Nutritional effects on chemical defense alter predator–prey dynamics

How Diet Leads to Defensive Dynamism: Effect of the Dietary Quality on Autogenous Alkaloid Recovery Rate in a Chemically Defended Beetle

The impact of different diets on chemical defense has been extensively studied in animals that sequester defensive chemicals from food. However, there are fewer studies of diet-mediated variation in autogenously produced defenses. Ladybird beetles, which use autogenously synthesized defensive alkaloids, are used as models in a wide diversity of studies of chemical defense, specifically in studies of intraspecific variation in color pattern and chemical defense. Many aphidophagous ladybirds consume a wide diversity of aphid prey, which vary in quality and thus could affect the synthesis of chemical defense. We measured alkaloid recovery rate after reflex bleeding by the ladybird Adalia bipunctata on two different aphid diets, the high quality Acyrthosiphon pisum and the lower quality Aphis fabae. Alkaloids reaccumulated in ladybirds more slowly when they were fed A. fabae than when they were fed A. pisum and females generally had more alkaloid than males, but reaccumulated alkaloid more slowly. Recovery times were more than 12 days. There appeared to be a weak positive relationship between alkaloid level and time since reflex bleeding for eggs of A. pisum- but not A. fabae-fed females. Our findings on diet and alkaloid synthesis in ladybirds suggest that chemical defense levels are very dynamic, indicating that studies conducted at a single point in time, such as those focused on ladybird color pattern, fail to consider a wide diversity of temporal variation that occurs in the field. This is likely true for many autogenously produced chemical defense systems in a diversity of other organisms.

Temperature Affects Chemical Defense in a Mite-Beetle Predator-Prey System

Temperature influences all biochemical and biophysiological processes within an organism. By extension, it also affects those ecological interactions that are mediated by gland-produced chemical compounds, such as reservoir-based chemical defense. Herein, we investigate how environmental temperature affects the regeneration of defensive secretions and influences the efficacy of chemical defense in a model predator-prey system: the oribatid mite Archegozetes longisetosus and the predaceous rove beetle Stenus juno. Through a combination of chemical analyses, non-linear regression modeling and theoretical simulations we show that the amount of defensive secretion responded to temperature in a unimodal optimum curve: the regeneration rate followed a positive, linear relationship up to 35 °C, but rapidly broke down beyond this temperature ("tipping point" effect). Using functional response simulations, there is an initially positive dampening effect on the predation rate when regeneration is optimal, but at higher temperatures chemical defense does not counteract the previously described effects of elevated predatory pressure. In a larger context, our results demonstrate the need to integrate relevant environmental factors in predator-prey modeling approaches.