Host plant specificity of the monarch butterfly (Danaus plexippus)

The preference-performance hypothesis explains host specificity in phytophagous insects, positing that host plants chosen by adults confer the greatest larval fitness. However, adults sometimes oviposit on plants supporting low larval success because the components of host specificity (adult preference, plant palatability, and larval survival) are continuous and not necessarily correlated. Palatability (willingness to eat) is governed by chemical cues and physical barriers such as trichomes, while survival (ability to complete development) is a function of nutrition and toxicity. Absence of a correlation between preference, palatability, and survival results in low performance hosts, supporting limited larval development. The importance and basis of survival and palatability have been under-explored. We conducted a review of 75 plant species claimed or tested to be hosts for the monarch butterfly (Danaus plexippus) and a meta-analysis testing if host status could be explained by properties of neurotoxic cardenolides or trichome density. We also conducted a no-choice feeding experiment to evaluate larval feeding behavior on two low performance, one high performance, and three non-hosts. We identified 31 high performance and 26 low performance hosts for monarchs. Neither cardenolide properties nor trichome density were significant predictors of host plant status although the mean cardenolide concentration was greater in high than low performance hosts. In the experiment, we identified two die-off events, the first of which (days 1-5) was caused by both low survival and palatability. The second die-off (~fifth instar) was caused by low survival, demonstrating that multiple factors can affect larval host plant specificity.