Effects of Folivory on Subcortical Plant Defenses: Can Defense Theories Predict Interguild Processes?

Abstract

The manner and extent to which physiological alterations resulting from folivory can affect the growth, survival, and resistance of trees against subsequent insects and fungi is poorly understood. Information obtained under natural conditions and with subcortically feeding insects is particularly lacking. We evaluated the effects of varying levels of natural feeding by a folivore on subsequent host growth, survival, and suitability to subcortical insects and fungi and also considered implications as to how source limitations caused by defoliation relate to current plant defense theories. The study system consisted of Choristoneura pinus pinus, the jack pine budworm, on Pinus banksiana Lamb., jack pine. We quantified a range of defoliation by C. pinus pinus in test trees and measured parameters of host defenses and colonization by bark beetles (Coleoptera: Scolytidae) and woodborers (Coleoptera: Cerambycidae). We also measured colonization and accompanying tree mortality attributable to these insects. Tree physiological parameters and insect colonization patterns were measured over a 24-mo period. Primary resin flow rates and the ability of active responses to confine the bark beetle's fungal symbiont Ophiostoma ips within the phloem, were reduced by high levels of defoliation. These patterns were further influenced by host seasonal phenology and time since defoliation. The predominant subcortical insects responding to defoliation by C. pinus pinus were Ips grandicollis and Monochamus carolinensis. Colonization by these insects together increased exponentially in relation to defoliation level, but the two species differentially exploited trees from particular defoliation levels. The loss of growth among surviving trees increased with defoliation intensity. However, growth and defensive capacity were not related, either negatively or positively, during any interval of this study. Depending on the length of time since defoliation stress, the relationship between plant defense and defoliation intensity was either parabolic, as predicted by growth differentiation theory, or inverse linear, as predicted by plant stress theory. Thus, differing models depicting how carbon allocation and overall carbon availability can influence secondary-chemical metabolism in plants may represent various stages along a temporal continuum. Our results suggest that the time since a stress is exerted is an important variant that should be incorporated into synthetic theories of plant defense. These results also suggest that integrative models of plant defense theory can be extended to describe impacts on community-level interactions.