You may have noticed that your hayfield looks like it has been through a frost but there has been no frost present in this hot weather. What you are seeing is bronzing. Bronzing is a result of chlorosis and necrosis in the top two to three leaves of your bermudagrass. The damaged leaves can easily be pulled from the sheath, and the end inside the sheath either shows evidence of insect damage or obvious decay. This is Bermudagrass Stem Maggot damage (BSM).
Yield Loss: The amount of yield loss caused by this feeding depends upon the stage of growth wherein the damage occurs. If the damage occurs once the bermudagrass is nearing harvest, the loss of those top two to three leaves may reduce the yield by less than 10% for that cutting. However, if the damage occurs during the early stages of regrowth, affecting less than 6 inches (15 centimeters) of new growth, yield losses can be severe. Yield losses in excess of 80% have been reported in bermudagrass hayfields in the latter part of the season.
Life Cycle: The life cycle begins with the BSM fly laying an egg on a bermudagrass leaf. The larva emerges approximately two to three days after the eggs are laid, and slips into the central whorl of the stem. Between the time when chlorosis is first observed and complete leaf deterioration, the larva exits the stem and moves to the soil for pupation. 7-10 days later the adult fly will emerge. Adult flies live for approximately 15 to 20 days under great conditions. Actual adult life spans are estimated to be 14 to 21 days. Based on these observations, the complete life cycle of the BSM is three to four weeks long with multiple offspring being produced by the fly during its adulthood. High populations develop as early as mid-June in central Florida, early July in south Georgia, mid-July in central Georgia, and late July in north Georgia.
Control:
Physical Control – Properly timed bermudagrass harvests can minimize the yield losses from the BSM. If signs of BSM damage occur near the end of a regrowth cycle (within 2.5 to 3 weeks after cutting or grazing), you should harvest or graze the field as soon as conditions become favorable. Once a stand that is 6 to 8 inches (15 to 20 centimeters) or taller has been damaged by BSM feeding, the only option is to cut and/or graze the stand to a height of 3 to 4 inches (7.5 to 10 centimeters) and encourage regrowth to occur. It is better to cut the field extremely early and accept the loss than to have a low-yielding, severely damaged crop that harbors a large fly population and leads to a further buildup. Ideally, the infected material would be removed from the field to prevent shading of any regrowth. The larvae do not appear to remain in cut stems. Within hours of cutting, larvae will exit damaged stems and travel to the soil. Flies in fields that have been harvested escape to field margins and neighboring bermudagrass fields. Prompt applications of chemical controls in fields following the harvest of a neighboring field can greatly reduce the risk of BSM damage.
Chemical Control – Chemical control of the BSM larva is challenging because it is inside the stem. Consequently, an insecticide with systemic activity would be needed to prevent larval feeding. However, none of the systemic insecticides currently approved for use in pastures or hay crops are labeled for (or effective at) controlling the BSM. The BSM fly is the target of chemical suppression efforts. A broad-spectrum insecticide timed when large numbers of adult flies are present provides the most suppression. Suppression of the BSM fly can be challenging because the flies are mobile. In our experience, the flies do not fly very high (usually less than 18 inches, or 0.5 meters, above the canopy) nor very far (no more than 10 feet, or 3 m) in any single instance of flight, even after being disturbed. Therefore, normal spray boom heights should be effective for chemical applications for BSM control. However, it is also important to understand the limits of a chemical application in canopy penetration. In our experience, the BSM flies tend to remain deep in the canopy except to move from one location to another or in response to a disturbance. Applications that do not penetrate the canopy may have limited success. It would be ideal to apply the insecticide in a volume of water in excess of 12 to 15 gallons/acre (112 to 140 L/ha) to ensure adequate canopy penetration. Suppressing the BSM can be effective when a recommended rate of an insecticide is applied after the bermudagrass has begun to regrow (7 to 10 days after cutting) following an affected harvest. A second application can be made 7 to 10 days later to suppress any flies that have emerged or arrived since the last application. This second application is usually only necessary when neighboring fields were harvested after the first application, the crop growth cycle has been extended due to dry weather, or a forecast of rain suggests that the hay harvest may be delayed. Chemical actions should be taken if there is a known history of BSM damage to the bermudagrass and the expense of the application(s) is justified by the forage yield saved. An individual application usually costs $2 to $3/acre for the insecticide and $5 to $10/acre for application. If the bermudagrass forage is valued at $100/ton ($90/metric ton), a corresponding yield savings of approximately 200 lbs DM/acre (225 kg DM/ha) would be necessary to warrant this investment. In July and August, bermudagrass hayfields may produce up to 6,000 lbs DM/acre (6,725 kg DM/ha) at a single cutting, so applications at this time of year are more likely to result in an economic benefit. Because bermudagrass yields in September or October may only be 1,500 to 2,000 lbs DM/acre (1,700 to 2,250 kg DM/ha), fall insecticide applications are much less likely to result in a return on that investment. Based on our current observations, BSM populations are not high enough to warrant chemical suppression prior to the first bermudagrass hay cutting (or equivalent timing if the crop is to be grazed) and population buildup may not occur until late into the regrowth cycle for the second cutting for the central latitudes of the Southeast U.S. or the third cutting for more northern areas where bermudagrass is grown.
Bermudagrass Stem Maggot
