Dectes Stem Borer Adults Are Active in Soybean

Figure 1. Field-collected Dectes stem borer (smaller gray beetles in the bag) and ash gray blister beetles (larger beetles in the bag). (Courtesy: Zachary Loomis)

Originally Submitted: July 11, 2024

We are again surveying soybean in South Dakota for Dectes stem borer adults through a grant from the South Dakota Soybean Research and Promotion Council. Last year we started observing Dectes stem borer adults in the middle of June, but this year the beetles were a little slower on emerging. However, large populations of beetles have been observed in soybeans this week (Figure 1). Our samples have also yielded large numbers of ash gray blister beetles, which are sometimes confused with Dectes stem borer. As a rule of thumb, always look at the length of the antennae, as ash gray blister beetles have shorter antennae and will also be much larger in size than Dectes stem borers. Last year, adults were collected from Bon Homme, Clay, Douglas, Turner, and Yankton counties. So far in 2024, we have also observed adults in Lincoln County.

Identification

Adult Dectes stem borers are light gray and approximately three-eighths of an inch long. They have long antennae with segments that alternate gray and black in color (Figure 2). The larvae are cream colored and often are referred to as having an accordion shaped body (Figure 3). The larvae range from one-half to five-eighths of an inch long. The larvae observed in soybean stems will be smaller than those observed in sunflower stems.