From foe to friend: The annual bluegrass weevil shifts from pest to beneficial suppressor of Poa annua as weed cover decreases

The annual bluegrass weevil (ABW) ( Listronotus maculicollis Kirby) is a key pest of short-mown golf course turf that preferentially oviposits and achieves greater larval densities on annual bluegrass ( Poa annua L.), which is less tolerant of ABW feeding than creeping bentgrass ( Agrostis stolonifera L.). Poa annua is also a persistent weed. Thus, withholding ABW-targeting insecticides can allow for the ABW to function as a biological control agent by selectively suppressing P. annua in mixed stands. However, higher P. annua abundance may increase ABW damage and reduce stand-level tolerance, leading to unacceptable turfgrass injury in highly managed fine turf and limiting its utility for biological control. We tested whether initial P. annua cover constrains this strategy. Two field experiments were conducted in mixed creeping bentgrass- P. annua fairway turf. Plots representing initial P. annua cover levels (0, 10, 25, and 50%) were managed under preventive, damage-threshold, or no-insecticide programs; threshold applications were triggered when turfgrass quality fell below 6 on a 1–9 scale. Withholding or delaying insecticide applications reduced P. annua cover relative to preventive programs (typically 20–50% relative reduction) but caused short-lived declines in turfgrass quality during peak ABW damage. Unacceptable turfgrass quality, when it occurred, was generally limited to 1–3 weeks and was primarily observed in plots with realized initial P. annua cover of 37% or greater, whereas plots with 33% cover or less generally maintained acceptable quality. Collectively, these findings suggest that ABW can be leveraged as a conditional biological control agent in mixed stands with relatively low P. annua abundance, highlighting opportunities to integrate weed and insect management while reducing insecticide use.