Sampling pollen beetle ( Brassicogethes aeneus ) pressure in oilseed rape: which method is best?

BACKGROUND: The pollen beetle (Brassicogethes aeneus) is the most abundant pests of oilseed rape in spring and potentially one of the most damaging. Adults feed on the pollen within closed flower buds and the damage leads to bud abscission, resulting in podless stalks and yield reduction. Several methods are currently used to monitor the pressure of this insect such as counting the numbers of adults on the plants, quantifying the number of buds damaged by the insect before flowering or counting the number of podless stalks before harvest. We conducted experiments to evaluate the robustness of these sampling methods and compared their results. We also describe how pollen beetles damage the plants to understand the limitations of the methods based on an estimation of the damage. RESULTS: We found that methods based on adult abundance lack robustness. We observed that most of the damage to buds is caused by pollen beetles feeding on small buds, and that this damage can be quantified later in the season, indicating the robustness of methods based on the count of podless stalks. Different methods gave consistent results and quantification of the pressure on the primary raceme can be a good proxy for data collected by assessing the whole plant. CONCLUSIONS: Standardized methods for assessment of pollen beetle pressure will enable comparison of pest management strategies between different studies and facilitate the development of alternative control strategies for this pest.