Interaction pathways between slugs and carabid beetles in strawberry agroecosystems: Implications for biological control

A central question in biological control is whether generalist predators can exert sufficient pressure on invasive pests. Due to the nature of predator-prey dynamics, generalists are often considered ineffective in biological control. Understanding the dynamics of predation is therefore important for examining the mechanisms of biological control between native predators and invasive pests. One example is the carabid-slug pathway, because the former has been reported as a major regulating force in some systems. However, the strength of this trophic linkage is poorly understood in North America. Observational studies of food webs are not always practical because generalist predators often forage at night and their rapid movements prevent accurate visual assessment. Therefore, COI molecular markers were developed to examine the strength of the trophic pathway between Harpalus pensylvanicus and Deroceras reticulatum in strawberry agroecosystems. Two treatments were used, a traditional bare ground planting and the addition of detrital subsidies to test the hypothesis that detrital subsidy would increase predator populations and translate into improved predation pressure on invasive mollusks. In laboratory experiments, the DNA of D. reticulatum was detectable in the gut of carabid beetles for up to 16h, a time period consistent with other (and optimal for these) experiments. Examination of gut-content from field-collected carabids revealed highly significant trophic pathways with invasive slugs, and variability in predation between different planting practices. Carabids have an impact on slugs worldwide and this research provided clear evidence for the level of connectedness and biological control potential within strawberry agroecosystems.