s Dectes Stem Borer And Lodged Soybeans M


DR. NICK SEITER URBANA, ILL.


any soybean growers have had problems with lodging at harvest this


year. The primary culprit for this (as for many of our woes this fall) was the extended pe- riod of unfavorable weather that we have suffered. How- ever, in parts of southern Illi- nois damage by the dectes


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December 9, 2016 / MidAmerica Farmer Grower•9


stem borer contributed to this problem. The adult dectes stem borer


(Figure 1) is a “long-horned” beetle that can often be found in soybean and on other plants. The adult female chews a hole into the surface of the plant (usually at the petiole), and lays her eggs in the pith. This often results in individual petioles wilting or falling com- pletely off of the plant, which is usually the first sign of an in- festation. Upon hatching, the larva (Figure 2)


tunnels


throughout the stem and feeds on the pith. As the plant nears maturity, the larva moves into the base of the stem where it will spend the winter. As awful as the bored-out stem of a soy- bean plant looks after being at- tacked by dectes stem borer (Figure 3), economic losses only occur if this damage leads to lodging. When preparing to overwinter, larvae will often girdle the base of the stem, causing the plant to break off and leading to harvest difficul- ties and reduced yield (Figure 4). This insect has one genera- tion per year, with the adults usually emerging from soy- bean residue beginning in late June in Illinois to start the cycle again. Insecticides are generally not


recommended for control be- cause the larvae are protected within the stem and the adults lay eggs over a long period of time in the summer (approxi- mately mid-July through Au- gust in Illinois). While there appear to be some differences in varietal susceptibility, these differences are not well docu- mented, and to my knowledge no soybean varieties have been characterized as resistant to dectes stem borer. However, there are some cultural man- agement options available to producers: Monitoring.While there


is no economic threshold established, finding adults at higher than average numbers will be the first indication of a problem. Note wilting or broken-off petioles, and split soybean stems toward the end of the season to gauge the level of infestation. In addition, ex- amine soybean stems in lodged areas to determine if dectes stem borer was part of the problem. If you have


Figure 1. An adult dectes stem borer at rest.


Figure 2. A larva of the dectes stem


borer removed from a soybean stem. Figure 3. Damage to the inside of a


soybean stem caused by a dectes stem borer larva.


Figure 4. Lodged soybeans due to dectes stem borer feeding. (Photo: Eric Alinger, Dupont Pioneer)


6• MidAmerica Farmer Grower / December 7, 2018


never done so before and you are in southern Illinois, the re- sults might surprise you. Timely harvest. Obviously,


we would harvest on time every year in every field if we could. However, if you note fields that are infested with dectes stem borer, put those fields as early as possible on the priority list to reduce the potential for lodging. Soybean stubble. Destroying


or burying soybean stubble in the fall reduces dectes num- bers locally, but the adults readily move from


Figure 4.


their overwintering sites to surrounding fields. Areas with a lot of no-till production are likely to have more issues with dectes stem borer. Alternate hosts. Dectes


stem borers feed on several other host plants, including sunflowers and giant ragweed. Areas with high populations of these plants could have higher populations of dectes stem borers as well. (As if you needed another reason to kill giant ragweed).


∆ DR. NICK SEITER: Research


Assistant Professor, Field Crop Entomologist, University of Illi- nois


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