2022 Dectes stem borer survey – larvae and stem tunneling

Nicholas Seiter , University of Illinois Department of Crop Sciences Research Assistant Professor, Field Crop Entomology | nseiter@illinois.edu | (812) 593-4317 2

Ashley Decker Research Specialist in Entomology

In September-October 2022, researchers from the University of Illinois sampled 18 soybean fields in southern Illinois during growth stages R6-R8 for dectes stem borer. The main stems of 25 soybean plants per field were split open, and the presence or absence of dectes stem borer larvae and/or their tunnels was recorded. In addition, eight no-till fields in Warren and Henderson Counties in Western Illinois were sampled post-harvest by examining soybean residue for dectes tunnels and larvae at the basal portion of the plant remaining after harvest (i.e. the portion of the plant where dectes stem borer larvae overwinter). These values were then used to determine the percent of plants infested for each field.

Figure 1. Dectes stem borer larva and tunnel in a soybean stem. (Nicholas Seiter)

Dectes stem borer is an insect pest that has gained a lot of attention in southern Illinois over the last 5-10 years. The larvae bore into the pith of the stem, giving the insect its name. While the effects of this stem tunneling on yield are generally limited, when the larva prepares to overwinter, it girdles the base of the stem resulting in lodging at harvest.

Figure 2. Map showing percent infestation of fields sampled for dectes stem borer larvae and tunneling in Illinois. (Nicholas Seiter)

The level of infestation ranged from 0-92% of plants infested with either tunnels or larvae in previously (2021) surveyed areas in south-central Illinois (last year’s survey results can be found at https://go.illinois.edu/2021PestPathogenARB). No dectes stem borer larvae were found in Warren or Henderson Counties. This is the second year of a planned multi-year survey to observe the distribution and spread of this insect. If you are interested in participating in future surveys, please email nseiter@illinois.edu with the subject line “Illinois dectes survey.”

“Dectes stem borer can be sort of a hidden pest (you won’t know you have it unless you go out looking for it), so we’re conducting this survey to better understand the limits of its distribution in Illinois,” says Seiter. “Our goal is to provide this information to soybean farmers so they can better understand their own risk of encountering this pest and what they can do about it.”

The Illinois Soybean Association provided funding for this effort.  We thank Andrea Kohring (Precision Conservation Management), Phil Krieg (Syngenta), Randy McElroy (Bayer CropScience), Talon Becker (University of Illinois Extension), and Chelsea Harbach (University of Illinois Extension) for their help identifying and/or surveying fields. In addition, we thank Dennis Bowman (U University of Illinois Extension) for preparing Fig. 2).