Is higher landscape heterogeneity associated with lower variation of abundances of pests and natural enemies?

Abstract: While the influence of landscape heterogeneity on responses of both crop pest and natural enemy populations have been extensively studied, impacts on the variation of these responses are not. In this study, we aimed to determine how landscape heterogeneity affects the variation of densities of the soybean aphid and of the guild of natural enemies, and whether temporal variations are observed. We hypothesized that 1) landscape heterogeneity is associated with lower variation of densities of pests and natural enemies; and 2) the effect is stronger during the period of highest aphid population level. Respectively 29 and 31 soybean fields were weekly sampled in 2011 and 2012 in Quebec (Canada). For each field, a coefficient of variation was calculated regarding the density of the soybean aphid (Aphis glycines), of the entire guild of natural enemies, and of the different sub-guilds (predators, parasitoids, entomopathogens) during 1) the population peak of the soybean aphid (August 15-20), and 2) two weeks before (August 1-6). Landscape heterogeneity indices were calculated at a scale of 1.5 km radius around the fields: Crop richness, Crop diversity, Margin density, Landscape patchiness, and Proportion of woodlands. Overall, when considering aphids, predators, pathogens or the entire guild, results are in accordance with our 1st hypothesis: less variation in densities observed in heterogeneous landscapes. Finally, in accordance with our 2nd hypothesis, landscape heterogeneity has a stronger effect on the variation of natural enemy densities (but not on aphid density) during the population peak of the soybean aphid.