Intra-tree variation in foliage quality drives the adaptive sex-biased foraging behaviors of a specialist herbivore |
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Authors: | Rob Johns Dan Quiring Don Ostaff Éric Bauce |
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Institution: | 1. Natural Resources Canada, Canadian Forest Service, Atlantic Forestry Centre, PO Box 4000, 1350 Regent Street, Fredericton, NB, E3B 5P7, Canada 2. Population Ecology Group, Faculty of Forestry and Environmental Management, University of New Brunswick, Fredericton, NB, E3B 6E1, Canada 3. Département des Sciences du Bois et de la Forêt, Faculté de Foresterie et de Géomatique, Centre de Recherche en Biologie Forestière, Université Laval, Sainte-Foy, QC, G1K 7P4, Canada
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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. |
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