Cultivar mixtures of soybeans have inconsistent effects on herbivore and natural-enemy populations

Mounting evidence suggests that increasing within-species (or genotypic) crop diversity by planting mixtures of cultivars is a promising tactic for managing herbivorous pests. The soybean aphid (Aphis glycines Matsumura) is an invasive species that has forced growers to increase use of insecticides across the Midwestern United States. There is a clear need for alternative and more sustainable management methods; therefore, the soybean-soybean aphid system provides a useful model for testing the utility of cultivar mixtures. We conducted a two-year field experiment with soybeans and soybean aphid to measure top-down and bottom-up ecological consequences of increasing plant genotypic diversity in crop fields. Soybean aphid populations can be suppressed by predators, but can also be influenced by aphid-tending ants. Specifically, we evaluated if cultivar mixtures influence soybean aphid populations, the associated arthropod community, predation, and crop yield. We compared monocultures of six soybean cultivars to all six possible five-cultivar mixtures using 82.8 m² plots. We also assessed if cultivar mixtures influence populations of another herbivore, potato leafhopper. Separately for day and nighttime, we evaluated predator and ant populations, and measured predation. We also assessed soybean yield. Over the course of the study, aphid populations remained low, and cultivar mixtures did not alter season-long aphid pressure or yield, but did produce intermediate yield and aphid populations compared to monocultures. Crop genotypic diversity increased leafhopper populations, likely because of non-additive interactions involving one highly susceptible or attractive cultivar. Diversity did not influence aphid-tending ants. In both years, predators helped suppress aphid populations, and predator abundance and predation was influenced by genotypic diversity during specific parts of the season and for some sampling times, but results were mixed and inconsistent. Of the most abundant taxa, genotypic diversity increased abundance of Harmonia axyridis in 2010 during the day, but not the night. Overall, results from this study demonstrated that under low pest pressure, cultivar mixtures do not produce clear benefits. Mixtures did appear to produce intermediate aphid populations and yield, and also influenced other herbivore species, which could influence natural-enemy control of soybean aphids. Effects on herbivores and predators could become more important under high pest pressure, although this remains to be determined. Our study also provides some evidence that plant genotypic diversity in crop fields can influence higher trophic levels, paralleling work in natural systems that has demonstrated system-wide consequences of diversity.