Bermudagrass stem maggot overview and mitigation strategies for South Carolina

The bermudagrass stem maggot [BSM; Atherigona reversura Villeneuve (Diptera: Muscidae)] is an invasive species in the southeastern USA. This insect was first reported in California in 2009 and in Georgia in 2010, but it is native to South Asia. It is unclear how BSM was introduced to the United States. The suspicion is that the pest arrived via cargo ships from international shipments, based on the geographical location of these early reports. The BSM prefers warm and humid climates, and its damage occurs at the last node of the stem where the leaves emerge because, after hatching, the maggots feed on the plant apical meristems, causing the leaves above the feeding area to die (Figure 1). The discoloration of the upper leaves causes the stand to look like there has been a light frost. If no control is conducted, the damage can drastically reduce forage yields. However, the BSM damage is often misidentified as nutrient deficiency, leaf spot, etc. The chlorosis due to BSM damage is restricted to the top two to three leaves, and the damaged leaves can be easily pulled from the stems; often, it is possible to see signs of insect feeding, decay or the actual maggot at the end inside the removed stem.

Figure 1. Damage caused by bermudagrass stem maggot (Atherigona reversura). Sources: Will Hudson, University of Georgia, Bugwood.org (left) and Liliane Silva, Clemson University (right).

Bermudagrasses (Cynodon dactylon) and stargrasses (C. nlemfuensis) are the only hosts of this insect pest in the United States. The entire life cycle of BSM lasts about 21 days, with multiple generations in one year, making control difficult. The life cycle of the maggot starts when the fly lays an egg on the bermudagrass leaf. The larva emerges 2 to 3 days after and starts to feed on the apical meristem and remains there until it moves to the soil for pupation. After 7 to 10 days, the new adult emerges and lives for 14 to 21 days. When bermudagrass is mowed, viable larvae are likely to move to the soil for pupation, which results in a flush of new flies approximately 7 to 10 days after the harvest occurs (Figure 2). Unfortunately, it is still unclear where BSM overwinters and how they manage to continue to return and spread (Baxter et al., 2014).

Figure 2. Proposed life cycle for the bermudagrass stem maggot with respect to a typical bermudagrass hay growth period. Adapted from Baxter et al. 2024.

No scouting has been developed for the larvae or pupae stages, and sweep nets are the best option to scout the adult fly in the field (Baxter et al, 2019). The adult flies tend to stay low in the canopy, not flying above 18 inches from the soil surface. The recommendation for scouting is to swing the new deep into the canopy at least 20 swings to collect the samples, and the ideal time is around 11:00 am, since the dew will be dried. Then, transfer the sweep sample to a plastic bag and place it in the freezer for 10 minutes before pouring it over a solid color surface to count the adult BSM flies. If there are at least 20 BSM flies in the sample, then mitigation strategies should start.

Only adult BSM flies can be suppressed through chemical applications, and it is virtually impossible to control an entire population of BSM in a field. The goal is to suppress the populations to reduce losses during regrowth and forage accumulation. Pyrethroid insecticide should be applied at the recommended rate at 7 to 10 days after harvest, and a second application should be done 7 to 10 days after the first application. This strategy targets the population that may emerge after the first application due to the life cycle of the insect. Additionally, malathion, carbaryl, chlorantraniliprole, and spinosad are not effective at controlling the BSM (Baxter et al., 2019). Insecticide application should be conducted in the morning before the dew dries, when flies are most active, with at least 12 to 15 gal/acre and the boom height set low to optimize product penetration into the canopy. Research has shown that forage production reduction usually occurs from July through September, and the BSM population rarely reaches damaging levels before July (Baxter et al., 2019). It is not economical to spray to suppress BSM every harvest, so the decision to spray should be made when there are expected losses in production due to its damage during the season. There are no effective systemic insecticides approved for use in pastures or hay crops; therefore, larval chemical suppression is not possible.

Management practices can be used to try to reduce losses caused by BSM. If BSM damage is observed in the field near the harvest date (within 7 days or so), it is recommended to cut. If a field is 6 to 8 inches and showing BSM damage symptoms, then the field should be grazed or cut before spraying chemicals to optimize the product to reach into the canopy; however, fields < 8-10 inches may not have sufficient material to be baled, therefore, if possible, it is ideal to graze it to remove the material from the field. Then, plan to apply timely treatment in the field for BSM suppression for the next harvest. Well-managed bermudagrass stands are generally less susceptible to BSM damage, and different varieties and breeding lines have shown different susceptibility levels to damage (Table 1).

Table 1. Typical yield loss from bermudagrass stem maggot damage on popular bermudagrass lines grown in the southeastern United States. Adapted from Baxter et al. (2024).

References

Baxter, L. L., Anderson, W. F., Hudson, W. G., Hancock, D. W., Prevatt, C. G., & Moore, Z. (2019). Quantifying the damage potential of the bermudagrass stem maggot. Crop Science, 59(5), 2280–2286. https://doi.org/10.2135/cropsci2019.04.0220

Baxter, L. L., Anderson, W. F., Hudson, W. G., Rios, E. F., Bowling, C. H., Hancock, D. W., Gates, R. N., & Burt, J. C. (2024). Improved management of the bermudagrass stem maggot. Crop, Forage & Turfgrass Management, 10(1), e20269. https://doi.org/10.1002/cft2.20269

By Liliane Severino da Silva, Forages Specialist.