Adult wheat stem sawflies emerging in Nebraska
Figure 1. An image taken with a macro lens attachment on the author's smartphone of a female (left) and male (right) sawfly. Note that even with this low-quality image you can see the slightly larger female with her somewhat triangular abdomen that terminates in an ovipositor. The male looks slightly smaller, has a more rounded abdomen and does not terminate in an ovipositor. Wheat stem sawflies are emerging in the Nebraska Panhandle. We collected our first sawflies in emergence cages and pheromone traps last week. On Tuesday in Banner County wheat fields we collected 15-30 adult wheat stem sawflies per 10 sweeps. We have not evaluated the sample for the number of females and males yet; however, the initial emergence of wheat stem sawflies is usually biased toward males. Research from Montana has indicated that 2 adult females per 10 sweeps can result in about 12% lodging. The numbers we are experiencing now will continue for the next two to three weeks, so the risk of sawfly infestation is high in fields that are at or beyond the wheat stem elongation stage. Lodging is dependent on a number of factors, but if sawfly densities remain at this level, lodging could become an issue in July.
Pesticides have not been successful in controlling adult wheat stem sawflies. The wheat stem sawfly adult does not damage wheat, but rather deposits eggs into the wheat stem which then develop into stem-feeding larvae. The larvae cause damage as they girdle wheat in July in preparation for overwintering within the wheat crown. Additionally, no chemical control technologies (including plant-systemic products) have shown any efficacy against the larvae and we have no known resistance against the wheat stem sawfly in our higher-yielding, hollow-stemmed varieties.
So, what can you do? My recommendation would be to scout your fields because the information you gain may help you. A multistate initiative (www.iWheat.org) will soon be launching scouting tools to help in this effort. I encourage you to log in and register to this website to gain access to these tools and to receive updates and product upgrades. For a specific sampling plan for adults, I would recommend the following steps:
- Take five 10-sweep samples (50 sweeps total) parallel to the eastern edge of your wheat field. (Note that you may get higher numbers on windy days as adult sawflies are not strong fliers and tend to stay put when it is windy.)
- Evaluate the average number of female wheat stem sawflies per 10 sweeps for a more accurate infestation risk assessment.
- Repeat steps 1 and 2 along parallel lines at 50 and 100 feet from the field edge.
- Consistently finding 2 adult females (or more) per 10 sweeps may suggest a risk of greater than 10% lodging within the area you sampled.