Consequences of diversity: comparing pest and natural enemy populations in monocultures and soybean variety mixtures

Substantial evidence indicates that ecosystems benefit from increased diversity. Unfortunately, modern agricultural monocultures tend to harbor less biodiversity than natural systems and benefit little from complex ecological interactions associated with high diversity. Recent evidence suggests that intraspecific (i.e., genotypic) diversity can be as valuable as plant species diversity in structuring arthropod communities and reducing herbivore populations. Using soybeans as a model system, we compared soybean aphid and natural enemy populations in high and low genotypic diversity plots using variety mixtures in a two-year field experiment. We created high diversity plots using all possible mixtures of five soybean varieties chosen from a pool of six varieties and created low diversity plots by planting a monoculture of a single variety. These treatments were planted in replicated 30 by 30 foot plots. Soybean aphid (Aphis glycines) populations were assessed through weekly counts. Natural enemies were sampled multiple times throughout the summer with pitfall traps, sweep netting and direct counts. In addition, we evaluated plant productivity by measuring yield in each plot. The arthropod community, including both herbivores and natural enemies, as well as plant yield varied between treatments. Our results thus far appear to indicate that increasing crop genetic diversity holds promise as an insect pest management strategy that would be readily accessible to growers. Our experiment also builds on a growing body of work in natural and agricultural systems that has shown the importance of intraspecific diversity.