| Insects face the challenge of locating hosts in a complicated olfactory landscape. Plants face a dilemma: luring pollinators through floral scent while remaining inconspicuous to herbivores. Through a combination of analytical chemistry and field research I have investigated how two species of thistle, Cirsium repandum, sandhill thistle, C. arvense, Canada thistle, and their insect floral visitors meet these challenges.; I chemically characterized the ontogenetic and temporal variability in volatile emission profiles of each thistle species and documented insect behavior under field conditions. I identified 42 compounds from the floral headspace of C. repandum. From C. arvense, which is dioecious (staminate and pistillate plants are separate), I identified 13 compounds and a five-fold difference in total emission rates between staminate and pistillate flowerheads. There were no repellent compounds emitted from either plant. In a manipulative pollination experiment, I determined that scent drops dramatically following pollination. Using scent-emitting insect traps in the field, I identified the components of C. arvense that are important in attracting both beneficial and detrimental insects. The two most abundant compounds of the C. arvense blend attracted both pollinators and herbivores, while the two least abundant blend components attracted pollinators only; other compounds attracted herbivores.; Combining results from each of my experiments, I compared patterns of scent emission with specific knowledge of the ecological significance of particular compounds. In both thistle species, attractive compounds are emitted only during the most reproductively receptive periods, and emission rates drop following fertilization, within flowerheads that contain developing seeds. In contrast, scent continues to be produced in older staminate flowerheads at a level higher than in any stage of ontogeny in pistillate flowerheads in C. arvense. This is not a critical stage in reproductive function, and herbivores may be attracted without consequence. The diel pattern of insect activity on C. repandum is not correlated with scent emissions. Scent in C. arvense tracks the peak in pollinator activity and declines when herbivores are active. In C. arvense through ontogenetic patterns, diel emissions and post-pollination scent changes, floral scent functionally targets pollinators and evades herbivores. |
