Is bigger better? Size and pheromone production in the mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera : Scolytidae)

Reproductive success in the mountain pine beetle, Dendroctonus ponderosae Hopkins, is determined by the production of aggregation and antiaggregation pheromones, as well as body size. In a laboratory experiment with beetles that emerged from naturally attacked hosts, there was no relationship between body size and the production of aggregation pheromones in either sex. In contrast, there were significant relationships between body size and the production of antiaggregation pheromones in males that were paired with females. Pheromone amounts decreased in paired females and antiaggregation pheromones correspondingly increased in paired males, suggesting that after pairing, males take over the role of pheromone production. Although males could potentially select large females by evaluating gallery size, and females could select large males on the basis of their strength in stridulation or physical courtship, we propose that mate choice occurs primarily by olfaction. Small individuals that produce large amounts of pheromone during initial attack could sabotage mate choice based on size-related criteria. This hypothesis is consistent with a lack of evidence for size assortative mating in 92 pairs of beetles. The production of antiaggregants by large male D. ponderosae after pairing with females appears to be an important factor in intraspecific resource partitioning, population regulation, and reproductive success.