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Evolutionary ecological theory predicts that among insect herbivores ‘mothers know best’ when selecting a plant to deposit their eggs. Host-plant selection is usually studied for the adult stage exclusively, although mothers have not always been reported to know best. Here, we investigate the host-plant selection behaviour of caterpillars, which are considered to be completely dependent on their mothers’ choices. We use a system that offers a biologically relevant framework to compare the degree of participation of adults and juveniles in host-plant selection. Our results show that neonate Pieris brassicae caterpillars can actively discriminate between conspecific Brassica oleracea plants with or without aphid (Brevicoryne brassicae) infestation. The caterpillars prefer aphid-infested plants on which their performance is significantly better, while their mothers, the female butterflies, did not discriminate. We compared caterpillar preferences of individuals released individually or in groups, because P. brassicae is a gregarious species. We found that the strength of the preference for aphid-infested plants was not affected by the degree of grouping. Caterpillar choices were made before contact with the plants, indicating that plant odours were used for orientation. However, the composition of the volatile blends emitted by plants with and without aphids did not show strong differences. Similarly, like with aphid-infested plants, plants treated with salicylic acid (SA) were also preferred by neonates over untreated control, indicating that the infestation by aphids may have rendered the plants more attractive to the neonates via changes related to interference with JA-signaling. The main parasitoid of the caterpillars did not discriminate between plants with hosts in the presence or absence of aphids, showing that top–down forces do not influence the relative suitability of the different food sources for the caterpillars. These data are discussed in the context of mothers and offspring having both important, but different roles in the process of host-plant selection. Butterflies may select the plant species patch, while their offspring adjust and/or update the choices of their mothers at the local scale, within the micro-habitat selected by the adult.