Insect pests in winter oilseed rape affected by field and landscape characteristics

Winter oilseed rape (OSR, Brassica napus) cropping is often associated with an intensive use of pesticides. The transformation of structurally rich landscapes into more monotonous landscapes may be partly responsible for this, because non-crop habitats believed to benefit natural enemies have been eliminated. We examined the influence of field (soil quality, nitrogen fertilization) and landscape characteristics (OSR area and isolation, non-crop area, landscape diversity, proportions of grassy fallows and woody areas) on the three major European OSR pest groups: pollen beetles, stem weevils, and brassica pod midges. Twenty-nine landscape sectors ranging from structurally poor to complex were studied at eight spatial scales (radii 250-2000m) centered in the studied OSR fields. Abundances of pollen beetles and stem weevils were significantly positively correlated with soil quality and negatively related to OSR area in the surroundings. Generally, abundances of all groups were positively related to woody areas but not related to grassy fallow area. Pod midges and stem weevils tended to respond primarily to landscape variables at small (250-500 m) and medium (1000-1250 m) scales, while pollen beetles responded at medium to large (1000-2000 m) scales. The results are discussed in relation to differences in overwintering strategies and mobility of pest insects. Strategies at the field and landscape level, aiming to reduce pest pressure in OSR fields, are also discussed. (c) 2007 Gessellschaft fur Okologie. Published by Elsevier GmbH. All rights reserved.