Monarch butterfly (Danaus plexippus) larval performance
Host selection among insect specialists has been studied extensively and relies on many physiological and environmental factors. Despite the fact that the monarch butterfly (Danaus plexippus), whose larvae feed almost exclusively on milkweeds in the genus Asclepias, is one of the best-known examples of an insect specialist, many online sources listing monarch host plants contain conflicting entries and lack citations. Additionally, research on oviposition behavior has shown that adult butterflies do not always select host plants that result in the greatest larval fitness. A wide variation in monarch larval performance has been documented on milkweed species commonly considered to be host plants, and limited survival data has been reported on less commonly utilized hosts, but few attempts have been made to explain the differences in larval performance on these plants. We present a comprehensive, evidence-based list of 123 plant species categorized by monarch larval performance, of which 41 were low performance hosts and 31 were high performance hosts. While we identified evidence for the role of higher cardenolide concentrations in the determination of high performance status, we found no evidence for the predictive effects of other cardenolide properties. No-choice feeding experiments using isolated tissue from plant species with varying larval performance found that all hosts exhibit two die-off events of varying magnitude. A die-off shortly after hatching was attributed to lack of stimulants or presence of deterrents (chemical or physical) in addition to toxicity or nutrient deficiency. The second die-off during the fifth instar was most likely due to plant toxicity or nutrient deficiency. We also observed a shorter development time in a high performance host when compared to a low performance plant. These results support monarch conservation efforts and lay the groundwork for further investigation into the basis of host plant palatability.