John A. Byers
Department of Plant Protection, Swedish University of
Agricultural Sciences, S-230 53 Alnarp, Sweden
The European bark beetles, Ips typographus and Pityogenes chalcographus, attracted by synthetic or natural pheromone to Norway spruce logs, Picea abies, preferred to colonize uninfested logs compared to logs occupied by these beetles, probably as a means of avoiding intra- and interspecific competition for the two-dimensional phloem layer under the bark.
To test for effects of intra- and interspecific competition, beetles of these two species were introduced at different densities to logs of Norway spruce. Intraspecific competition was observed in each species since females produced less offspring with increasing attack densities of conspecifics. When both species were introduced simultaneously to host logs, only the smaller P. chalcographus (2-mm long, 0.1 mg fresh weight) produced increasingly fewer offspring per female as densities of the larger I. typographus (6-mm long, 10 mg) were increased. However, introduction of P. chalcographus either 6 or 3 days earlier than the larger beetle reduced reproductive output of I. typographus.
During evolution there could be a natural selection for semiochemical mechanisms allowing individuals to avoid such competition. In fact, I. typographus may avoid P. chalcographus by detecting the aggregation pheromone components of P. chalcographus, chalcogran and methyl (E,Z)-2,4-decadienoate, because these components inhibited the attraction response of I. typographus to its pheromone components (methyl butenol and cis-verbenol). However, these components of I. typographus did not reduce attraction of P. chalcographus to its pheromone. Nevertheless, verbenone, released from colonized bark of I. typographus does inhibit pheromonal response of P. chalcographus as well as that of I. typographus.
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