WITHIN-POPULATION VARIATION IN RESPONSE OF RED OAK SEEDLINGS TO HERBIVORY BY GYPSY-MOTH LARVAE

The potential for an evolutionary response to gypsy moth (Lymantria dispar L.) herbivory was investigated in red oak (Quercus rubra L.), a preferred host. Seedlings of nine open-pollinated families were grown in a greenhouse and experimentally defoliated by fourth instar larvae in the summer of 1991 to assay for intraspecific variation in resistance to and recovery from herbivory. Defoliation increased mortality and reduced growth differentially among the nine families. Family differences in plant size before defoliation explained much of the variation in mortality and growth following defoliation. Different allocation patterns among families explained some of the variation in seedling growth. Absolute growth rate of larvae was significantly different among families and higher on individuals with larger leaf area, but resistance to defoliation and larval growth efficiency were not significantly different among families. Since we have not measured the additive genetic variation in seedling traits, the magnitude of the potential evolutionary response cannot be projected from this study, but intraspecific variability in red oak does exist for tolerance and response to defoliation and possibly for resource allocation. Phenotypic selection on traits related to response to defoliation may result in evolutionary change in natural populations of red oak.