Landscape-dependent effects of varietal mixtures on insect pest control and implications for farmer profits

Intraspecific plant diversity can significantly impact insect herbivore populations in natural systems. Yet, its role as an insect pest control strategy in agriculture has received less attention, and little is known about which crop traits are important to herbivores in different landscape contexts. Moreover, empirical economic analyses on the cost-effectiveness of varietal mixtures are lacking. We used varietal mixtures of Brassica oleracea crops on working farms to examine how two metrics of intraspecific crop diversity, varietal richness and number of plant colors (color richness), affect crop damage and the incidence and abundance of two insect pest species: Pieris rapae and Phyllotreta spp. We evaluated the context-dependency of varietal mixtures by sampling early- and late-season plantings of B. oleracea crops in farms across a gradient of landscape composition. We developed crop budgets and used a net present value analysis to assess the impact of varietal mixtures on input and labor costs, crop revenues, and profit. We found context-dependent effects of varietal mixtures on both pests. In early-season plantings, color richness did not affect Phyllotreta spp. populations. However, increasing varietal richness reduced Phyllotreta spp. incidence in simple landscapes dominated by cropland, but this trend was reversed in complex landscapes dominated by natural habitats. In late-season plantings, color richness reduced the incidence and abundance of P. rapae larvae, but only in complex landscapes where their populations were highest. Varietal richness had the same effect on P. rapae larvae as color richness. Unexpectedly, we consistently found lower pest pressure and reduced crop damage in simple landscapes. Although varietal mixtures did not affect crop damage, increasing color richness corresponded with increased profits, due to increased revenue and a marginal reduction in labor and input costs. We demonstrate varietal mixtures can significantly impact pest populations, and this effect can be mediated by intraspecific variation in crop color. However, the strength and direction of these effects vary by season, landscape composition, and pest species. The association between varietal color richness and profitability indicates farmers could design mixtures to enhance economic returns. We recommend additional research on the benefits of intraspecific trait variation for farmers.