BROOKINGS, S.D. - A recent, informal survey of sunflower fields in Hughes and Sully Counties in South Dakota revealed the presence of Dectes stem borer in roots or at the base of many of the sunflower stems.
Ruth Beck, South Dakota State University Extension Agronomy field specialist, who conducted the informal survey, says although Dectes can be found in sunflower fields most years, in many cases, the stalk boring by Dectes in sunflowers has no measurable impact on seed yield or oil content.
But, significant losses can occur when the insect girdles the sunflower plant and causes lodging. The survey she conducted this fall showed that in most infested sunflower fields, the borer had girdled the plants.
Beck attributes this to the drier growing conditions of 2016. "Dectes larvae cannot physically girdle a radius greater than about half an inch," she says. "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 girdling will begin.
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"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," says Adam Varenhorst, assistant professor and SDSU Extension field crop entomologist.
Under these conditions, Varenhorst says few larvae complete the girdling process and some do not girdle at all. Therefore, lodging does not become such a problem.
Greatest risk
Typically, oilseed dryland sunflowers are more at risk because they are planted at higher populations than confection types. In many years, sunflowers can tolerate Dectes infestations without sustaining any losses.
To avoid issues, Beck encourages sunflower growers to carefully control plant spacing to manage stalk radius and other cultural practices that preserve soil moisture.
Beck also says Dectes can be a pest in soybeans and is hosted by a number of common weeds including ragweed and cocklebur.
"Information from Kansas suggests that wild sunflower is resistant to this pest," she adds.
White larva
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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," says Patrick Wagner, SDSU Extension entomology field specialist. "At this time, the larvae (only one per stem) will terminate feeding and prepare for winter by cutting a disk-shaped incision about half an 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 Beck's survey, "nearly 100 percent of the Dectes found in surveyed fields were sealed in a chamber at the base of the plant just under the soil surface," she says of the evidence that girdling had occurred.
