By Dr. Lisa Baxter, UGA State Forage Extension Specialist
Bermudagrass hay producers throughout the region are currently battling the bermudagrass stem maggot, or BSM. This pest has damaged bermudagrass forage each summer since it was first discovered back in 2010.
The adult flies are most easily found with an insect sweep net. Your local Extension agent should be able to help you scout if needed. They are most active during the cooler periods of the day such as early in the morning and late in the evening.
Pyrethroid insecticides are still the only effective mode of action available on the market today. At this point, all pyrethroids are equally effective on the BSM. Populations of the BSM fly can be suppressed by applying the recommended rate of a pyrethroid insecticide at seven to ten days after harvest and a second application seven to ten days after the first application. Apply the insecticide early in the morning before the dew dries and when the flies are most active. The boom height should be set as low as possible since the flies rarely move above 18 inches. Finally, apply the insecticide with at least 12 to 15 gallons of water per acre to also ensure canopy penetration.
Two strategically timed insecticide applications are needed in most instances to sufficiently suppress the BSM population. The bermudagrass is most vulnerable to the BSM in the early stages of growth following harvest. The faster your grass grows, the less susceptible the stand will be to BSM damage.
Unfortunately, several hay producers are struggling with controlling the BSM this season. So, what is going wrong?
Our recommendation says that you should apply the insecticide early in the morning before the dew dries and when the flies are most active. Most of the time, this is any time before 10:00 to 11:00 in the morning. However, the dew dries off much earlier when your farm is under heat advisories or a heat dome. In some instances, spraying after 8:00 in the morning would be too late!
In theory, you could also spray in the evening when the dew returns to achieve the same results. However, pyrethroids require a period of rainfast to ensure the chemical is effective. Our regions is known for pop up showers in the late afternoon and early evening. The risk of these rainfall events on most weeks is simply too high for this to be a viable solution for the majority of us spraying for the BSM.
Recent research at UGA-Tifton has shown that time of day is likely more important than the number of days that the insecticide is applied after harvest. Although we have not yet identified the ideal amount of dew or temperature for applying the insecticide, it does seem that it is better to be a day late than an hour late when treating the BSM.
For more information on managing the BSM, please see the new management guide here: https://acsess.onlinelibrary.wiley.com/doi/epdf/10.1002/cft2.20269
