Chemical Control of the Bermudagrass Stem Maggot By Dr. Lisa Baxter, UGA State Forage Extension Specialist

Expert advice Chemical Control of the Bermudagrass Stem Maggot

Lisa Baxter, Ph.D., UGA State Forage Extension Specialist, University of Georgia – Tifton Campus

The two most dominant pests that we see in our hayfields and pastures are the bermudagrass stem maggot (BSM; Atherigona reversura Villenueve) and the fall armyworm (FAW; Spodoptera frugiperda). The BSM impacts only bermudagrass; the FAW can feed on any forage species, except generally bahiagrass. This article will briefly outline the scouting and control protocols for these two pests, according to UGA Extension recommendations.

Scouting for the BSM

Currently we are only able to chemically control the adult BSM fly. The BSM fly is easier to find and identify than the larva or pupae because it occurs outside of the pseudostem and has distinct coloration. They have transparent wings, a gray thorax, and a yellow abdomen with at least one pair of black spots. Adult BSMs are about 1/8 inch (around 3.0 to 3.5 millimeters) in length.

Sweep net estimates have been found to be relatively accurate predictors of actual fly populations in the field. It is not uncommon to find 50-80 flies in a sample of 10 sweeps during July and August (peak BSM damage season). This translates to ~300500,000 flies per acre! Be sure to sweep deep into the canopy, as the flies do not fly very high. While the flies are more active in the morning hours, it is difficult to sweep if dew is present. Plan to scout your fields just after the dew dries off the grass, no later than 11 a.m. to noon. Transfer your sample (about 10-15 sweeps) to an “insect cube” or a plastic bag and place in a freezer for 5-10 minutes. Remove from the freezer and count the number of flies. If you have observed a significant (30 percent) level of damage in your field and find at least 40-50 flies in your sample, then it’s time to employ the appropriate control strategy.

How to Suppress the BSM

If the BSM damage occurs near the end of a regrowth cycle (~3 weeks after the previous cutting or grazing), the yield loss is estimated to be less than 10 percent, so you can harvest as normal. However, if the hay crop damaged at an early stage of regrowth (e.g., 6-8 inches), it is unlikely to further develop. If the crop is damaged at this point, it is crucial to remove (mow and harvest, if possible) the damaged grass to enable new growth to occur.

Suppressing the BSM can be effective when a recommended rate of a pyrethroid insecticide is applied after the bermudagrass has begun to regrow (7-10 days after cutting) following an affected harvest. A second application should be made 7-10 days later to suppress any flies that have emerged or arrived since the last application. This second application is crucial! Remember, the pyrethroid is not a systemic insecticide and it will only be effective on the adult BSM fly. This second application is required to sufficiently suppress the BSM population and protect your bermudagrass stand through the most sensitive regrowth phase.

Pyrethroids are the only effective mode of action at this time. Apply the insecticide in at least 12-15 gallons/acre to ensure adequate canopy penetration. Spraying earlier in the day should be more effective than later in the afternoon since the flies are more active in the early morning hours. Remember, overuse of a single mode of action to combat a pest may eventually result in a buildup of resistance. Preventing overuse and uneconomical use of insecticides is a crucial educational objective to combat the potential for resistance of the BSM to pyrethroid insecticides.

Scouting for FAW

Small worms tend to stay down in the canopy during the day, and their feeding is less damaging and harder to see. The youngest larvae do not eat much, but the mature larvae can eat more than all other ages put together. Large armyworms are fairly easy to find because of their size. They are typically most active in the early morning or late afternoon when temperatures are cooler, but they will feed throughout the day. Most producers will identify armyworms at the fully grown larvae stage. They can be up to 1.5 inches long and are light green to nearly black with light and dark stripes that stretch horizontally along the body of the worm. One of their most distinct characteristics is an inverted Y-shape on the head of the worm.

An insect sweep net can help you identify low populations of FAW before significant feeding damage occurs. Sweep early in the morning and late in the afternoon when the FAW are more prevalent. Some birds (cattle egrets) may serve as an indicator of armyworm activity; however, this is not always guaranteed – especially in the Coastal Plains, where the birds are always present. Preventing and Controlling the FAW

Textbook treatment thresholds are typically recommended at three armyworm larvae per square foot, but weather and forage demand may dictate a more conservative threshold. Harvesting a hay crop may be the best option for armyworm control if hay is close to cutting time. If insecticides are required, be sure to follow all label instructions. Insecticides are labelled for specific life stages of the FAW. In general, small caterpillars are easier to control than larger worms. Several insecticides are available, but we should pay careful attention to grazing or harvesting restrictions related to these products. You may want to consider a residual control option. Products vary in their length of residual control and application cost, so you will need to determine the potential returns on this investment.

For More Information

The Extension bulletin “Managing Bermudagrass Stem Maggots” is available online at http://www.caes.uga.edu/ extension-outreach/commodities/forages. html. If you need more specific insecticide recommendations, please consult the Georgia Pest Management Handbook and your local county Extension agent. For more information on forage management, please visit our website, www. georgiaforages.com.

Insecticide options for fall armyworms. For specific recommendations, please contact your local County Extension Agent and refer to the Georgia Pest Management Handbook.

Chemical Example Trade Name Grazing Interval (d) Harvest Interval (d)

Residual Activity Carbaryl Sevin SL 14 14 medium

Methomyl Lannate 2.4 LV 7 3 low

Cyfluthrin Tombstone 0 0 low

Beta-cyfluthrin Baythroid XL 0 0 low

Lambda-cyhalothrin Zeta-cypermethrin Warrior II w/ Zeon 0 7 low

Mustang Maxx 0 0 low

Diflubenzuron Dimilin 2L --- 1 medium

Methoxyfenozide Intrepid 2F 0 7 medium

Spinosad Blackhawk 0 3 low

Chlorantraniliprole Chlorantraniliprole + Lambda-cyhalothrin Prevathon 0 0 high

Besiege 0 7 high Photos: (L) Bermudagrass Stem Maggot, Lisa Baxter, UGA. (R) fall armyworm, Russ Ottens, UGA.

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