Consumptive and non-consumptive effects of predatory arthropods on billbug (Coleoptera: Dryophthoridae) pests in turfgrass.
Generalist predators affect pest populations through direct consumption or by non-consumptive effects, whereby predators induce changes in prey behavior which represent a cost to prey. A diverse community of predatory arthropods has been described in turfgrass, contributing to the direct mortality of pests including black cutworm, fall armyworm, and Japanese beetle. Billbugs are a major pest of turfgrass in the Intermountain West, but the composition of the local predatory arthropod community and whether predators aid in billbug suppression through consumptive or non-consumptive effects is unknown. First, we catalogued the predatory arthropod community on Utah and Idaho golf courses using linear pitfall traps. Then, we assessed adult billbug consumption by resident predators in the field. Using a series of lab assays, we assessed the most abundant predators' consumption of billbug life stages, including adults, sentinel waxworm larvae at varied soil depths, and eggs in turf stems. Finally, we assessed the non-consumptive effects of these abundant predators on adult billbug activity (mating, oviposition, thanatosis). We found that the predatory arthropod community consisted primarily of carabids (Pterostichus melanarius, Harpalus sp., Amara aenea, and Anisodactylus sp.) and spiders (lycosids), representing 60% and 28% of all predators, respectively. In the field and in lab assays, adult billbug mortality from predation was generally low at <6%. While predators readily consumed sentinel larvae in petri dish arenas, larvae escaped predation at 1 cm soil depth. The greatest consumptive effects of predators were on billbug eggs, with Anisodactlylus sp. feeding on 46% of eggs. Predator exposure reduced overall billbug activity by 56%, and for hunting billbugs, specifically, reduced mating activity by 28%. Our new understanding of the consumptive and non-consumptive effects of predators on billbugs supports the importance of conservation biocontrol in turfgrass and assists in planning for enhancement of specific predators.