LOCAL ADAPTATION AND AGENTS OF SELECTION IN A MOBILE INSECT

The deme-formation hypothesis states that selection can produce adapti ve genetic variation within and among phytophagous insect populations. We conducted three field experiments and tested this prediction by tr ansferring eggs and measuring performance of a mobile leafmining insec t, Stilbosis quadricustatella. In Experiment 1, we compared the rate o f mine initiation of leafminers transferred to natal and novel sites. In Experiment 2, we compared mine-initiation rate of leafminers transf erred to natal and novel host-plant species. In Experiment 3, we compa red the mine-initiation rate, mine-completion rate, and sources of mor tality of miners transferred to neighboring natal and novel Quercus ge minata trees. In the first, second, and third experiments, leafminer l arvae initiated significantly more mines at the natal site, on the nat al plant species, and on the natal e. geminata tree, evidence for adap tive differentiation. Furthermore, plant-mediated mortality was signif icantly lower among miners transferred to natal Q. geminata trees. Thi s result supports a key assumption of the deme-formation hypothesis: i nsects adapt to the defensive phenotypes of individual trees. However, natural-enemy mortality was significantly higher among miners transfe rred to natal trees, essentially reversing the plant effect. Therefore , rates of successful mine completion were similar on natal (19%) and novel (17%) trees. This experiment suggests that host plants and natur al enemies may represent opposing forces of selection. Leafminers adap ted to individual trees may realize a selective advantage only when na tural-enemy densities are low.