Dietary mixing within the crown of a deciduous conifer enhances the fitness of a specialist sawfly

► Dietary mixing of different foliage types was reported for a larch-feeding sawfly. ► We tested in field and laboratory experiments two key hypotheses to explain diet mixing. ► Results supported the 'complementary diet' and 'ontogeny' hypotheses. ► This is the first example of adaptive diet mixing of foliage in a deciduous host. ► Predictable patterns of heterogeneity in conifers may select for mixing in insects.