Nitrogen fertilisation of winter oilseed rape: impact on insect pests, their parasitoids and plant diseases

Abstract: Nitrogen fertilisation is essential for maximized oilseed rape yields. However, very little is known about the impact of fertilisation on insect pests and diseases. We studied the impact of additional nitrogen on winter oilseed rape pod number, disease abundance and the oviposition activity of the insect pests Meligethes aeneus and Ceutorhynchus assimilis and their parasitoids. Insect sampling, pod counts and disease assessments were carried out on plots with seven different N-fertilizer levels (0, 60, 80, 100, 120, 140 and 160 kg/ha). Fertilisation increased significantly the number of pods and decreased the disease scores of the most severe plant disease Alternaria brassicae. There was no clear correlation between fertilisation and insect damage. The least preferred plants for M. aeneus were in plots with 140 kg/ha and with no additional nitrogen fertilisation. There was no significant difference between plots in the number of damaged pods caused by C. assimilis. The parasitation rate of M. aeneus was low whereas C. assimilis larvae were 100% parasitizised. This study showed that intermediate nitrogen fertilisation did not attract more insect pests but was favourable to parasitoids and might even reduce disease problems, such as dark pod spot.