We’ve seen the symptoms of this pest for nearly a month now (see blog post on 11 July). However, the hot and dry conditions, as of late, have favored its development, especially in the Coastal Plain. Both Jim Dunphy, NC State soybean specialist, and I have been diagnosing these fields. If you have it, 1) how do you know and 2) what can you do about it.
1) Some of what both Jim and I have been seeing is a result of threecornered alfalfa hopper. According to Jim, this sort of injury has been more prevalent in the Piedmont while lesser cornstalk borer injury has been more prevalent in the Coastal Plain. This makes sense, as lesser cornstalk borer development is favored by hot and dry conditions and it seems to injure soybeans more on droughty soils. I have seen threecornered alfalfa hopper injury in the Coastal Plain as well, though. So how do you know what you have?
Threecornered alfalfa hopper injury typically occurs as a girdle around the stem above the soil line. This likely occurred when the plant was small (under 10 inches). These insects will continue to feed as the plants grow, but will feed on and injure petioles on the main stem. This can block the vascular tissue of the beans, causing indirect yield loss. More often, loss is direct when plants lodge during harvest. The breaking point from the lodge is often a distinct and clean break.
Lesser cornstalk borer injury is found near their place of feeding, at or below the soil surface. Sometimes you can see the feeding galleries, if you pull back the base. This lodging that results from these pests is usually irregular and at the soil surface.
2) If you have threecornered alfalfa hopper injury, the damage is probably already complete. Plant compensation when soybean is over 10 inches tall will generally suffice to maintain yield, unless population densities are extremely high. Next year, scout beans under 10 inches tall and use a pyrethroid spray if densities exceed 1 per sweep.
If you have lesser cornstalk borer injury and even if you have lesser cornstalk borer larvae present, a remedial treatment will be ineffective. Things that can be done next year include avoiding droughty soils, planting a resistant variety (see the links in this post), burning weeds down two weeks prior to planting, and incorporating a granular in-furrow application of Lorsban or a directed in-furrow spray of Lorsban at planting.