About a decade ago, hay producers in Georgia began noticing a decline in their bermudagrass fields characterized by a bronze appearance. This bronze color was caused by the chlorosis and death of the top two to three leaves of the plant, which showed obvious evidence of insect damage and decay when examined more closely. The insect responsible for this was identified as Atherigona reversura Villeneuve, commonly known as the bermudagrass stem maggot (BSM).
The bermudagrass stem maggot does the most damage as a larva. In this stage, the BSM is white, cylindrical, and about 1/8 long. As they mature, their color darkens to brown. The adult phase of the BSM is a fly with a gray thorax, yellow abdomen, and one pair of black spots. Adult flies are about 1/8 inch in length, with the females larger than the males. The entire life cycle of BSM is estimated to be 3-4 weeks long. The life cycle of the BSM begins when the BSM fly lays an egg on a bermudagrass leaf. The larvae will emerge in 2-3 days and bore into the upper stem of the grass. Once in the stem, the larvae will chew the vascular tissue at the first node it encounters, which is why the top leaves of the grass die off. Between the first signs of damage and the complete death of leaves, the larvae exit the stem and moves to pupate in the soil. Seven to ten days later the adult fly will emerge. It is important to note that when the grass is cut (mowed or grazed) any larvae will exit the plant and move to pupate, resulting in a swarm of flies 7-10 days after cutting.
BSM is native to Southeast Asia and has spread across the southeast, causing significant damage to hay production. The yield loss of BSM is highly dependent on the stage of growth of the grass when infected. Bermudagrass that is close to harvest may experience a 10% yield loss for that cutting, but for more immature fields, yield losses can be over 80%. This yield loss can represent over $600/acre in economic damage strictly on loss of product. Furthermore, some research shows that BSM damage decreases the Relative Feed Quality of bermudagrass hay. Lower quality hay can bring lower premiums, causing additional economic losses.
The best prevention for BSM is for producers to scout their fields regularly and look for telltale signs of infestation. Mitigation strategies follow the integrated pest management approach. Biological control options are limited as the pest is not native to Georgia, but some varieties of bermudagrass are more resistant to infestation and may be considered if establishing new hay fields. Additionally, the interseeding of alfalfa has been shown to reduce BSM damage. Cultural options include maintaining appropriate soil fertility as it has been shown that stands receiving high rates of Nitrogen and low rates of Potassium fertilizer can be more susceptible to BSM. Stands with disease pressure are also more susceptible to BSM damage. Harvesting or grazing at the correct time is essential for helping reduce yield loss. It is typically better to cut the field early and accept yield loss than to have a low-yield, damaged crop with a large fly population. The last option for control of BSM is chemical. Unfortunately, there are no systemic insecticides currently labeled for the control of the BSM larva, as it resides in the stem of the plant. Therefore, our efforts are to target the adult BSM flies with insecticidal control. Typically, a spray application from a normal boom sprayer with enough pressure to penetrate the canopy of the field can be effective. Insecticides should be applied 7-10 days after an affected harvest, and an additional application should be made 7-10 days after the first. Chemical control should only be used when yield losses reach the economic threshold for the cost of the insecticide and application. Most of the time, this threshold is reached in July and August, but is not as clear later in the season. Prior to the first bermudagrass hay cutting, the BSM population is usually not high enough to warrant chemical suppression.
If you have any questions about the BSM, it’s impact, scouting, or insecticide recommendations, please let the extension office know at uge3181@uga.edu or 706-359-3233.
