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

Dietary mixing is an important adaptation used by many animals to account for nutritional heterogeneity in their environment, but it remains largely unstudied for specialist insects within crowns of deciduous hosts. We carried out field and laboratory studies to explain dietary mixing by larvae of the larch sawfly, Pristiphora erichsonii (Hartig) (Hymenoptera: Tenthredinidae) of dimorphic foliage types (i.e. long shoots versus fascicles) within a deciduous conifer, Larix kaempferi (Lamb.) Carr. First-instar larvae fed mainly on long shoots, and late instars fed mainly on fascicles; however, during all instars, some individuals could be observed feeding on long shoots or fascicles. A mixed diet yielded higher overall survival due to benefits accrued during both early and late instars. Late instars had similar prepupal mass and developmental rate when provided a mixed or fascicle-only diet, but they suffered significant reductions in both performance parameters when forced to feed exclusively on long shoots. Overall fitness of females was 15–22% higher for larvae provided a mixed diet than for larvae provided one foliage type. Our results support the ‘ontogeny’ hypothesis, which attributes dietary mixing to changes in nutritional needs of larvae over the course of development, as well as the ‘complementary diet’ hypothesis, which contends that mixing of different foliage types is associated with nutrient balancing or toxin dilution. Our results suggest that architectural patterns of growth and nutritional quality within crowns of deciduous conifers exert selective pressure similar to that of evergreens on insect herbivores to diet-mix during juvenile development.