Soybean insect activity - Dectes stem borers

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In the last week, we scouted many soybean fields around north central Kansas and found no significant infestations of early-season defoliators. Of course, that can change relatively quickly once/if defoliators get started.

However, a relatively large number of adult Dectes (soybean) stem borers were sampled (Figure 1). The holes in the leaves were made by adult bean leaf beetles and are of no consequence, nor were the numbers of adult bean leaf beetles sampled. However, the Dectes stem borer adults were observed in relatively large numbers, but still only around the edges of the fields. This is where they usually aggregate for a week or two after emerging as adults, mate (note the 2 beetles at the top of the leaf in Figure 1), then disburse to deposit eggs in stems throughout the field for the next few weeks. The numbers revealed in sampling this past week were higher than usual. Thus, growers throughout the western two-thirds of Kansas need to be monitoring their fields that have high infestation levels in August/September and try to harvest those fields as soon as possible to try to prevent lodging due to soybean stem borer girdling.

Figure 1. Dectes stem borer adults from one sweep sample. The holes in the leaves were made by adult bean leaf beetles. Photo by the Department of Entomology, K-State Research and Extension.


Jeff Whitworth, Extension Entomologist

Tags:  soybeans insects dectes stem borer