Effect of nitrogen fertilization, cultivar and species on incidence of two major pests of winter oilseed rape (Brassica napus L.): the pollen beetle (Meligethes aeneus F.) and the stem weevil (Ceutorhynchus napi Gyl.)

Abstract: Pollen beetle and stem weevil are among the most important insect pests in winter oilseed rape crops (WOSR). Moreover recent monitoring programs have shown the important development of metabolic resistance to pyrethroïds resulting in inefficient insecticide treatments. A better comprehension of the relationships between crop management and pest damage has been investigated in order to adapt new control strategies. To understand the effects of crop management, we measured population dynamics and damage caused by both pollen beetles and stem weevils on a split plot testing three nitrogen supply levels and two cultivars as main factors. Plant species effects were investigated in an experimental trial of winter oilseed rape with turnip rape borders. A cultivar with high isothiocyanate concentrations was more attractive than a cultivar with low isothiocyanate concentrations. Crop attractiveness is function of nitrogen supply and its effect on different crop variables. Indeed, stem weevil selects its host plants at stem elongation and is affected by crop height, whereas pollen beetle is sensible to dry weight. Furthermore a major role of growth stage development on host selection was found: the more advanced stages were the more colonized. These effects have not been previously reported at field level. Our results confirmed a high attractiveness of the turnip rape due to growth stage. No effect of nitrogen supply and cultivar was reported on the number of damaged buds. However, clear effects of nitrogen on stem weevil damage were recorded. Nitrogen fertilization interacts with pollen beetle damage by compensation mechanisms principally acting on seed weight. Compensation capacities are determined by time and rates of nitrogen applications and by pest population dynamics. These results bring new challenges for crop management, particularly for organic crops, by trap crop strategies and adapted nitrogen applications.