Interacting effects of wind direction and resource distribution on insect pest densities

Considerable effort has been made to investigate how landscape composition and spatial structures of habitats influence distribution patterns of species. In particular, specialist insect herbivores are known to be affected by spatial and temporal accessibility of their host plants. We studied three important insect pests of oilseed rape (OSR): Meligethes aeneus, Ceutorhynchus pallidactylus and Dasineura brassicae. In a landscape with northwest winds prevailing, we analysed their densities by comparing the predictive power of OSR area in differently orientated landscape sectors. Regression analyses showed that OSR area downwind from a sample site explained up to 72% of the variance in the density of M. aeneus, whereas OSR area in other directions had little effect. The correlation between downwind OSR area and M. aeneus density was negative and observable up to a distance of 1250m. In contrast, the densities of C palliclactylus and D. brassicae showed little response to OSR area in whatever direction. We suggest that mainly resource detection mechanisms and dispersal capabilities are responsible for the detected patterns: Odour-orientated upwind anemotaxis apparently drives directional dispersal of mobile species (M. aeneus). However, OSR area along the dispersal path seems to reduce pest density in upwind direction, because the majority of individuals detect resource patches early during the dispersal process. For less mobile species (C pallidactylus and D. brassicae), similar effects were not detectable at the landscape scale because dispersal capabilities probably were too short. (C) 2008 Gesellschaft fur Okologie. Published by Elsevier GmbH. All rights reserved.