Authors: Buyung Hadi, Jeffrey Bradshaw, Robert J. Whitworth, J. P. Michaud and Phillip E. Sloderbeck
Dectes stem borer, also known as soybean stem borer (Dectes texanus), is a member of longhorned beetle family native to the USA. It is widely distributed across North America east of the Rocky Mountains.
Identification
The full grown larva of the Dectes stem borer is cream colored, about 1/2 to 2/3 in (15 mm) in length, legless with clearly visible segmentation. The adult body is tapering toward posterior. The adults are grey with long antennae. Two prominent spines protruding from the sides of the thorax are usually visible when viewed from above.
Life Cycle and Seasonal History
The dectes stem borer overwinter as larvae within stalk below the girdle. Tje beetle also utilizes a number of native weedy hosts including cocklebur (Xanthium strumarium), common and giant ragweed (Ambrosia artemisifolia and Ambrosia trifida). The larvae pupate in early summer and adults begin to emerge over an extended period, beginning in late June. Mating occurs 10-12 days after emergence and mated females lay their eggs during July and August within the petioles (leaf stalk) of soybean plants. Eclosing (emerging) larvae feed on the petiole pith and eventually tunnel down into the main stem. The petiole and the trifoliate leaf then wilt and drop prematurely, a useful indicator of infested plants early in the season. Multiple eggs may be deposited in a single soybean plant, and even in a single petiole, however only one survivor usually prevails per infested plant by the end of season as a result of cannibalism. As the mature soybean plant dries down, the larva tunnels down to the plant base and girdles the stalk (making a circular incision) around the interior of the main stem. This predisposes the plant to lodge during heavy winds or at the slightest touch of farm machinery. The hollow core of the stalk is typically plugged with chewed fibers below the girdle, sealing the insect in its overwintering chamber.
Plant Injury and Damage
Area-based estimates suggest that tunneling by larvae may decrease yield by as much as 10% but plant-by-plant estimates have not confirmed this. Most of the economic loss due to dectes stem borer is associated with lodging of mature plants. Dectes stem borer is a significant problem in south and central Kansas and an occasional pest in Nebraska. In other more northerly states such as South Dakota, North Dakota, and Iowa, dectes stem borer is present in soybean, but not in damaging populations.
No-till cultivation favors the dectes stem borer because of enhanced larval survival. The soybean stem borer also utilizes cultivated sunflower which is actually a superior host. Thus, soybean fields planted in or near rotations that include sunflower are especially at risk of stem borer infestation.
Management Approaches
Chemical control is presently inadvisable. The larval stage remains protected within the plant and adults have an extended period of emergence. Thus it is difficult to time pesticide applications to coincide with peak adult activity and a single application is unlikely to provide sufficient control season-long. However, some cultural methods can assist with suppression of dectes stem borer damage.
Cultural Methods
Fall tillage has been shown to kill a significant portion of overwintering larvae and decrease the subsequent summer population, but stalks must be buried to a depth of at least 4-6 inches and this approach is not compatible with minimum or no-till agriculture. Because soybean stem borer also use weeds, good management of known weed hosts in fallow fields and fencerows may help reduce local populations during periods when cultivated host plants are not available.
Crop rotation away from soybean and sunflower can help reduce populations locally as the dectes stem borer is not a strong flyer and long range dispersal is limited. Prevention of soybean infestation can be obtained by planting six rows of cultivated sunflowers around the field perimeter. Sunflower is strongly preferred over soybean and field trials with this approach have reduced soybean infestation to less than 5% of plants. Yield losses are exacerbated when dry soil conditions persist post-maturity because stalk desiccation cues early onset of girdling by larvae. In areas with a history of stem borer problems, plants should be sampled across transects in fields prior to maturity and the presence and severity of stem borer infestation assessed. Timely harvest of fields with a high percentage of plants infested remains the best rescue approach for limiting losses to girdling and lodging.
Other Online Resources
For information regarding labels of chemical control options, please visit Agrian.com
