Declaring war on bermudagrass stem maggot
|By Lisa Baxter|
The author is an extension forage specialist with the University of Georgia who is based in Tifton.
The bermudagrass stem maggot (BSM; Atherigona reversura Villenueve) has severely damaged bermudagrass pastures and hayfields throughout the southeast U.S. since it was first discovered in southern Georgia. Since the 2010 discovery, the BSM has spread throughout the Southeast, damaging bermudagrass hayfields and pastures as far north as North Carolina and Kentucky and as far west as Texas.
Current control protocol
Although pyrethroid insecticides are the most effective way to control the adult BSM, they are not effective on the larva or pupae. It is the larval feeding at the uppermost node of the plant that will kill the top two to three leaves on the stem, giving the forage canopy the characteristic “frosted” appearance.
If the BSM damage occurs near the end of a regrowth cycle (within a week of harvest), the yield loss is estimated to be less than 10 percent, so harvest can proceed as normal. However, if the hay crop is damaged at an early stage of regrowth (for example, 6 to 8 inches tall), it will not grow out of the damage. When this occurs, it is crucial to remove (mow and harvest, if possible) the damaged grass before making an insecticide application.
The adult BSM can be controlled with two applications of pyrethroid insecticides, one sprayed seven to 10 days after harvest and one sprayed seven to 10 days after the first application. Each chemical is different, so be sure to read and follow the labeled rate and instructions.
Two applications are necessary to effectively suppress the BSM population and protect the bermudagrass during the most sensitive phases of regrowth. While this recommendation is certainly effective, recent research has discovered simple ways to improve the effectiveness of the insecticide applications and fine-tune the recommendation for your own farm.
Scout early and oftenYou can easily use sticky traps or sweep nets to collect and identify the adult BSM fly in the field. To date, sticky traps have only been useful for alerting the producer to the presence of the BSM. Fly counts on sticky trap cards have not been observed to correlate with fly populations. If sticky traps are used, secure the traps to stakes at 8 inches above the soil surface.
Sweep net estimates have been found to be relatively accurate predictors of actual fly populations in the field. It is not uncommon to find 50 to 80 flies in a sample of 10 sweeps during July to September (peak BSM damage season). This translates to about 300,000 to 500,000 flies per acre.
When using a sweep net, sweep deep into the canopy, as the adult BSM does not fly very high. While the flies are more active in the morning hours, it is difficult to sweep if dew is present. Scout your fields just after the morning dew dries off the grass, or around noon.
Transfer your sample (about 10 to 15 sweeps) to an “insect cube” or a plastic bag and place it in a freezer for five to 10 minutes. Remove the cube/bag from the freezer and count the number of flies. If you have observed a significant (30 percent) level of damage in your field and find at least 40 to 50 flies in your sample, then it’s time to employ the appropriate control strategy.
Don’t spray until necessary
Research has shown that the BSM only significantly reduces herbage mass from late July to September each year. This would generally correspond to the third, fourth, and/or fifth harvest of the year for bermudagrass hay producers in the Deep South states. If the BSM population is not large enough to significantly reduce herbage mass outside of this peak season, then it is not necessary or economical to spray pyrethroid insecticides during the early (May to June) and late (October) harvests.
Scouting for the adult BSM flies with sweep net samples can help determine when you need to start spraying. Deferring insecticide applications until at least July would not only reduce economic inputs but would also slow the potential resistance of the BSM to pyrethroids by preventing their overuse.
Penetrate the canopy
In our experience, the BSM do not fly very high or far (less than 10 feet) in any single instance of flight, even after being disturbed. Preliminary results from on-farm research collaborations with county extension agents and coordinators have shown that the BSM fly is most active 8 inches above the soil surface and before 11 a.m. Therefore, normal spray boom heights should be effective for chemical applications.
Since the BSM flies tend to remain deep in the canopy, applications that do not penetrate the canopy may have limited success. Apply the insecticide in at least 12 to 15 gallons of water per acre to ensure adequate canopy penetration.
Currently, there are more than a dozen active research trials at the University of Georgia (UGA) related to various aspects of BSM management. The Georgia Beef Commission has supported much of our past and current research efforts. The goals of these trials are to better understand the BSM’s life cycle, screen new insecticides, and fine-tune the management recommendations. In addition, the USDA-ARS forage breeding team in Tifton, Ga., has screened potential new lines for their tolerance to BSM damage. Six potential lines were selected from the group and are undergoing additional testing for yield and forage quality.
A new UGA Extension bulletin, “Managing bermudagrass stem maggots,” is available online at www.georgiaforages.com or for immediate download by scanning the QR (Quick Response) code provided. This is the most complete paper available on the BSM and contains a detailed
This article appeared in the April/May 2019 issue of Grower on pages 22 and 23.
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