Landscape structure and intraspecific diversity in Brassica oleracea crops significantly influence herbivore dynamics

Plant intraspecific diversity has been shown to slow the spread of plant pathogens and has been used in agricultural disease control programs for many years. In contrast, the effect of crop intraspecific diversity on the abundance of insect herbivores has received relatively little attention. Recently, studies in natural ecosystems have demonstrated that plant intraspecific diversity can play as important a role as plant species diversity in regulating herbivorous insects. In this study we assessed the effect of crop intraspecific diversity on economically important herbivores of Brassica oleracea crops, including the imported cabbage worm (Pieris rapae) and flea beetles (Phyllotreta spp). We collaborated with 12 farms in the Finger Lakes region of NY State that grow different varieties of B. oleracea crops, such as cabbage and broccoli. We selected plots with low and high levels of intraspecific B. oleracea diversity and visually sampled herbivore abundance five times from July through September in 2013. Preliminary analyses suggest that the effect of intraspecific diversity on Phyllotreta spp. depends on landscape context, specifically the percent of pasture within a 500 m radius of the plot. These analyses also indicate a negative correlation between intraspecific diversity at the plot level and the incidence of P. rapae larvae. Our results suggest that enhancing intraspecific crop diversity can suppress insect herbivore populations and that understanding plot-level dynamics as well as landscape structure is vital to effective herbivore management.