Adaptation to Host Plants in the Fall Cankerworm (Alsophila pometaria) and Its Bearing on the Evolution of Host Affiliation in Phytophagous Insects

Abstract

Certain parthenogenetic genotypes of the foliage-feeding moth Alsophila pometaria are associated with stands of red maple (Acer rubrum), and others with oak (Quercus spp.). A maple-associated genotype and an oak-associated genotype were compared with respect to their behavioral responses to maple and oak, and their efficiency of conversion of ingested and digested food. Dispersal from maple foliage, used as an assay of host preference, was greater for the oak-associated genotype; the response to oak did not differ between genotypes. Efficiency of utilization of consumed maple did not differ between genotypes; the maple-associated genotype displayed higher efficiency an oak than did the oak-associated genotype. This study illustrates an instance in which incipient divergence in host utilization occurs via differences in behavior rather than physiological adaptation to the host plant. If, as in this case, incipient divergence in host utilization commonly entails genetic differences in behavior but not in physiological adaptation, an important condition for sympatric speciation may hold infrequently for phytophagous insects. In such cases, moreover, the evolution of host specificity would not be attributable to a greater efficiency of specialized than generalized genotypes in detoxifying host-specific plant allelochemicals. Specialized physiological adaptations to the host in host-specific insects would then be the consequence, rather than the cause, of host specificity.