Landscape complexity and farming practice influence the condition of polyphagous carabid beetles

Previous work has suggested that the condition of polyphagous predatory car abid beetles can be influenced by landscape structure and farming practice. However, earlier studies aimed at testing the effect of farming practice o n insect condition and reproduction have not been designed to test this acc urately because farms representing different cultivation systems have not h ad similar landscape complexity. In this study we measured the condition of the polyphagous predatory carabid beetles Harpalus rufipes, Pterostichus c upreus, P. melanarius, and P. niger from five pairs of conventional and org anic farms in central east Sweden. The farms within each pair were near to each other and had similar size, land use, and landscape structure. The con dition of the beetles was measured as the residuals from the regression bet ween 1n(elytra length) and 1n(body mass). We show that the residuals for P. melanarius were positively correlated with their fat reserves. For P. mela narius, residuals were higher on farms where the fields had high perimeter- to-area ratio. The pooled residuals of H. rufipes, P. cupreus, and P. niger were positively correlated with the perimeter-to-area ratio and crop diver sity. Pterostichus melanarius from organic farms had better condition than conspecifics at conventional farms, and for H. rufipes, P. cupreus, and P. niger combined there was a tendency toward better condition on organic farm s than at conventional farms. Harpalus rufipes, P. cupreus, P. melanarius, and P. niger; which are all polyphagous predatory carabid beetles, are natu ral enemies of pest aphids in the agroecosystem. We conclude that, by manip ulating landscape structure and composition, improvement in the condition o f polyphagous predatory carabid beetles could be achieved, thereby potentia lly increasing populations of these beneficial insects.