Mating and fitness consequences of breeding aggregations in pine engraver bark beetles, Ips pini (Coleoptera: Scolytidae)

The function of breeding aggregations in non-aggressive bark beetles (Scolytidae) is unknown. I examined whether breeding aggregations of Ips pini were due to increased mate attraction, at least for some individuals. Mate acquisition decreased with increasing aggregation density for both large and small males. However, pioneer males, both small and large, had slower mate attraction than males in aggregations. This apparent benefit of joining aggregations rather than pioneering disappeared when offspring competition was considered. A biologically explicit simulation model of offspring competition determined that pioneers produced more offspring than joiners, except when pioneers were small and had limited time for mate attraction. Thus, I reject the hypothesis that Ips pini form aggregations for the purposes of maze attraction. Male settlement patterns among aggregations were not according to the Ideal Free Distribution, indicating that aggregations were not forming due to limited habitat. I discuss alternative explanations for aggregation formation.