Many 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 period of unfavorable weather that we have suffered. However, in parts of southern Illinois damage by the dectes 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 completely off of the plant, which is usually the first sign of an infestation. 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 soybean plant looks after being attacked 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 difficulties and reduced yield (Figure 4). This insect has one generation per year, with the adults usually emerging from soybean residue beginning in late June in Illinois to start the cycle again.
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)
Insecticides are generally not recommended for control because the larvae are protected within the stem and the adults lay eggs over a long period of time in the summer (approximately mid-July through August in Illinois). While there appear to be some differences in varietal susceptibility, these differences are not well documented, and to my knowledge no soybean varieties have been characterized as resistant to dectes stem borer. However, there are some cultural management 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, examine soybean stems in lodged areas to determine if dectes stem borer was part of the problem. If you have never done so before and you are in southern Illinois, the results 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 numbers locally, but the adults readily move from 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).
Nick Seiter firstname.lastname@example.org (217) 300-7199
Research Assistant Professor, Field Crop Entomology
University of Illinois Department of Crop Sciences
Authors: Nick Seiter, Joe Spencer, and Kelly Estes
Based on degree day accumulations, western corn rootworm egg hatch should be underway in much of Illinois (roughly south of Peoria as of May 29; you can view your specific location using the degree day calculator here: https://www.isws.illinois.edu/warm/pestdata/sqlchoose1.asp). We are probably just over a month away from seeing the emergence of the first adult beetles. With low rootworm populations for the last several years, there has been a renewed interest in adult sampling. The only way to determine if larval densities will be high enough to justify a control action in a specific cornfield next spring is to monitor adults in the field this summer. Doing this correctly will require some preparation to obtain the correct materials. Now is a good time to review your monitoring procedures for western corn rootworm adults.
The most common monitoring tool for western corn rootworm adults is a 5.5 × 9-in yellow card trap coated in sticky material (e.g., Pherocon® AM No-Bait trap, Trécé, Inc., Adair, OK). The yellow color attracts the beetles, and when they land on the sticky substance they become trapped (Fig. 1). We recommend placing 12 of these traps uniformly throughout each field that you are monitoring beginning in late July. If the field you are monitoring is planted to corn this season (i.e., continuous corn), simply place each trap on a corn plant just above an ear. If you are monitoring a soybean field this season that will be rotated to corn next year, you will need to place each trap on a stake so that it will sit just above the soybean canopy. PVC pipes (½” diameter) are a relatively cheap and easy material that you can use to make these stakes, but wooden, metal, or plastic stakes also work. Use poles that are long enough to allow trap height to be raised as the soybean crop grows taller. Replace each trap once a week for 3-4 weeks, count all western corn rootworm adults stuck to the trap upon collection, and determine the average number of adults collected per trap per day.
Figure 1. Yellow sticky card trap used to monitor western corn rootworm adults.
We recommend using the economic thresholds recently updated by our colleagues at Iowa State University  to determine if a control action is needed in corn the following spring (Table 1). If the beetle numbers you see on your traps are above these thresholds, a corn hybrid with Bt traits targeting corn rootworm or a soil insecticide is justified in that field when corn is planted the following spring. While monitoring for western corn rootworm takes some effort, it is the only way to get field-specific information on the economic need for a control tactic the following year.
Table 1. Economic thresholds for western corn rootworm in continuous or rotated corn.
|Sticky Trap Location
|Corn (continuous corn)
||2 beetles per trap per day
|Soybean (rotated corn)
||1.5 beetles per trap per day
1. Dunbar MW, Gassmann AJ. Abundance and Distribution of Western and Northern Corn Rootworm (Diabrotica spp.) and Prevalence of Rotation Resistance in Eastern Iowa. Journal of Economic Entomology. 2013;106(1):168-80.