Non-consumptive predator effects influence pest suppression and optimal timing of predator release in augmentative biocontrol

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Jessie Mutz, Brian D. Inouye and Nora Underwood, Biological Science, Florida State University, Tallahassee, FL, Jennifer S. Thaler, Entomology, Cornell University, Ithaca, NY

Background/Question/Methods
Biocontrol in agriculture is the use of natural enemies to suppress pest populations and minimize crop damage and economic injury. Decisions about what kinds of biocontrol agents to use and optimal release strategies have focused on enemy traits such as degree of specialization and attack rate, which reflect the enemy’s capacity for consumption of target prey. However, predators can also have non-consumptive effects on prey, where uneaten prey respond to predator presence with changes in life history, physiology, and behavior. To address how consumptive and non-consumptive predator effects combine to influence prey populations, we investigated the effect of the predator Podisus maculiventris on short-term population dynamics of an agricultural pest, the Colorado potato beetle (Leptinotarsa decemlineata, or CPB). To do this, we developed a demographic model incorporating empirical measurements of both consumption and non-consumptive Podisus-induced shifts in CPB reproductive strategy, where predator-exposed adults have fewer, higher-quality offspring. Our model was structured by life stage, larval condition, and predator exposure. We asked: (1) What are the net effects of consumptive and non-consumptive predator effects on short-term population dynamics?, (2) Under which predation regimes are CPB densities minimized?, (3) Are CPB populations more sensitive to qualitative or quantitative non-consumptive predator effects?
Results/Conclusions
Our simulations show that cumulative CPB density is lowest when predator densities are high in each of two CPB generations within a growing season. Initial conditions and CPB population parameters influence the specific shape of the relationship between predator densities in each generation and cumulative CPB density. In particular, increasing the initial proportion of predator-exposed adults weakens the effect of predator density during the first generation and strengthens the effect of predator density during the second generation, suggesting that a higher percentage of predators should be released during the second CPB generation to minimize cumulative CPB density when total predator release is limited. Similarly, when or whether CPB density exceeds an economic threshold of 1.5 adults / m2 depends on the proportion of predator-exposed adults and on the mortality rate of high-quality larvae. At low levels of predator augmentation, CPB populations are most sensitive to quantitative non-consumptive effects (e.g., number of clutches). With increasing predator density, CPB populations are most sensitive to qualitative non-consumptive effects (e.g., mortality rate of high-quality larvae). Our results suggest that non-consumptive predator effects can influence the efficacy of predator augmentation and which predator release strategies minimize pest densities across the cropping season.