Intra-tree variation in foliage quality drives the adaptive sex-biased foraging behaviors of a specialist herbivore

Abstract

Some herbivorous insects enhance their fitness using foraging strategies that allow them to find and colonize the best of available resources within heterogeneous plants. The yellowheaded spruce sawfly, Pikonema alaskensis (Roh.) (Hymenoptera: Tenthredinidae), is a common defoliator that oviposits and feeds on the developing foliage of young open-grown black spruce [Picea mariana (Mills) B.S.P.]. While female and male eggs are both laid throughout the crown, most eggs laid in the upper crown are female, and more female than male late-instar larvae disperse acropetally, from the lower and mid-crown to the upper crown, to complete juvenile development. Here we present results from 4 years of manipulative sleeve-cage experiments that were carried out to evaluate the hypothesis that sex-biased oviposition-site selection and acropetal dispersal by P. alaskensis are adaptive responses to intra-tree variation in foliage quality. Survival and proportion of survivors that were female were either the same (2 years) or significantly higher (2 years) for groups placed in the lower or mid-crown during early instars, and then transferred acropetally during late instars to complete development in the upper crown, compared with those forced to feed exclusively in the lower crown. This suggests that females benefited most from acropetal dispersal. Sex ratios of survivors that had been forced to develop exclusively in the upper crown were usually more female biased than those of survivors that developed exclusively in the lower crown, suggesting higher survival for female than for male larvae emerging from eggs laid in the upper crown. Sex-biased egg allocation and larval dispersal appear to increase the survival of P. alaskensis by accounting for differential effects on male versus female larvae of phenology-independent temporal and spatial variations in the quality of foliage within the heterogeneous crown of black spruce.