The adaptive value of autumn colours is still a puzzle for evolutionary biology. It has been suggested that autumn colours are a warning signal to insects that use the trees as a host. I show that aphids (Dysaphis plantaginea) avoid apple trees (Malus pumila) with red leaves in autumn and that their fitness in spring is lower on these trees, which suggests that red leaves are an honest signal of the quality of the tree as a host. Autumn colours are common in wild populations but not among cultivated apple varieties, which are no longer under natural selection against insects. I show that autumn colours remain only in the varieties that are very susceptible to the effects of a common insect-borne disease, fire blight, and therefore are more in need of avoiding insects. Moreover, varieties with red leaves have smaller fruits, which shows that they have been under less effective artificial selection. This suggests a possible trade off between fruit size, leaf colour and resistance to parasites. These results are consistent with the hypothesis that autumn colours are a warning signal to insects, but not with other hypotheses.