Selective pressures exerted on the feeding behaviour of animals have been extensively studied to understand their foraging patterns. In herbivores, specific within-plant patterns of resource exploitation have been reported, but their determinants remain poorly understood. Here, we describe and decipher the determinants of the foraging pattern of the pollen beetle Brassicogethes aeneus, a pollinivorous insect that is a pest of oilseed rape Brassica napus. This insect feeds from flowers for almost all of its life cycle, except for a couple of weeks preceding blossoming. During this period, only flower buds are available and the insect destroys them to feed from the pollen they contain, causing serious yield losses. We found that during this critical period, pollen beetles exhibit a stereotypic intra-inflorescence feeding pattern that depends on flower bud maturity. To explain this pattern, we first deciphered the selective pressures driving pollen beetles' feeding behaviour. Using a set of manipulative laboratory experiments, including behavioural experiments on plant tissues and artificial substrates, chemical characterization of plant tissues and performance experiments, we show that the pollen beetles' feeding behaviour does not seem to be driven by specialized metabolites or an attempt to reach an optimal nutrient balance, but rather by a process of maximization of total macronutrient intake. Next, using optimal diet choice models, we found that one aspect of the intra-inflorescence feeding pattern, the preference for young over old flower buds, could be well-explained through the lens of total macronutrient intake maximization per unit of time to access the resource. Our study provides new insights into small-scale foraging patterns and highlights the need to characterize and assess the relative influence of several components of diet quality when deciphering selective pressures driving foraging patterns. Read the free Plain Language Summary for this article on the Journal blog.