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.

Ontogeny and Stand Condition Influence the Dispersal Behavior of a Defoliating Specialist Caterpillar

“…For example, the spruce bud moth (Zeiraphera Canadensis Mutuura and Freeman), another tortricid that attacks young white spruce (Picea glauca (Moench) Voss), disperses acropetally so that it may feed twice on young foliage, Þrst in the lower crown where buds tend to burst Þrst and then in the upper crown where buds burst later (Quiring 1993). Similarly, the late-instar larvae of a tenthredinid sawßy, Pikonema alaskensis (Rohwer), also disperse acropetally to feed on more nutritious shoots in the upper crown of young spruce hosts (Johns et al 2009(Johns et al , 2010. Based on the L2 overwintering and subsequent defoliation patterns of other Choristoneura Nealis andLysyk 1988, Volney 1992), we suspect that these other species may also disperse acropetally in the spring, probably also by walking up the tree trunk.…”

Evidence for Female-Biased Dispersal in the Protandrous Hermaphroditic Asian Seabass, Lates calcarifer

“…Competition among kin and inbreeding avoidance can also produce sex-biased dispersal in mammals (Handley & Perrin 2007), birds [6], [10] and some fish species [11], [12], because competition among kin diminishes the inclusive fitness. Whether avoidance of kin competition can result in sex-biased dispersal is associated with the possibility that the two sexes compete for the same resources [13]. In polygamous mating systems, the breeding success of males may be limited mainly by the number of mating partners (i.e.…”

Plant module size influences the intra‐tree distribution and abundance of a shoot‐boring sawfly in young balsam fir

“…Johns et al. (), for instance, found that higher larval survival of a free‐feeding sawfly in the upper vs. lower crown of young black spruce, Picea mariana (Mill.) BSP, was correlated with 21% lower monoterpene content in the upper crown, despite only a 10‐mm difference in average length between the two crown levels.…”

“…Although our study did not include foliar chemical analyses, several past studies have shown significant variation in insect performance and/or shoot nutritional chemistry among both crown levels and shoot types within young conifers. Johns et al (2010), for instance, found that higher larval survival of a free-feeding sawfly in the upper vs. lower crown of young black spruce, Picea mariana (Mill.) BSP, was correlated with 21% lower (Carroll & Quiring, 1994).…”

“…In our study, adult females may have been very selective of where they laid eggs, selecting intermediate-sized shoots, perhaps due to leverage constraints on oviposition (cf. Floate & DeClerck-Floate, 1993) or to confer performance benefits to progeny (Johns et al, 2009(Johns et al, , 2010. Alternatively, the adult shoot borers could have laid eggs indiscriminately, with progeny laid on intermediate-sized shoots simply surviving better than those laid on relatively small or large shoots.…”