Is quantity or quality of food the issue in feeding site choice in oilseed rape by the pollen beetle?

International audienceAccording to the optimal foraging theory, diet selection by herbivorous insects should result from a trade-off between food quality and food quantity. Indeed, these insects are expected to optimize their food choice by balancing nutrient intake on the one hand, while minimizing toxicity from specific secondary metabolites in their host plants on the other. While optimal foraging theory has been used to understand food foraging and selection by some generalist herbivore insects, it has seldom been applied to specialist herbivores to understand how they choose among different plant parts or tissues. Yet, within-plant variability of primary compounds such as carbohydrates and proteins can highly influence tissue nutritional quality. The same variability also exists for secondary toxic compounds. Profitability of plant tissues also depends on architecture, organ size and physical defenses. These three factors (nutritional quality, toxicity, morphology) are rarely studied together but all could condition within-plant feeding patterns. We tried to explain feeding patterns of the pollen beetle (Meligethes aeneus), a pollinivorous insect and a pest of oilseed rape (Brassica napus) in Europe. We observed a clear feeding pattern, with insects preferring young buds in the center of the inflorescence. However, this pattern seems rather counterintuitive because of the low quantity of pollen present in these non-mature buds. We used behavioral and performance experiments paired with chemical analyses of plant tissues and insects to understand how constraints such as nutritional quality, organ toxicity and accessibility of the pollen may explain such paradox