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Abstract Plants have evolved multiple defensive traits in response to herbivory; in turn, herbivore specialists evolved adaptive behaviours to avoid or tolerate such defences. Here, we employ milkweeds (Asclepiasspp.) to test two defences, latex and trichomes, for their independent and interactive effects on behaviour and performance of monarch caterpillars (Danaus plexippus).Latex exuded upon damage and the density of leaf trichomes positively correlate across milkweed species, suggesting they may have evolved together as synergistic defences. Nonetheless, the complementary roles of these two traits have been little‐studied. We focus on two behaviours: shaving, or the removal of trichomes, and chewing, which encompasses both deactivation of latex and leaf consumption.In an experiment with seven milkweed species, with and without manipulated latex flow, we found latex to be the primary determinant of reducing chewing, while both defences positively predicted shaving behaviour in the first instar. Next, we conducted a factorial experiment throughout the first three instars, manipulating latex and trichomes on a high‐latex, high‐trichome species, the woolly milkweedAsclepias vestita. On plants with latex and trichomes intact, caterpillars spent the most time shaving and least time chewing of all treatment groups, suggesting a possible synergism. These defence‐driven behavioural effects decreased later in larval development.Latex and trichomes both impacted monarch performance, additively increasing mortality and reducing growth of survivors. Thus, latex and trichomes represent two important plant defences with effects on specialist herbivore behaviour and implications for insect fitness. 

Abstract Despite long‐standing theory for classifying plant ecological strategies, limited data directly link organismal traits to whole‐plant growth rates (GRs). We compared trait‐growth relationships based on three prominent theories: growth analysis, Grime's competitive–stress tolerant–ruderal (CSR) triangle, and the leaf economics spectrum (LES). Under these schemes, growth is hypothesized to be predicted by traits related to relative biomass investment, leaf structure, or gas exchange, respectively. We also considered traits not included in these theories but that might provide potential alternative best predictors of growth. In phylogenetic analyses of 30 diverse milkweeds (Asclepiasspp.) and 21 morphological and physiological traits, GR (total biomass produced per day) varied 50‐fold and was best predicted by biomass allocation to leaves (as predicted by growth analysis) and the CSR traits of leaf size and leaf dry matter content. Total leaf area (LA) and plant height were also excellent predictors of whole‐plant GRs. Despite two LES traits correlating with growth (mass‐based leaf nitrogen and area‐based leaf phosphorus contents), these were in the opposite direction of that predicted by LES, such that higher N and P contents corresponded to slower growth. The remaining LES traits (e.g., leaf gas exchange) were not predictive of plant GRs. Overall, differences in GR were driven more by whole‐plant characteristics such as biomass fractions and total LA than individual leaf‐level traits such as photosynthetic rate or specific leaf area. Our results are most consistent with classical growth analysis—combining leaf traits with whole‐plant allocation to best predict growth. However, given that destructive biomass measures are often not feasible, applying easy‐to‐measure leaf traits associated with the CSR classification appear more predictive of whole‐plant growth than LES traits. Testing the generality of this result across additional taxa would further improve our ability to predict whole‐plant growth from functional traits across scales. 

Summary The chemical arms race between plants and insects is foundational to the generation and maintenance of biological diversity. We asked how the evolution of a novel defensive compound in an already well‐defended plant lineage impacts interactions with diverse herbivores.Erysimum cheiranthoides(Brassicaceae), which produces both ancestral glucosinolates and novel cardiac glycosides, served as a model.We analyzed gene expression to identify cardiac glycoside biosynthetic enzymes inE. cheiranthoidesand characterized these enzymes via heterologous expression and CRISPR/Cas9 knockout. UsingE. cheiranthoidescardiac glycoside‐deficient lines, we conducted insect experiments in both the laboratory and field.EcCYP87A126 initiates cardiac glycoside biosynthesis via sterol side‐chain cleavage, andEcCYP716A418 has a role in cardiac glycoside hydroxylation. In EcCYP87A126knockout lines, cardiac glycoside production was eliminated. Laboratory experiments with these lines revealed that cardiac glycosides were highly effective defenses against two species of glucosinolate‐tolerant specialist herbivores, but did not protect against all crucifer‐feeding specialist herbivores in the field. Cardiac glycosides had lesser to no effect on two broad generalist herbivores.These results begin elucidation of theE. cheiranthoidescardiac glycoside biosynthetic pathway and demonstratein vivothat cardiac glycoside production allowsErysimumto escape from some, but not all, specialist herbivores.