Bermuda Stem Maggots (BSM) is believed to be native to Southeast Asia, which it was where it was first discovered. Since the 2010 discovery in southern Georgia, BSM has spread throughout the Southeast, damaging bermudagrass, turfgrass, hayfields, and pastures as far north as North Carolina and Kentucky and as far west as Texas. The larvae of BSM bore into the pseudostem, the stem-like structure made up of leaf sheathes, where it macerates the vascular tissue. It feeds on the sap and microbial soup that it creates from the macerated tissue. The feeding occurs outward from the last node of the plant, which cuts off the water and sap flow to sand from the top two to three leaves. This will leave the top of the bermudagrass stems with a bronze color, similar to color from the lack of water. 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 for hay, 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 of new growth, yield loss can be severe. Yield losses in excess of 80% have been reported in bermudagrass in the later part of the season. Preliminary research has shown that BSM damage decreased relative forage quality (RFQ) of late season bermudagrass by 7% on average. This decrease was attributed to lower total digestible nutrients (TDN) and slightly lower dry matter intake (DMI).
The adult stage of the BSM is a fly. The BSM fly is easier to find and identify than the larva or pupae because it occurs outside the pseudostem and has distinct coloration. The flies have transparent wings, a gray thorax, and a yellow abdomen with at least one pair of black spots. Adult BSM are about 1/8 inch in length. The females are typically larger than the males, and the female abdomen is longer, more pointed, and curves under the fly’s body. In contrast, the male’s abdomen is more round and shorter.
The larvae are white, cylindrical, and about 1/8 inch long when fully grown. As they mature, their color gradually darkens to a tan or brown. The larvae have mouth hooks that are barely visible to the naked eye. It is presumed these mouth hooks enable the BSM to macerate the walls of the pseudostem. The cycle begins with the BSM laying an egg on a bermudagrass leaf. The larva emerges approximately two-three days post-oviposition, or after the eggs are laid and slips or bores into the central whorl of the pseudostem. Once in the pseudostem, it macerates the vascular tissue at the first node it encounters. The lack of sap flow causes the top two to three leaves to become chloritic, or yellowed due to insufficient chlorophyll. Within one to two days after the feeding begins, the first signs of damage are observed, and the affected leaves die prematurely. Between the time when chlorosis is first observed and complete leaf senescence, or deterioration, the larva exits the stem and moves to the soil for pupation. The metamorphosis occurs in an orange-colored puparium over the course of 7-10 days and culminates with the emergence of the adult fly. It has also been shown that when the pseudostem is cut (with a hay mower or grazed, for example), any viable larvae will exit the stem and move to the soil to pupate. As a result, adult flies begin to emerge in a sizeable flush 7 to 10 days after cutting. These findings have helped refine the timing for insecticide applications for suppressing adult populations during the first two to three weeks following a cutting.
Adult flies live for approximately 18 to 20 days when kept in enclosures and provided sugar water. Actual adult life spans are estimated to be 14 to 21 days. Based on these observations, we believe the complete life cycle of the BSM to be three to four weeks long with multiple offspring being -produced by the fly during its adulthood. For more information regarding the Bermudagrass Stem Maggot, please contact your local Extension Office at 706-468-6479.