The larvae of insect herbivores feed on plants that may vary qualitatively and/or quantitatively during their immature development. In this study we investigated the effects of both quantitative and qualitative differences on insect performance in an insect herbivore (gregarious Pieris brassicae caterpillars) that can be confronted with both variation in plant nutrition and plant availability during immature development. In the first experiment we compared larval performance until pupation of caterpillar on plants that differed in their nutritional quality. Plants (Brassica nigra) had been exposed to aphid infestation, pathogen infection or were in or outbred, factors that have previously been shown to affect nutritional quality of plants including B. nigra. These three factors had no or only marginal effects on pupal mass and development time until pupation. We continued by examining the effects of quantitative food constraints during immature development on pupal mass, which correlated strongly with adult size. Female pupal mass and fecundity were negatively correlated with the duration of starvation during larval development. Starvation of the male mating partner had no significant effect on female reproduction. The results showed that caterpillars of P. brassicae were more affected by quantitative than by qualitative constraints in terms of pupal mass, which strongly correlated with female reproduction.