Guess the Pest! Week 21 Answer: Dectes Stem Borer

David Owens, Extension Entomologist,

Congratulations to Matt Morris for correctly answering last week’s Guess The Pest Challenge. Matt has earned a sweep net which will come in handy in soybean fields and diffusing awkward encounters with strangers, because deep down inside, everyone thinks sweep nets are cool.

Dectes stem borer adults emerge from their overwintering cell in last year’s soybean, ragweed, cocklebur, and/or sunflower between June and early August and lay eggs in leaflet petioles. As soon as the egg hatches, larvae burrow into the petiole and start moving to the central stem. When they enter the stem, the petiole will sometimes wilt (Figure 1). Gently tugging at a wilted petiole will reveal a red tunnel entrance in the center of the petiole scar (Figure 2). Gently splitting open the petiole stem will reveal tunneling, and splitting open the plant stem below the petiole will often reveal the Dectes larva (Figure 3).

Figure 1. Wilted petiole caused by Dectes stem borer

Figure 2. Tunnel at the center of the petiole scar

Figure 3. Dectes larva in tunnel

I say often, because sometimes the larva disappears, most likely falling out of the plant and dying on the ground surface. Other times they will start to work on the main stem and then go back up the petiole. And sometimes you may have more than one larva in a plant. This is why they girdle plants. The root can only support one overwintering larva, so the first one down seals the stem off to any that try to follow it, cuts the plant, and leaves following Dectes out in the cold to die in the winter. Larvae are also cannibalistic if they meet each other in the plant. Right now, larvae can be found in the main stem, few have gone down to the soil line in soybean. Plant senescence and stem drying is their cue to begin overwintering preparation.

Once inside a plant, Dectes will not be affected by insecticides. Our only management tactics are cultural: timely harvest, crop rotation, and fall disking or plowing which can reduce winter survivorship.