Many earlier planted fields in the reproductive stages and under irrigation have very good canopies. The primary defoliators are bean leaf beetles, Japanese beetles, and green cloverworm. Some fields are at the cusp of the 10-20% defoliation. Unfortunately, there aren’t very clear answers on the economics of treating vs not treating R-stage beans at this level of defoliation if going over the field with a fungicide. Dr. Dominic Reisig from NCSU cautions in his blog (https://soybeans.ces.ncsu.edu/2018/07/when-to-treat-earworms-in-soybeans/) that recent work in the Midsouth suggests that vegetative thresholds are very conservative. Other work from Virginia Tech stresses canopy development. If the canopy is thick, actively growing, and closed, beans can tolerate quite a bit of defoliation in the reproductive stages (https://pubs.ext.vt.edu/444/444-203/444-203.html). There is a very good extension bulletin from Purdue that discusses defoliation and for evaluating economics of defoliation and crop stage and may be of interest: https://extension.entm.purdue.edu/fieldcropsipm/insects/bean-leaf-beetle.php. If it is a concern for you and you have the flexibility, try a simple experiment on your farm. Treat the field as you normally would and on one or two of the last passes, put an insecticide in. Compare those areas at harvest; and let us know what you think.

Dectes is widespread. Spider mites were beginning to cause significant defoliation last week. Plants are recovering, but as Bill mentioned in his post last week, rain does not automatically ‘cure’ mite populations. Continue to check your fields and evaluate canopy health. Fields with large numbers of Dectes and that have had a history of lodging losses should be prioritized for timely harvest to reduce likelihood of foul weather causing lodging. Easier to say now in July than in October.

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