Relative importance of plant quality and habitat structure on consumptive and non-consumptive effects

Plant nutritional status and predation have strong effects on herbivores. When resource quality is poor, herbivores are expected to respond less to predators and take more risks. Increasing habitat complexity can lead to more effective biological control by providing resources to natural enemies. Here we examine the effects of plant nutrition and habitat complexity on the interaction of a herbivore (Pieris rapae, Lepidoptera: Pieridae) and a lady beetle predator (Hippodamia convergens, Coleoptera: Coccinellidae). In 2015, we investigated in a greenhouse experiment the impact of nitrogen and cover crop to collards on herbivore performance, preference, consumptive and non-consumptive effects. Butterflies laid more eggs per gram biomass on plants with low nitrogen but the effect of cover crop or predator was not significant. Larvae grew larger on the high nitrogen plants compared to low nitrogen plants, again with no significant effect of cover crops or predators. Predators consumed more larvae on low nitrogen plants than high nitrogen plants supporting the “slow-growth-high mortality” hypothesis. However, the effects of cover crops on larval survival were inconsistent. More larvae survived on high nitrogen plants with cover crops, while fewer larvae survived in the presence of cover crops on low nitrogen plants. The presence of lady beetles reduced larval weight on the high nitrogen plants but not on the low nitrogen plants. Our results indicate that the interactions between plant and habitat traits have different outcomes for biological control and we will conduct further studies in the future to elucidate this relationship.