Longhorned Beetle
longhorned beetle
Longhorned beetle

The Longhorned Beetle (Dectes texanus) is also referred to as the Long-horned Sunflower Stem Girdler. More commonly known, however, as the ‘Dectes', this insect has been most prolific in the High Plains, especially in Kansas. The adult is 3/8 to 5/8 inch long and is pale bluish gray with extended antennae that are longer than the body. The larvae are creamy white when fully grown and corrugated in appearance. The Dectes is also known as the soybean stem borer. But the insect prefers sunflower as a host plant. In the last few years, the Dectes have been commonly found throughout SD and in south central ND in addition to Kansas and other High Plains states.
dectes larva
Dectes larva

Life Cycle: Adults appear in mid-June to early July in the High Plains. The adults have an extended period of activity in the field. Eggs are laid on leaf petioles. The hatched larvae tunnel into the stalk to feed. The mature larvae girdle the inside of the base of the stalk and overwinter. There is one generation per year. Other host plants include soybean, cocklebur and ragweed.
dectes girdled stalk
Girdled stalk

Damage: An infected plant is susceptible to lodging breaking off at the base of the plant making harvesting of the downed plant nearly impossible.

Economic Thresholds: There are no thresholds established.

Scouting Method: None has been established.

Management: This insect does not appear to be of economic consequence unless the infected plant lodges. According to KSU research, early or at least timely harvest is the best management tool. Lodging is more likely when plants are under 10% seed moisture. Delayed planting has not proven to be a management tool. Field observation and research indicates a thick stalk will withstand lodging. In years with adequate soil moisture stalks are larger and lodging is less likely. Insecticide control of the adult has not been efficacious due to the extended emergence period. The larvae are well protected in the stalk and insecticide treatment has not proven efficacious.

Research: The NSA has funded a post doctoral scientist for the 2008-2011 seasons to work on developing host plant resistance to the Dectes and other insects. Perennial sunflower species are resistant to stalk infestation indicating that resistance is a possibility. Testing of breeding material has been ongoing in Kansas for several years. The goal is to identify and refine this material and get it into the commercial hybrid seed company breeding programs as soon as possible.

Source: NDSU Extension Bulletin 25 Sunflower Production, NDSU Extension Service, September 2007, High Plains Production Handbook June 2005 and personal communication with Dr. JP Michaud, KSU Research Entomologist.

More information about insects can be found in the Archive section of The Sunflower magazine.
Additional Resources
High Plains Sunflower Production Handbook  file size: 1554 kb

NDSU Extension Bulletin 25 - Revised 9/2007 file size: 5591 kb

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