Causes and consequences of phenotypic variation in pheromone use of the pine engraver beetle

Bark beetles employ a pheromone communication system for location of host material and attraction of mates. In the pine engraver beetle, there is high genetic variation in the blend of ipsdienol enantiomers used as pheromones. This high variance is paradoxical in a trait that is so obviously linked to fitness but could be understood if there is disruptive selection, inverse frequency-dependent selection, or spatial variation in fitness surfaces that produce a migration-selection balance. Factors that could influence beetle fitness as a function of pheromone blend include competition for phloem and mortality from predators that exploit pheromones for prey location.;I evaluated the potential for competition and predation to influence larval mortality by placing semi-permeable screen around host material to manipulate the abundance of predators and competitors. Female reproduction decreased with increasing density of conspecifics. However, mates had increased mating success at higher colonization densities, suggesting a conflict between the sexes with respect to optimal colonization density. There were additional, smaller, effects on reproductive success from woodborer larvae that compete for phloem and clerid predators.;I employed hierarchical sampling to characterize spatial variation in pheromone preferences on a scale from 100 in to 100 km. Phenotypic distributions of pheromone preference varied temporally and spatially at all spatial scales sampled, but were bounded in that pheromone blends of 25% (+)-ipsdienol were always less attractive than blends with higher concentrations of (+)-ipsdienol. The hierarchical accumulation of spatial variation argued against the role of drift or migration-selection in maintaining genetic variation across the landscape.;Measurements of mating success for males that produced different pheromone blends indicated that males with rare pheromone blends had higher fitness than males producing common blends by virtue of attracting more female mates. This disruptive selection can explain variation among males in the enantiomeric blend of ipsdienol that is produced. It is less clear why the range is bounded from about 50%-(+): 50%-(-) ipsdienol to 75%-(+): 25%-(-) ipsdienol, but it seems likely that preference of some specialist predators (histerids) for low %(+)-ipsdienol blends and others (clerids) for high ipsdienol blends produces a regional equilibrium around which populations fluctuate.