Spatial heterogeneity within an evergreen conifer promotes foliage-age dietary mixing by a specialist herbivore

Dietary mixing of different food items enhances the performance of many animals; however, few studies have tested the mechanisms influencing mixing within individual plants by herbivores. We investigated the mechanisms promoting apparent foliage-age dietary mixing by a specialist sawfly, Pikonema alaskensis (Roh.) (Hymenoptera: Tenthredinidae), within juvenile black spruce (Picea mariana (Mill.) B.S.P.). Field defoliation surveys indicated that larvae generally prefer developing foliage, and most defoliation occurs in the upper crown of trees. However, when upper-crown developing foliage becomes scarce, many larvae consume mature foliage in the upper crown rather than dispersing to lower branches, where preferred developing foliage remains abundant. Manipulative field experiments were conducted to compare the performance of early and late-instar larvae fed mixed versus individual age classes (i.e. developing or mature) of foliage in the upper and lower crown. In contrast to all past studies, foliage-age mixing by P. alaskensis was not associated with nutrient balancing or toxin dilution. Instead, mixing was associated with an expansion of feeding breadth in larvae as they developed, coupled with high levels of variation in resource quality between upper and lower crown levels, which enabled older larvae consuming mature foliage in the upper crown to achieve similar survival as those consuming developing foliage in the lower crown. Our study emphasizes the importance of considering spatial heterogeneity in studies of foliage-age preference–performance relationships for herbivores within trees and demonstrates how a herbivore can use dietary mixing to dramatically expand its resources while simultaneously accounting for heterogeneity within large, architecturally complex plants.