Timing of arrival at a resource often determines an individual’s reproductive success. Tree-killing bark beetles aggregate by producing aggregation and antiaggregation pheromones to overcome defenses of the host tree and successfully reproduce. Beetles that arrive early at a tree have to contend with its defenses, and produce enough pheromones to attract more beetles, but have a head start on gallery construction and egg-laying. Beetles that arrive late may be impeded by competition and diminishing resources, but may save costs associated with pheromone production and countering tree defenses. We investigated the effects of timing of arrival on pheromone production and fitness in the southern pine beetle, Dendroctonus frontalis. In field experiments, we captured beetles that arrived early (pioneers) and late (scroungers) on slash pine trees, Pinus elliottii, and measured pheromone amounts in their hindguts. We marked gallery entrances of beetles as they landed on a tree and measured their reproductive success after attack terminated. We found no difference in pheromone amounts between pioneers and scroungers. Most beetles arrived at the middle of the attack sequence, and excavated longer galleries per day than early arrivers. The number of offspring produced per day by beetles that came in at the middle was higher than those that came in early or very late in the sequence. Our results suggest that beetles do not exhibit adaptive phenotypic plasticity with respect to quantitative variation in pheromone production, depending on the condition of the tree under attack. It appears that stabilising selection favours beetles that attack in the middle of the sequence, and contributes to attack synchrony on trees that is imperative before population booms characteristic of tree-killing bark beetles can occur in nature.

Keywords: Southern pine beetle, Cooperation, Timing of arrival, Stabilising selection, Attack synchrony