Why does longleaf pine have low susceptibility to southern pine beetle?

Pine forests throughout the world are subject to disturbance from tree-killing bark beetles, but pine species differ in their susceptibilities. In the southeastern United States, Pinus palustris Mill. suffers far less mortality from the southern pine beetle, Dendroctonus frontalis Zimmermann, than do its sympatric congeners. We tested the commonly invoked hypothesis that P. palustris has relatively low susceptibility because it has higher oleoresin flow than other pines, especially Pinus taeda L. However, seven studies in three states over 6 years refuted the hypothesis that P. palustris and P. taeda differ in their constitutive resin flow or in their capacity to replace resin depleted by either experimental wounding or natural beetle attacks. Additionally, surveys of natural beetle attacks revealed that P. taeda and P. palustris were equally likely to be attacked and killed when they cooccurred in front of growing infestations. Thus, the relative susceptibility of these two species changes with the spatial scale at which they are mixed, and the strong landscape-scale pattern of low mortality in P. palustris is not because individual trees are physiologically less susceptible. Ultimately, the conspicuous differential impact of D. frontalis on P. taeda and P. palustris may be the product of coevolution between tree defenses and beetle behavior.