Interacting Selective Pressures in Conifer-Bark Beetle Systems: A Basis for Reciprocal Adaptations?

Conifer-bark beetle interactions provide a useful model system for evaluating potentially reciprocal selective pressures between plants and insects. The phloem-feeding bark beetles that infest living conifer stems are a major source of host mortality, and their successful reproduction is usually contingent on the death of the tree. Trees respond to invasion by producing a series of localized secretions and biochemical alterations that can contain the insect and associated microorganisms. We describe the relative advantages and disadvantages of two beetle reproductive strategies: overwhelming trees with a synchronized mass attack; and selecting weakened trees that cannot offer strong resistance. Differences in the defensive physiology of grand fir, Abies grandis (Douglas) Lindley, and lodgepole pine, Pinus contorta var. latifolia Engelmann, may be partially responsible for differences in the behavior of the fir engraver beetle, Scolytus ventralis LeConte, and the mountain pine beetle, Dendroctonus ponderosae Hopkins, respectively. Both tree species react to controlled inoculation with fungal symbionts of the beetle by producing monoterpenes and other defensive chemicals that may or may not be present in uninjured tissue. Grand fir, however, undergoes a more extensive series of biochemical conversions than does lodgepole pine. The relative extent of these responses appears to have resulted in different optimal behaviors by the two bark beetle species. Because they are not strongly repelled by host monoterpenes, mountain pine beetles enter trees that can be made suitable only by the arrival of additional colonizers. Fir engravers are repelled by cues associated with strong defensive capacity and orient solely to weakened trees. Their aggregation behavior is somewhat cooperative, but it also includes elements of social parasitism. Even though colonization by bark beetles usually leads to tree death, the effect on host fitness varies with age and ecological status. Because of factors affecting both resistance and substrate suitability, most trees are able to produce cones for several decades before attack. Where lodgepole pine is seral, there is probably little selection for resistance once late-successional species have grown into the understory. We examine the theory that a synchronous decline in resistance capacity among old trees in even-aged stands increases the likelihood of beetle epidemics and subsequent fires, thereby favoring reestablishment of lodgepole pine. Later-successional species, like grand fir, are apparently not favored by beetle outbreaks, and all age groups sampled seem physiologically capable of defending themselves against attack. We discuss the interactions between beetle and conifer traits and whether these can affect reproductive rates. We propose a model in which several feedback processes operating among groups of traits, rather than strictly deterministic pathways or single biochemical causes, structure the plant-insect interaction. Finally, we discuss the general implications to coevolutionary theory.