If growers have noticed small dead patches in their bahiagrass that get bigger each year, billbugs may be the cause. Billbugs are a group of beetles in the weevil family (Curculionidae). If left unchecked, they are capable of killing stands of bahiagrass. Adult billbugs are recognizable by their long snout and black or gray bodies. The larvae are legless grubs, which distinguish them from common white grubs. Both the adults and larvae can cause damage.
Females lay their eggs into feeding holes right above the crown of the grass. Newly hatched larvae begin to feed within the grass stem. The stems become hollowed out and can sometimes easily be pulled out by hand. As the larvae grow and get too big for the stem, they will migrate into the soil and feed on the roots, causing further damage.
Sphenophorus coesifrons is the billbug in Alabama’s region that can cause problems in bahiagrass pastures and hayfields. The adults do not fly, so they often infest fields by crawling across paved surfaces into the grass. They can also be moved accidentally by vehicles or farm equipment.
Scouting and Control
In the spring and summer, billbug damage can often be misidentified as grass that has failed to green up. This damage is also often confused with disease or drought. These areas will not respond to watering as the larvae have damaged the roots so severely that the grass cannot obtain water and nutrients from the soil. Damaged patches will start out as irregularly shaped or rounded areas of brown or yellowing grass. These damaged areas will sometimes begin near sidewalks or walkways.
There is a lack of a solid management option for this pest other than a timely spray when the adults have emerged. The easiest way to check a potentially infested field for billbugs is with pitfall traps to catch the adults as they emerge from the ground. Using a plastic water bottle with the top cut off, or any size plastic cup, dig a hole and place the cup in the ground so the top is flush with the ground’s surface. Add a liquid such as soapy water or propylene glycol to the cup, mark the location in the field with a flag, and check weekly for adults. Once adults are captured in the cup, it is time to apply an approved insecticide such as pyrethroids or carbaryl. A second application may be necessary later in the summer to control the infestation.