Host plant specificity of the monarch butterfly Danaus plexippus: A systematic review and meta-analysis

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 non-binary
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) depends upon nutrition and
toxicity. Absence of a correlation between the components of host
specificity results in low-performance hosts supporting limited larval
development. Most studies of specificity focus on oviposition behavior
leaving the importance and basis of palatability and survival
under-explored. We conducted a comprehensive review of 127 plant species
that have been claimed or tested to be hosts for the monarch butterfly
Danaus plexippus to classify them as non-hosts, low performance, or high
performance. We performed a meta-analysis to test if performance status
could be explained by the properties of neurotoxic cardenolides or
trichome density. We also conducted a no-choice larval feeding experiment
to identify the causes of low performance. We identified 34 high
performance, 42 low performance, 33 non-hosts, and 18 species with
unsubstantiated claims. Mean cardenolide concentration was greater in
high- than low-performance hosts and a significant predictor of host
status, suggesting possible evolutionary trade-offs in monarch
specialization. Other cardenolide properties and trichome density were not
significant predictors of host status. In the experiment, we found, of the
62% of larvae that attempted to eat low-performance hosts, only 3.5%
survived to adult compared to 85% of those on the high-performance host,
demonstrating that multiple factors affect larval host plant specificity.
Our study is the first to classify all known host plants for monarchs and
has conservation implications for this threatened species.