Warming Alters Prey Density and Biological Control in Conventional and Organic Agricultural Systems

Studies have shown that organically farmed fields promote natural predator populations and often have lower pest populations than conventional fields, due to a combination of increased predation pressure and greater plant resistance to pest damage. It is unknown how pest populations and predator efficacy may respond in these farming systems as global temperatures increase. To test these questions, we placed enclosures in eight alfalfa fields farmed using conventional (n = 4) or organic (n = 4) practices for 25 years. We stocked enclosures with pea aphids and 0, 2, or 4 predaceous ladybeetles. Half of the enclosures per field were then either left at ambient temperature or plastic-wrapped to warm them by 2 °C. Aphid abundances were similar in conventional and organic fields under ambient conditions, but were significantly more abundant in conventional than in organic fields when enclosures were warmed. Predator efficacy was reduced under low predator abundance (Hippodamia convergens = 2) in conventional fields under warming conditions; predation strength in organic fields was unaffected by warming. Alfalfa biomass increased with increased predators in all farming and temperature treatments. Our study suggests that biological control may be more easily maintained in organic than in conventional systems as global temperature increases.