Verdure: Tinder weevil
Beth Guertal, Ph.D.
Annual bluegrass weevil (ABW) is a serious pest of golf course turfgrass throughout much of eastern North America. Overwintered adults invade in spring, marauding onto fairways from the rough, where the females lay eggs in grass stems. Hatched larvae bore in the stem of the grass plant, develop, and then exit to feed more and eventually pupate. As their name suggests, ABW favor annual bluegrass (both annual and perennial biotypes), but they can also be found in creeping bentgrass.
The belief that ABW prefers to feed on Poa annua has come from observations in the field where islands of dead P. annua can be seen among seemingly healthy creeping bentgrass. However, previous work done by the researchers featured in this article showed that, in fact, larvae are found in both turfgrass species, yet the damage thresholds are three to four times lower for P. annua. So, like drunken college sophomores, are ABW not particular about where they hang out? Will they accept a fairly wide range of grass hosts, and, if so, how do they decide on whose (metaphorical) sofa to crash?
If a preference for a host could be determined, it could be used as a tool to help manage ABW. Knowing where ABW are headed to lay eggs (or feed) would allow the development of targeted monitoring and management. This is what Ben McGraw, Ph.D., of Penn State University and Albrecht Koppenhöfer, Ph.D., of Rutgers University set out to examine, studying the spatial distribution of various growth stages of ABW at three golf courses in New Jersey over a four-year period.
The fairways at each course were a mix of P. annua and creeping bentgrass (Agrostis stolonifera) maintained at a ½-inch (1.25-cm) height. Large blocks of turf (30 feet × 30 feet, or ~9 meters × 9 meters) were laid out, with one side of each block parallel to the border of the fairway, at the beginning of the rough. In each block, 900 samples were taken at 1-foot (30-cm) intervals. In May of each year, the percent turfgrass species damaged and the amount of turfgrass damage were assessed at each of the 900 points. At the same time, a soil sample was collected from each point, and larvae, pupae and adult ABW were extracted from each core. Using the data collected, the researchers created detailed maps that showed the spatial distribution of the ABW (at a variety of life stages) throughout each block, as affected by turfgrass species.
The researchers found a wide range of ABW populations across the plots, from low levels of 11 individuals per square foot (11/0.1 square meter) to damaging populations of 44 per square foot. The spatial relationships were examined by life stage to determine whether females were drawn to or avoided patches of P. annua that might already contain eggs laid by another female. The clear answer was no — females did not avoid laying their eggs on patches of turfgrass already occupied by ABW eggs, an action that could place their eggs in direct competition with others. Also, this was not a function of the grass species, and laying eggs where other eggs were already present was not related to the amount of P. annua that was present.
Annual bluegrass weevils did not appear to prefer P. annua as a place to lay eggs when compared with creeping bentgrass. No strong spatial relationships were found between ABW larvae and P. annua. The data from this work showed that creeping bentgrass was a readily accepted host, even when P. annua was readily available. The authors mentioned that this lack of preference may explain why ABW can be a problem in areas where the predominate turf stand is creeping bentgrass.
Source: McGraw, B.A., and A.M. Koppenhöfer. 2015. Spatial analysis of Listronotus maculicollis immature stages demonstrates strong associations with conspecifics and turfgrass damage but not with optimal hosts on golf course fairways. Entomologia Experimentalis et Applicata 157:307-316.
Beth Guertal, Ph.D., is a professor in the Department of Crop, Soil and Environmental Sciences at Auburn University in Auburn, Ala., editor-in chief for the American Society of Agronomy, and president-elect of the Crop Science Society of America. She is a 20-year member of GCSAA.