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.
Based on your scouting results, you may want to consider one of the following management options:
- Do nothing. You may not need to take any action if 1) you do not have a history of lodging in this field; 2) the infested area is small; or 3) your yield expectations are low in the sampled area.
- Swath harvest. Swathing equipment might be hard to come by, but you could swath and windrow your wheat while it is still somewhat green at the end of June. Doing so would cut the wheat before the sawfly has a chance to cut it.
- Get low. Harvest at a normal time with a low-cutting, floating-head combine. However, this harvesting method may not be compatible with some residue-management operations.
I’m providing these management options now as some of these may require more time to plan than conventional insecticide management options.
We continue to monitor the statewide status of this quickly evolving pest. Additionally, we are working with a company to provide producers a pheromone and a trap design that may save time in scouting. We hope to be able to produce a higher resolution map of the prevalence of this insect with the aid of the pheromone-based traps.
Once the adults die, I will provide additional sampling plans in CropWatch that will be specific to larvae.