Dectes Stem Borer In Sunflowers

By Ruth Beck
SDSU Extension Agronomy Field Specialist

About Dectes Stem Borer

A recent informal survey of sunflower fields in Hughes and Sully Counties revealed the presence of Dectes stem borer in roots or at the base of many of the sunflower stems.

During the summer, the Dectes stem borer, also referred to as the long horn stem borer, lives inside the sunflower stalk as a white larva. The larvae arise when adult stem borers lay eggs in the sunflower plant. The female Dectes stem borer chews a hole in the stalk just under the leaf petiole and places its eggs in the hollow cavity at the core of the petiole. Upon hatching, in early July, the legless larvae bore down the petiole and into the core of the main stalk. The larvae will only feed on the moist pith and will not consume dry material. Therefore, stalk desiccation will signal the end of larval feeding. At this time the larvae (only one per stem) will terminate feeding and prepare for winter by cutting a disk-shaped incision about ½ inch in radius from the center of the stalk. The larvae then plug the tunnel below the girdle with plant frass or shavings to seal itself in the chamber for the winter. During the above mentioned survey almost 100% of the Dectes found in surveyed fields were sealed in a chamber at the base of the plant just under the soil surface.

Impact on Sunflowers

In many cases, the stalk boring by Dectes in sunflowers has no measurable impact on seed yield or oil content. However, significant losses can occur when the insect girdles the sunflower plant and causes lodging. Dectes larvae cannot physically girdle a radius greater than about ½ inch. Therefore, lodging will occur more frequently in situations of high plant density and drought conditions that lead to a small stalk diameter. Hot, dry conditions that result in stalk desiccation will trigger the larvae to stop feeding and begin the girdling activity. In situations where soil moisture remains high close to harvest, sunflower stalks can remain damp and the larvae will continue to feed until cool temperatures limit their feeding. Under these conditions, few larvae complete the girdling process and some do not girdle at all. Therefore, lodging does not become such a problem.