The evolution of glandularity as a defense against herbivores in the tarweed clade

Abstract Premise Glandular trichomes are implicated in direct and indirect defense of plants. However, the degree to which glandular and non‐glandular trichomes have evolved as a consequence of herbivory remains unclear, because their heritability, their association with herbivore resistance, their trade‐offs with one another, and their association with other functions are rarely quantified. Methods We conducted a phylogenetic comparison of trichomes and herbivore resistance against the generalist caterpillar, Heliothis virescens , among tarweed species (Asteraceae: Madiinae) and a genetic correlation study comparing those same traits among maternal half‐sibs of three tarweed species. Results Within a tarweed species, we found no evidence that herbivore growth rate decreased on tarweed individuals or maternal sib groups with more glandularity or denser trichomes. However, tarweed species with more glandularity and fewer non‐glandular trichomes resulted in slower‐growing herbivores. Likewise, a trade‐off between glandular and non‐glandular trichomes was apparent among tarweed species, but not among individuals or sib groups within a species. Conclusions Our results suggest that this key herbivore does not select for trichomes as a direct defense in tarweed species. However, trichomes differed substantially among species and likely affect herbivore pressure on those species. Our results demonstrate that trade‐offs among plant traits, as well as inference on the function of those traits, can depend on scale.