Since the bermudagrass stem maggot (BSM) first hit southern Georgia in July 2010, it has infested and damaged bermudagrass throughout the southeastern U.S. Little was known about this tiny invasive pest before it showed up. In the years since, we’ve learned much about this Central and Southeastern Asia native.
The adult fly of the BSM lays its eggs on bermudagrass leaves. Upon hatching, a larva (maggot) slips into the stem and chews on the vascular tissue at the uppermost node. The top two to three leaves die one to three days later.
Affected leaves slip easily out of the sheath and show obvious damage at their base. Between when the leaves first discolor and they completely die, the larva exits the stem and moves to soil for pupation. After pupating for seven to 10 days, the adult fly emerges. Flies escape from harvested fields to field margins and neighboring bermudagrass fields.
In severe infestations, more than 80% of tillers in a given area may be affected. The 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. Based on our experiences so far, damage appears to begin toward the end of first cutting through the middle stages of second cutting. We now expect this to be an annual occurrence.
In general, the pest cannot be fully contained. However, mechanical and/or chemical controls may suppress populations enough to prevent severe economic damage. Timing is critical.
If BSM damage occurs near the end of a regrowth cycle – two and a half to three weeks after cutting or grazing – harvest or graze fields as soon as possible. Earlier growth-cycle damage could substantially reduce the crop’s agronomic performance. Stands at 6-8” or taller when damaged should be cut and/or grazed to 3-4”; bermudagrass is unlikely to outgrow this damage even if the BSM population is suppressed. Ideally, infected material would be removed from the field to prevent regrowth shading.
Because the BSM larva is inside the plant’s pseudostem, chemical control is challenging. Insecticides with systemic activity would be needed to prevent larval feeding, but aren’t approved for use in pastures or hay crops.
Suppressing the mobile BSM flies is also difficult, and it’s unclear to what degree the flies travel from one field to another. In our experience, flies don’t move more than 10’ in any single flight, even after being disturbed. One must also consider whether chemicals will effectively penetrate the crop canopy. We’ve noticed that BSM flies tend to remain deep in the canopy, except to move from locations or in response to a disturbance.
To suppress the fly population, apply a recommended rate of an inexpensive pyrethroid insecticide when the grass is regrowing, seven to 10 days after harvest. A second application may be necessary seven to 10 days later to suppress any flies that have emerged or arrived since the last application.
Take chemical action if there is a known history of BSM damage to bermudagrass. The expense of the two applications – usually less than $15/acre for both – is justified by the forage yield saved. Based on our current observations, BSM populations aren’t high enough to warrant chemical suppression before the first hay cutting, or equivalent timing if the crop is to be grazed. Populations shouldn’t build until late in the regrowth cycle for the second cutting in most of the bermudagrass-growing region.
Our research has shown that the BSM damages finer-textured varieties more than coarser varieties. Generally, the most susceptible varieties are common, Alicia, Coastal, Russell and Tifton 44. Tifton 85 and bermudagrass x stargrass hybrids have fewer tillers damaged by the BSM and relatively less yield loss. Well-fertilized plants (75 lbs nitrogen/acre) or those with leaf-spot damage suffer more BSM damage than those that are not subjected to these factors.
Much remains unanswered about the BSM, but with these basic suppression techniques and timely action, producers can do much to manage it. For more information, visit www.georgiaforages.com.