Plants make scents: Variation in plant volatile organic chemical emission at multiple scales

Substantial evidence indicates that herbivore-induced plant volatile organic chemical (VOC) emission can serve as an indirect plant defense by attracting the natural enemies of herbivores to sites of plant damage. A thorough understanding of the function and evolution of plant VOC emission requires detailed knowledge of the underlying variation that influences the trait. This dissertation investigates genetic variation in plant VOC emission at multiple scales: intraspecific variation within a population, intraspecific variation among plant populations, and variation among species within a genus. I used field and common garden experiments to study the potential role of VOC emission as an indirect defense in plants of the genus Asclepias (milkweeds) against an important specialist herbivore, Danaus plexippus Linnaeus (monarch caterpillar). I found that herbivore-damaged milkweed plants attracted more natural enemies than did undamaged plants, and that VOC emission varied among plant genotypes within a single population. Although VOC concentration and blend varied among genotypes, natural enemies attacked decoy caterpillars with equal frequency on all plant genotypes. To evaluate variation in VOC emission among plant populations, I tested the hypothesis that the indirect defense would vary predictably across a latitudinal gradient. Contrary to a predicted pattern of more intense defense at low latitudes, I found that herbivore-induced VOC emission increased with increasing latitude. Finally, I used phylogenetic analyses to investigate macroevolutionary patterns in VOC emission among plant species in the genus Asclepias. I tested the hypothesis that this indirect defense would exhibit a pattern of phylogenetic escalation to compensate for the phylogenetic decline of direct defenses in the same system. I observed macroevolutionary escalation in the number of VOCs emitted upon herbivore damage and a directional change in herbivore-induced VOC blend, which may indicate that Asclepias species have evolved to target more effectively the natural enemies of their herbivores. I also observed a phylogenetic decline in total constitutive and induced VOC concentrations among Asclepias lineages. These experiments provide substantial evidence that plant genotype influences variation in VOC emission at multiple scales.