DISSERTATION RESEARCH: Defensive chemistry underlying latitudinal patterns in plant-herbivore interactions

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The distribution of crop pests is not constant in all places always. If scientists could predict where or when pests would be worst, it could prevent financial losses. This project will provide direct insight into the specific role of the environment (climate, summer length, etc.) on pest pressure. The researchers will address this question in a wild plant species rather than a crop species. This is because pesticides and agricultural practices hide the influence of natural factors. The focal species in this study is pokeweed (and associated pests). Pokeweed is a culturally significant perennial plant native to the eastern USA. Prior research has shown that pokeweed is heavily eaten by caterpillars in the southern vs. northern US. This proposal checks if pokeweed's defenses against insects are stronger in the South. That pattern would suggest that the plants in the South have adapted to a long history of stronger pest pressure. In addition, the research will answer basic questions about how the environment influences relationships between plants and insects. It will also help to address the long-standing mystery of why ecosystems closer to the equator have more species than ecosystems at higher latitudes. The project will also enhance K-12 education through the creation of videos on latitudinal diversity for distribution to teachers.This project tests the hypothesis that plants are better chemically defended at lower latitudes. The researchers will use a generalist caterpillar (Spodoptera exigua) to bioassay defense, and correlate caterpillar performance with leaf traits. Preliminary trials show that mortality is greater for S. exigua raised on a diet of pokeweed leaves from lower vs. higher latitude plants, suggesting that low-latitude plants are better defended. Lower latitude leaves are tougher, so to disentangle mechanical and chemical defense traits, ground leaves will be incorporated into agar for bioassays that test the effects of chemical defenses only. Liquid and Gas Chromatography Mass Spectrometry will characterize leaf chemistry profiles, and the ratio of carbon to nitrogen will serve as an index of leaf nutritional value. In sum, the project asks which leaf traits predict caterpillar performance over a latitudinal gradient, advancing our understanding of geographic variation in plant defense and addressing recent controversy about the generality of stronger biotic interactions at lower latitudes.