Beyond preference and performance: host plant selection by monarch butterflies, Danaus plexippus

The connection between adult preferences and offspring performance is a long-standing issue in understanding the evolutionary and ecological forces that dictate host associations and specialization in herbivorous insects. Indeed, decisions made by females about where to lay their eggs have direct consequences for fitness and are influenced by interacting factors including offspring performance, defence and competition. Nonetheless, in addition to these attributes of the offspring, a female's choices may be affected by her own prior experience. Here we examined oviposition preference, larval performance and the role of learning in the monarch butterfly, Danaus plexippus, which encounters diverse milkweed host species across its broad range and over the course of migration. Monarch females consistently preferred to oviposit on Asclepias incarnata subspecies pulchra. This plant, however, was associated with poor caterpillar growth, low sequestration of toxins and the highest plant defences (latex and trichomes). We examined flexibility in this apparently maladaptive preference by testing the impact of previous experience and competition on preference. Experience laying on an alternative plant species enhanced preference for that species in contrast to A. i. pulchra. In addition, presence of a (competing) conspecific caterpillar on A. i. pulchra had a strongly deterrent effect and reversed host plant preferences. Thus, monarch butterflies exhibit preferences contrary to what would be expected based on offspring development and sequestered defences, but their preferences are altered by learning and competition, which may allow butterflies to shift preferences as they encounter diverse milkweeds across the landscape. Learning and perception of threats (i.e. competition or predation) may be critical for most herbivorous insects, which universally experience heterogeneity among their potential host plants.