Efficacy of tree defense physiology varies with bark beetle population density: a basis for positive feedback in eruptive species

Abstract

We evaluated the ability of constitutive and inducible defenses to protect trees and restrict herbivore reproduction across the endemic, incipient (i.e., transitory), and eruptive phases of a native bark beetle species. Host defenses were major constraints when mountain pine beetle (Dendroctonus ponderosae Hopkins) populations were low, but inconsequential after stand-level densities surpassed a critical threshold. We annually examined all lodgepole pines (Pinus contorta Douglas var. latifolia) in six 12–18 ha stands for 3–6 years for beetle attack and establishment as beetle densities progressed through various population phases. We also assayed a suite of tree physiological and chemical attributes and related them to subsequent attacks during that year. Rapidly inducible defenses appeared more important than constitutive defenses, and total monoterpenes were more important than particular constituents. Trees that exude more resin and accumulate higher monoterpene concentrations in response to simulated attack largely escaped natural attacks when populations were low. In stands where beetles had reached incipient densities, these defenses were ineffective. Larger diameter trees had more pronounced defenses than smaller diameter trees. As populations increased, beetles selected increasingly larger, more resource-rich trees, despite their better defenses. When populations were too low for cooperative attack, beetles exploited trees weakened by lower-stem insects. Behavioral plasticity allows beetles to persist at endemic levels until conditions shift, after which positive feedbacks predominate.

Insect outbreak shifts the direction of selection from fast to slow growth rates in the long-lived conifer Pinus ponderosa

“…As the MPB population increases to the epidemic stage, host defenses are more easily overwhelmed and allow beetles to behaviorally switch from weakened, stressed trees toward larger, resource-rich hosts, even if such hosts are better defended (35). This behavioral switch is consistent with the plant-vigor hypothesis, posing that more vigorous, highly nutritious hosts are preferred by insects (44).…”

“…We suggest it may relate to delayed induced defense responses (47). Induced defenses are critical during bark beetle outbreaks (35), are under genetic control (48) and, importantly, often show a trade-off with constitutive defenses (49). Therefore, higher constitutive defenses in faster growing trees and families at our site (45, 50) may be associated with lower induced defenses.…”

“…Our results point to a delayed induction effect (i.e., at age 36 y). Although the underlying mechanisms remain unclear, induced chemical defenses in pines have been shown to increase with MPB density (35), and it could be that the strong induction effect at age 36 y was a response to high MPB densities following high tree mortality and brood production at age 35 y.…”

“…We also asked: Does this change arise because of a growth-survival trade-off? Defenses against MPB are critical when beetle populations are low to moderate, but their role is thought to decrease at the epidemic stage when mass attacks can overwhelm even well-defended individuals (35). Although resistance to bark beetles is under genetic control (33,36), it is not known whether genetic effects on tree resistance change during the course of the outbreak.…”

Strong Induction of Minor Terpenes in Italian Cypress, Cupressus sempervirens, in Response to Infection by the Fungus Seiridium cardinale

“…1) were unexpected. Concentration decreases for several compounds also have been observed, however, in other pathosystems (Boone et al 2011;Davis and Hofstetter 2011), and at least one general decrease in terpene concentration also has been reported (Bonello et al 2008). At day 10, the few compounds that increased in concentration showed an abrupt increase in proportion, and they were the same compounds that were most induced throughout this study, such as α-thujene, sabinene, terpinolene, manool, diterpene 2, and diterpene 5.…”