Caterpillars on a phytochemical landscape: the case of alfalfa and the Melissa blue butterfly

Modern metabolomic approaches that generate more comprehensive
phytochemical profiles than were previously available are providing new
opportunities for understanding plant-animal interactions. Specifically,
we can characterize the phytochemical landscape by asking how a larger
number of individual compounds affect herbivores and how compounds covary
among plants. Here we use the recent colonization of alfalfa (Medicago
sativa) by the Melissa blue butterfly (Lycaeides melissa) to quantify
plant metabolites and the performance of caterpillars as affected by both
individual compounds and suites of covarying phytochemicals. We find that
survival, development time and adult weight are all associated with
variation in nutrition and toxicity, including biomolecules associated
with plant cell function as well as putative anti-herbivore action. The
plant-insect interface is complex, with clusters of covarying compounds in
many cases encompassing divergent effects on different aspects of
caterpillar performance. Individual compounds with the strongest
associations are largely specialized metabolites, including alkaloids,
phenolic glycosides and saponins. The saponins are represented in our data
by more than 25 individual compounds with beneficial and detrimental
effects on L. melissa caterpillars, which highlights the value of
metabolomic data as opposed to approaches that rely on total
concentrations within broad defensive classes.