Dectes stem borer is making its presence known
This year I suspect the above average pod load is also increasing the lodging. Also, when the field is harvested, these girdled plants often break over and don’t make it into the header of the combine. We have documented as much as 25-30% yield loss in fields with severe infestations.
Literature sources and work we have done here in AR as well as TN, MO, KY and KS indicates no impact by main stem tunneling. However, all the literature agrees that the yield loss does occur from lodging. Damage caused by adults is not a concern and is limited to light foliage feeding.
The female is reported to produce about 50 eggs. The great majority of these eggs are laid on the petioles. However, they can be laid on the main stem or other stems. Larvae overwinter in the soybean stem below the girdle they cause. They will pack the top of the tunnel at the girdle with a frass plug. The larvae pupate in May-July and adults will begin to emerge from June to September. They emerge from the stem by chewing their way out of the stem below the plug they formed below the girdle. Adults will mate within 5 days of emergence. There is a preovipositional period lasting 10-14 days. Males will survive 2-3 weeks and females 4-8 weeks.
Control of Dectes
Because of the extended emergence time starting in late-May through June and July, foliar applications of insecticides have not been shown to be effective. Peak emergence is hard to pinpoint and can vary from year to year based on environmental conditions. Foliar applications of insecticides such as pyrethroids will control the adults present, however it would take multiple applications to be effective. Single applications are not considered effective.
Currently, the only recommendation for reducing populations is by tillage and burial of infested stubble to a depth of 2 inches or more. Crop rotation might reduce numbers as it appears the worst infested fields are those that are in a monoculture of soybeans. However, as these are mobile insects, effectiveness of crop rotation could be reduced depending on surrounding fields and wild hosts in the area.
What the Grower Should Know About Dectes Stem Borer
- Controlling adults during the season is very difficult because of the extended emergence time (late-May through July). Foliar applications of pyrethroids aimed at this pest could possibly make other pests like bollworm and looper worse.
- Foliar applications are not effective for controlling larvae inside the stem. You just cannot reach them.
- Harvest soybeans as timely as possible. The longer the soybeans stay in the field the more susceptible they are to lodging. Our observations indicate that the worst lodging is associated with fields that have been ready to cut for an extended period of time before cutting. Increased time for the girdled area to break down and weather events such as high winds and heavy rain will increase lodging on mature plants.
- The only recommendation for reducing populations is by tillage and burial of infested stubble to a depth of 2 inches or more. Crop rotation might reduce numbers as it appears the worst infested fields are those that are in a monoculture of soybeans.