Effect of within-field and landscape factors on insect damage in winter oilseed rape

It was investigated whether damage on winter oilseed rape (Brassica napus) caused by three major pests (rape and cabbage stem weevil, pollen beetle, brassica pod midge) was affected by within-field (soil quality, nitrogen fertilization level, plant development, stand density) and landscape factors (percentage B. napus area, isolation of B. napus fields, number of B. napus fields, average distance between study field and surrounding B. napus fields—all within a radius of 2000 m around the study fields). Damage and within-field/landscape relationships were analyzed on 29 landscape sectors using stepwise multiple regression analyses. Damage caused by stem weevil larvae was assessed by measuring the length of damaged stem pith on dissected B. napus stems, pollen beetle damage was assessed on top racemes by calculating the percentage of podless peduncles, damage by pod midge larvae was calculated as the percentage of yellow and prematurely split pods in the top racemes. Stem weevil and pollen beetle damage was significantly positively correlated with respective pest abundances, however no such relationship could be observed for pod midge. Oilseed rape yield was for all three damage measures significantly negatively related to the degree of damage. Multiple regression analyses revealed that pollen beetle and pod midge damage was negatively related to B. napus area in the surrounding landscape, while stem weevil damage showed a positive relationship with soil quality, plant development and stand density. The results indicated that pollen beetle and pod midge damage was mainly influenced by the amount of host plants in the landscape while stem weevil damage seemed to be more affected by within-field characteristics that might have altered the nutritional quality of the oilseed rape crop and/or the stand microclimate.