Predator densities alter the influence of non-consumptive effects on the population dynamics of an agricultural pest

1. Augmentative release of predators is used in agriculture to suppress pest populations and minimise crop damage. However, predators affect prey populations not only through consumption but by inducing changes to prey morphology, physiology, behaviour and life history. These risk-induced trait responses can increase prey fitness, potentially reducing the efficacy of certain predators as biocontrol agents.2. We investigated how non-consumptive effects contribute to the overall influence of predators by modelling the effect of the predator Podisus maculiventris on the population dynamics of a major agricultural pest, the Colorado potato beetle (Leptinotarsa decemlineata). We developed a demographic model incorporating both consumption and non-consumptive Podisus-induced shifts in beetle reproductive strategy. Our model combines data from multiple mechanistic experiments showing that predator-exposed beetles produce fewer offspring but increase offspring provisioning, and that provisioned larvae have increased survival when predators are present. We simulated beetle densities across multiple beetle generations and a range of predator densities.3. The influence of non-consumptive effects depended on predator density. At low predator densities, non-consumptive effects strengthened the negative effect of predators by up to 45%, and risk-induced changes to offspring provisioning had a stronger effect on prey densities than reduced offspring quantity. In contrast, at high predator densities, non-consumptive effects changed the effect of predators by <5%, likely because beetle densities were very low both with and without non-consumptive effects.4. Our results suggest that even adaptive responses to predation risk by prey do not necessarily reduce the ability of predators to suppress pest populations.