Breeding performance of Tomicus destruens at different densities: the effect of intraspecific competition

In many bark beetles (Coleoptera: Curculionidae: Scolytinae), intraspecific competition is a limiting factor governing their population dynamics. The aim of this study was to evaluate the effect of increasing infestation densities on the breeding performance of Tomicus destruens (Wollaston), one of the main pests of Mediterranean pine forests. Six densities (ranging from 0.25 to 1.50 females dm-2) were tested under laboratory conditions on stone pine logs, assessing the mortality of eggs, larvae, and pupae, and recording the number of emerging adults. Density significantly affected the fertile tract of the maternal galleries, i.e., those containing eggs, which was shorter at densities higher than 0.75. Numbers of eggs, larvae, pupae, and adults per female decreased markedly when density increased from 0.25 to 1.5, although differences were statistically significant only at a density of 0.75 and higher. The lowest mortality was found in larvae reared at a density of 0.25 (17.7%) and the highest in pupae at 1.5 (84.8%). Increasing density affected the pre-emergence stages in different ways, with pupae and larvae suffering the highest increases in mortality (39.5 and 163.6%, respectively). The breeding performance of T. destruens (number of eggs laid reaching adulthood) progressively decreased from 17 to 4% with increasing densities, although at low infestation, the mean number of emerging adults per dm2 of bark increased with density, showing a positive trade-off between female fecundity and female density. In conclusion, breeding densities higher than 0.75 female dm-2 decreased female fecundity and elicited high mortality, mainly in larvae.