Dectes Stem Borer in Soybeans

Joanne Whalen, Extension IPM Specialist
Phillip Sylvester, Kent County Agricultural Extension Agent

With the delayed harvest conditions this fall, we have received a number of calls regarding lodged soybean plants that are not being picked up by the combine. Although it is most common and severe in full season soybeans, we are also seeing a recent increase in lodging in double crop soybeans.

As everyone knows, lodging can be caused by many agronomic and weather related factors. However, if you are seeing plants laying on the ground after harvest or lodging in your field before harvest, you should consider the Dectes Stem Borer as a potential cause of this damage. In Delaware, this insect has been a pest of soybeans since the early 1980s. Although some have asked if we are seeing an expansion in the range of infestation in Delaware, it may be more related to the increase in full season soybean acres planted in 2013 and 2014. In many cases, low levels are probably present in many fields but in certain years, especially under delayed fall harvest, we tend to see and hear about more lodging losses. We do know that this insect is more of a problem in areas and fields that are not rotated i.e. continuous soybean production.

Insecticide applications have generally not been successful in significantly reducing the damage caused by this insect pest. The only developmental stage that can be targeted for control is the adult beetle because the larvae never leave the host plant. Temporary reductions in beetle populations have been achieved with an insecticide application; however, because adult emergence occurs over a large window, fields can be quickly re-infested. There has also been mixed results with multiple insecticide applications. In addition, the research from other areas of the US indicates that adult control is not highly correlated with levels of larval infestation at the end of the season. So that leaves cultural control options as you make plans for the 2015 season.

Although cultural control practices will not provide 100% control, research results from Delaware and other regions have indicated that with the adoption of a combination of the following cultural control strategies, the risk of yield loss from the DSB can be reduced.

• Early/Timely Harvest: Early or timely harvest has been shown to significantly reduce lodging losses. Sampling techniques and field history can be used to identify fields with high infestations of DSB that should be harvested in a timely manner.

• Crop Rotation: Adult DSB are not considered “strong” fliers so crop rotation can help in reducing populations. However, unless a field is isolated or this approach is adopted area wide, this strategy may not be effective in reducing DSB populations in regions with significant acreage planted in soybeans.

• Fall Plowing: Research has indicated that plowing stubble lead to a reduction of adult beetle emergence in the summer.

• Weed Control: Giant ragweed, common cocklebur and wild sunflowers are alternative host plants for DSB. Therefore, weed control within and around the perimeter of fields is important.

• Row Spacing: In general, the severity of an infestation or number of lodged plants does not appear to be affected by row spacing. However, in fields planted in narrow rows, lodging losses may be reduced because the lodged plants are held up to some degree by the surrounding plants.

• Variety Selection: Experience in Delaware has indicated that lodging loss is typically more severe in shorter season varieties (Group II, III and early Group IV’s). Soybean Cyst Nematode (SCN) resistant varieties may have fewer lodged plants compared to SCN susceptible varieties. Field sampling has confirmed that the larvae will infest most varieties; however, some are more prone to lodging, especially in drought situations.

Dectes Stem Borer larva in soybean stem

Source:udel.edu