Using New Molecular Tools to Explore How Organic Farming Impacts Top-Down and Bottom-Up Regulation of Potato Herbivores

Herbivorous agricultural pests are simultaneously threatened by predatory natural enemies and by toxic plant defenses. Both these "top down" and "bottom up" forces might be enhanced under organic farm management. For example, reductions in the use of broad-acting insecticides, typical of some organic systems, can lead to dramatically higher predator density and biodiversity on organic compared to conventional farms. Similarly, efforts by organic farmers to increase soil organic matter might lead to more robust plants better able to activate effective anti-herbivore defenses. We took advantage of modern molecular approaches to track predation and quantify the deployment of plant defenses for potatoes grown under organic or conventional practices in eastern Washington, USA. Chapter 2 reports significantly higher densities of predatory Geocoris and Nabis bugs in organic potato fields; because per-capita predation rates on spider mites were generally similar in the two farming systems, we would expect stronger top-down suppression of spider mites in organic than conventional potato fields. Chapter 3 reports greater defense-gene activity for potato foliage on organic than conventional fields of the potato variety Norkotah, although no such effect was seen for the variety Alturas. Chapter 4 reports heightened survivorship of Colorado potato beetles in the greenhouse on potato plants grown in organic than conventional soils, but was unaffected by the presence of aphids. In contrast, aphids reached significantly higher densities when reared alone than when paired with Colorado potato beetles, and were unaffected by soil type. Across the field and greenhouse studies reported in Chapters 3 and 4, we saw weakly heightened soil microbial activity in organic compared to conventional soils, but few differences in plant chemistry. Thus, further work is needed to explore the ecological basis of differences in plant defenses or herbivore survivorship mediated by soil management practices. Altogether, the work described in my dissertation suggests that the potential to alter and exploit organic versus conventional management practices to maximize plant deployment of natural defenses could form an exciting new approach to sustainable agriculture and host-plant resistance.