Invasive species are often exposed to strong natural selection in their new environments, making them ideal for studying rapid evolution. Invasive species often exhibit differences in important traits like development rate, body size and reproductive capacity as they colonize new habitat. However, it is not clear if these differences are due to genetic change in the invasive population, or to underlying flexibilty in their traits. The ability of an organism to express different traits depending on the environment (called phenotypic plasticity) allows organisms to cope with a broad range of conditions. It is not known whether invasive species colonize new environments by rapid evolution or by phenotypic plasticity. Biological invasions are therefore a living laboratory for studying how organisms cope with new environments. This is particularly true for highly successful invaders that have colonized more than one region. By comparing multiple invasion events, we can learn whether the mechanisms and patterns of adaptation are general to all biological invasions, or are unique to each invasion event. My research on invasive cabbage white butterfly (Pieris rapae) provide an ideal foundation for studying rapid evolution in an invasive species. The cabbate white butterfly is native to Europe, but has become an invasive pest on every continent but Antarctica. It was accidentally introduced to Eastern Canada by1860, and rapidly colonized North America, quickly spreading to the west coast in just 25 years. The cabbage white independently colonized Japan by the early 1700s. Because it is an agricultural pest, it is the target of biological control programs in both regions using the imported European parasitic wasp, Cotesia glomerata. The wasp lays its eggs in cabbage white caterpillars, and the developing wasp larvae kill the host caterpillar. Attack by parasitic wasps is the largest source of mortality for P. rapae caterpillars. However, attack rates of C. glomerata vary with climate and season My research is on how the cabbage white has adapted to climate and parasites in Japan and in North America, and whether the patterns of growth, development and immune function are different in northern or southern climates. My field studies in Japan and North America demonstrate that the butterfly has evolved differences in body size and growth rate in both North America and Japan, and that these traits are temperature dependent and vary regionally. Specifically, some populations grow to be larger in warm temperatures, while others mature at a smaller size at warm temperatures. My NSF fellowship not only provided me with a unique opportunity to study teh Japanese population of cabbage white butterfly, it also allowed me unprecidented ability to work with Japanese researchers and engage in international collaboration. I hope to return to Japan to work on some theoretical projects that I developed with colleagues while doing my field work at Kyoto University.