Aggregation and its termination

Production and release of pheromone attracts other conspecific beetles of both sexes and aggregation or mass attack is underway. In Ips, males attack first and additional males as well as females arrive. Males initiate new boring holes. Females locate established males, enter their boring holes and mate. In Den-droctonus, pheromone release is modified as a tree is colonized. The female arrives first and releases exo-brevicomin and myrene, which preferentially attract males. Male arrivals release frontalin near female entrance holes and the blend attracts both males and females in equal numbers. The release of ver-benone and trans-wQxbtnol then deters further arrivals (Wood, 1982). 

Two questions about aggregation behavior appear to be paramount. One is what stimuli beetles use to orient to trees under attack, then to locate the entrance hole to gain entry and to mate. The second question concerns the mechanism of terminating or shifting attacks to different parts of the tree, or to 

It seems likely that bark beetles orient anemotactically in flight to attractive trees (see Carde, Chapter 5), though there is no experimental evidence to support this. The upwind walking response of beetles to a source of pheromone in the laboratory indicates that they can follow an odor plume to its source, at least while walking (Wood et al., 1966 see also Bell, Chapter 4). Increasing concentration of pheromone near a source may be used for final orientation. There appears to be a maximum pheromone concentration above which beetles do not continue orientation, but rather land and initiate new attacks. This explains why trees are often killed in groups around one susceptible tree that is attacked first. 

Visual and chemical stimuli may be integrated. Thus, D. brevicomis responded in much higher numbers to pheromone traps associated with a vertical silhouette similar to a standing tree trunk, than to traps without a silhouette (Tilden, 1976). Conversely, D. ponderosae, which frequently attacks fallen timber, could be caught in greater numbers on horizontal traps. 

The spacing of attacks over a log surface is fairly even, but how the males determine where to bore when they arrive at a tree is not known. The decision may be a function of pheromone concentration or simply visual distance from neighbouring frass piles. In any case, the final result is a log with attacks all over its surface which has ceased to be attractive to other beetles. 

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