This has been a common question the last several weeks. The short answer is you should NOT spray for threecornered alfalfa hoppers (TCAH) in flowering soybean. Populations have been high all year, and there are currently reports of 1-2 TCAH per sweep. This is above our established treatment threshold of one TCAH per sweep, so why am I saying not to spray?
- Frankly, the previous threshold was based on limited and old data. No one was comfortable with treatment recommendations.
- Some recent research by Dr. Fred Musser at Mississippi State University, where very high populations were caged on soybean, indicated that TCAH did not cause yield loss in flowering soybean. They did girdle the upper portion of the main stem and leaf petioles. They did not find evidence of feeding on pods or pod stems.
- Insecticide applications that are unnecessary can backfire. They often wipe out beneficial insects and create secondary outbreaks of serious pests such as soybean loopers or corn earworm. The risk of spraying for threecornered alfalfa hoppers appears to outweigh any minor threat they pose to yield. Several surrounding states no longer recommend treatment for TCAH once beans are 10-12 inches tall. MSU Extension increased their threshold to 2 per sweep (and really they are not suggesting treatment at any level unless other pests require treatment).
I’ll confess to not being 100% confident that we should never treat for mid and late season infestations of TCAH. A little more data would sooth my nerves, but the most recent data and my experience indicate that potential yield loss from infestations is very small or non-existent.
TCAH still pose a significant threat in soybean that are less than 8-10 inches tall. Please don’t apply the above comments to fields of seedling soybean.