Fine-scale plant defence variability increases top-down control of an herbivore

Herbivore populations are regulated by a combination of plant defences and natural enemies. While plant defence can suppress herbivore populations, these defences can also adversely affect natural enemies, thereby releasing herbivores from top-down control. Over their life spans, herbivores and their natural enemies may experience substantial variation in plant defence. Recent studies have demonstrated that individual herbivores feeding on diets containing variable concentrations of plant toxins suffer substantially reduced fitness compared to herbivores feeding on a constant toxin concentration, even when both groups of herbivores experience equivalent means. However, the impacts of defence variability on natural enemies and top-down control of herbivores are unknown. Using artificial diets, we independently manipulated the mean concentration and variation of a plant toxin experienced by individual Trichoplusia ni caterpillars and its parasitoid Copidosoma floridanum. Additionally, by combining the performance of individual caterpillars on different constant diet concentrations of toxin, we were able to estimate the effect of toxin variability between herbivores using nonlinear averaging. Increases in the mean toxin concentration in the diet of parasitized T. ni hosts decreased the fitness of C. floridanum, while variance in individual diets did not impact parasitoid fitness, even though both mean and variance decreased the fitness of T. ni caterpillars. Increased variability in encountered plant defences suppressed individual herbivore fitness with no perceptible cost to top-down control. At the population level, however, increased variability between individual herbivore diets decreased the success of parasitoids relative to herbivores, thus reducing the strength of top-down control. Our study highlights the importance of defence variability at different scales in regulating herbivore performance. Variability in plant defence has the potential to reduce herbivore populations through a combination of bottom-up and top-down effects, but only at small spatial scales experienced by individual herbivores.