Effects of density on the reproductive success of pine engravers: is aggregation in dead trees beneficial?

1. The pine engraver bark beetle Ips pini (Say) (Coleoptera: Scolytidae), aggregates primarily on dead or dying pine trees. In this study pine engravers were laboratory‐reared on logs at a range of low densities to determine whether there was a fecundity advantage of breeding aggregations. 2. Mean reproductive success for both males and females declined exponentially with increasing density. 3. Female pine engravers had shorter egg galleries at higher densities, suggesting that they left high‐density breeding sites earlier. This would reduce the number of eggs that failed to survive due to larval competition. 4. Some pine engravers colonized the logs voluntarily during the experiment. These volunteers settled independently of the original density. 5. The fungus Ophiostoma sp. was present on the logs and may be competing with the pine engravers for limited bark area. 6. Aggregation resulted in a considerable cost to pine engraver reproductive success even at low densities. Thus, it remains perplexing why pine engravers aggregate actively in nature.