Not the Usual Suspect

August 28, 2023 10:01 PM
Blog Post

While it is no secret soybean disease has been more prevalent in soybean fields in southwest Iowa this month. However, not all the problems in soybeans can be attributed to disease incidence. I have received and observed an unusual pest this month, Dectes stem borer! Look at your soybeans carefully to determine the cause of premature death.

Dectes stem borer is native to North America and can be found in sunflower, giant ragweed, and cocklebur to name a few common weeds. The borer is a small grey, flat faced, longhorned beetle. The antennae are longer than the body and have black and grey rings. The life cycle of Dectes stem borer is that they emerge from mid-June to early-October. Peak emergence is from mid-July to early-August. Adults can live as long as 60 days and feed on tender epidermises of stems. Eggs laid by adults in the pith of leaf petioles will hatch 3-5 days later. As they mature into the second and third instar, they move to the main stem of soybeans and work towards the base of the plant where they form an overwintering chamber. During the following spring, the larvae create an exit hole for emergence as an adult. 

Damage caused by Dectes stem borer can be like other diseases or insects without close inspection. The presence of wilted leaves in then upper canopy are just one clue. The key feature to look for in wilting plants is the exit hole in the petiole or split the stem and look for frass in the stem and the larva. Management is difficult due to the fact the larvae are contained inside the stem, so insecticides are rarely effective. One big feature is to identify the pest and where practical, harvest early.


Aaron Saeugling Field Agronomist in SW Iowa

Aaron Saeugling is a field agronomist in southwest Iowa for Iowa State University Extension and Outreach.