Pollinators are often assumed to be the primary selective agents on floral structures in plants. However, many insect visitors to flowers do not pollinate, but rather only feed on nectar or prey on herbivorous insects. In the absence of a need for pollinators, do other floral visitors – including insect herbivores and their predators – drive evolution of floral traits? In the permanent translocation heterozygosity (PTH) mating system of Oenothera biennis (Onagraceae), flowers self-fertilize before opening; because the PTH system also lacks recombination, plants typically produce seeds that are clones of the parent. Thus, this plant species presents an opportunity to evaluate floral visitor preferences for distinct genotypes in a system with no need for pollinators. In a preliminary study, we assessed the degree of variation in floral traits in a common garden of 25 genotypes and allowed natural colonization by insect visitors including a specialist herbivore, Schinia florida. We also induced plant defenses by applying jasmonic acid to half of all plants to test effects of induction on plant growth and floral traits.
Jasmonic acid had a positive short-term effect on herbivore damage in rosettes, and effects on plant size that varied by genotype. In a subset of annual genotypes, there were trends toward increased floral display size and decreased nectar volume in flowering plants induced by jasmonic acid. However, the effect of simulated herbivory on these attractant and reward traits differed by genotype. Schinia florida oviposition also varied by genotype, and occurred most often on the genotype with the greatest concentration of linalool in floral volatiles. We conclude that Oenothera biennis genotypes vary in floral traits considered important for pollinator attraction and reward, and may induce changes in these traits in response to herbivory. However, we have yet to elucidate the importance of this variation for non-pollinating insect visitors. To do so, future experiments will partition the interactions of plants with herbivores and predators. Specifically, we will assess the attraction of floral traits to these two different trophic levels and the consequences for plant fitness.