Long-term effects of defoliation on red pine suitability to insects feeding on diverse plant tissues

Abstract

Evidence that defoliation can induce long-term responses in perennial plants that can regulate insect population dynamics is based largely on studies using deciduous trees and folivores, particularly Lepidoptera. Studies with evergreen trees, and with insects feeding on other plant parts, have yielded more variable results. This study examined the effects over several seasons of controlled defoliation on the suitability of an evergreen conifer, red pine (Pinus resinosa) to insects and pathogens that exploit foliage, stem phloem, and root tissue. Test insects included a folivorous sawfly (Hymenoptera) and two species of stem- and root-colonizing beetles (Coleoptera). Each of these species undergoes periodic population irruptions.

Controlled defoliations were administered to 10-yr-old trees at levels that mimic naturally occurring sawfly outbreaks. Additional studies were conducted on seedlings in a glasshouse. Foliar suitability to the redheaded pine sawfly (Neodiprion lecontei) varied in a nonlinear fashion with defoliation intensity one year after treatment.

However, not all parameters of sawfly success were equally affected. Female cocoon mass was related to foliar concentrations of nutrients, but not monoterpenes or diterpene acids. Suitability to the stem boring pine engraver (Ips pini) increased 2 yr after defoliation. Moderate defoliation reduced stem resin flow rate by 50% and increased stem phloem colonization rate by the beetle’s mutualistic fungus, Ophiostoma ips. Defoliation also affected host suitability to adult pales weevils, Hylobius pales. Weevil feeding increased 1 yr after defoliation, but this response dissipated 2 yr after treatment.

The observation that intermediate defoliation stress can either decrease or increase foliar suitability in an evergreen conifer, depending on intensity, is consistent with elements of both the plant stress and carbon:nutrient balance hypotheses. Moreover, host responses to a single stress agent may significantly and differentially influence the population dynamics of insects and pathogens that exploit different plant tissues. Implications of these results for plant stress and carbon:nutrient theories, plant-herbivore interactions, and chronic forest declines are discussed.