The influence of increased crop genotypic diversity on arthropods in agroecosystems

Higher levels of plant diversity within fields, farms, and landscapes helps counteract the vulnerability of crop fields to insect pests by promoting both plant-mediated (bottom-up) and natural enemy-mediated (top-down) control of insect herbivores. Cultivar mixtures increase crop genotypic diversity and present a tactic to diversify agricultural systems that is more feasible than increasing plant species diversity in fields. Encouragingly, mixtures of plant genotypes can influence arthropod communities and increase plant productivity in both natural and agricultural systems.;I examined the potential use of cultivar mixtures to suppress insect pests in field crops using a combination of field, greenhouse, and laboratory experiments with two model systems: soybean aphid (Aphis glycines Matsumura) and soybean, and bird cherry-oat aphid (Rhopalosiphum padi L.) and wheat. I also characterized the natural enemy community available to suppress soybean aphids in PA as well as ants that may serve as mutualists of the aphids. I further tested determined if the arthropod community sampled in the foliage at night was distinct from the community captured during the day. Soybean fields in central Pennsylvania hosted a diverse arthropod community that contained many taxa identified in previous surveys across the U.S., but also a number that were previously missed, omitted, or identified at a coarser level. The community of natural enemies collected at night was distinct from the day-collected community. Specifically, a number of predators and ant taxa were more abundant in the foliage at night and have been under-represented in previous work. Under low pest pressure, soybean cultivar mixtures did not host smaller aphid populations than monocultures, but did produce intermediate aphid populations and influenced natural enemies and predation. Similarly, genotypic diversity per se did not reduce R. padi populations in the greenhouse on spring wheat. Populations on individual mixtures rarely differed from those on the individual components of the mixtures grown in monocultures. However, aphid populations on mixtures were intermediate in mixtures compared to the monocultures, and on individual plants of certain varieties, genotypic diversity influenced aphid populations. Interactions between plants or source-sink population dynamics likely increased or decreased aphid populations on some varieties in mixtures compared to monocultures. The spotted lady beetle (Coleomegilla maculata De Geer), an aphid predator, was influenced by plant genotypic diversity. It was more abundant on or attracted to aphid-infested plants in mixtures than in monocultures. In winter wheat, plant-plant interactions in diverse neighborhoods reduced aphid offspring production across tested varieties. However, the specific effect of neighborhood diversity depended upon variety identity. In addition, the influence of neighborhood diversity was most pronounced when plants were adequately watered and not drought-stressed.;My results indicate that while genotypic diversity per se does not always suppress pests and reduce pest populations, cultivar mixtures can significantly affect both herbivores and their natural enemies and the "correct" mixtures could play a role in suppressing pests. Both bottom-up and top-down forces can mediate effects of crop genotypic diversity on herbivores. Simple mixtures are now widely used for managing pest resistance to transgenic Bt corn, providing an opportunity to consider moving beyond these simple mixtures and genotypic monocultures in other crops. My findings indicate that producing effective mixtures will require careful development and will likely benefit from cooperation between entomologists and plant breeders to produce mixtures that are acceptable to growers and contain relevant diversity to suppress pests.