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Prior research from trapping experiments using synthetic pheromone components suggested the existence of local and geographical variation in the pheromone system of the spruce beetle, Dendroctonus rufipennis (Kirby). To test hypotheses concerning variation in the spruce beetle's aggregation pheromone blend, we extracted volatiles from the hindguts of individual spruce beetles sampled from sites in eastern Canada (Nova Scotia and Newfoundland) and western Canada (British Columbia and Alberta) and quantified the amounts of four known aggregation pheromone components within each sample. Chiral analyses were performed on a subset of samples. Frontalin (1,5-dimethyl-6,8-dioxabicyclo[3.2.1]octane) was more dominant in western Canada, whereas MCOL (1-methyl-2-cyclohexen-1-ol) and seudenol (3-methyl-2-cyclohexen-1-ol) were more dominant in the east. Verbenene (4-methylene-6,6-dimethylbicyclo[3.1.1]hept-2-ene) was prevalent only in one of our six sites (Rocky Mountain House, AB). Female beetles in Rocky Mountain House (AB) also produced a higher proportion of (+)-frontalin than females from the other sites, and a comparison to previously published data suggests additional geographic variation that was not captured among our sites. When paired with a male in a gallery, female spruce beetles produced pheromone blends with more frontalin and less MCOL compared to solitary female beetles. Our results show that variation in the spruce beetle's pheromone blend exists at both large and small spatial scales, possibly a consequence of local selective pressures and assortative mating. [ABSTRACT FROM AUTHOR]

We examined the role of population density in host selection behavior of an eruptive insect herbivore, the spruce beetle Dendroctonus rufipennis. We conducted field and laboratory experiments on spruce beetles from 29 endemic and eruptive populations in Alaska and Utah, USA, and Yukon, Canada. Beetles from both population phases colonized trees that had been felled to remove host defenses. However, only beetles from eruptive populations colonized defended, healthy trees. A series of laboratory assays using host-based media amended with varying concentrations of phytochemicals identified several factors affecting population-dependent responses to hosts. First, beetles were repelled by high concentrations of the predominant spruce monoterpene, alpha-pinene, but intermediate concentrations elicited entry and gallery construction. Second, heritability assays suggested high genetic variance of host selection behavior within populations, and between-population differences persisted following rearing in a common environment. Third, beetles from eruptive populations were less likely to enter medium amended with phytochemicals and constructed shorter galleries, which disagreed with our prediction and seemingly contradicted the field observations. However, fourth, beetle avoidance of high concentrations of alpha-pinene decreased with the addition of other beetles, and this effect was more pronounced among beetles from eruptive populations than among those from endemic populations. This interaction broadened the host range of eruptive beetles. We propose that such density-dependent behaviors can maintain heterogeneity among population phases and contribute to positive feedback in herbivore population dynamics. A conceptual model suggests how heritable and environmentally induced variation in host selection behavior can affect bimodal equilibria and numerical thresholds in eruptive species. [ABSTRACT FROM AUTHOR]

Abstract. In a pattern that is typical for bark beetles, the lateralis medius flight muscle of male pine engravers, Ips pini Say (Coleoptera: Scolytidae), was found to decrease four-fold in volume (from mean ± SE = 1.36 ± 0.06 × 10-2 mm3 to 0.34 ± 0.06 × 10-2 mm3 ) within five days of the initiation of breeding galleries, and then to regenerate gradually to functional capacity during subsequent weeks. Although there was considerable variation in the timing and extent of flight muscle regeneration in males, this variation was not a consequence of differences between small (length < 4.0 mm) and large (length ≥ 4.0 mm) males. Two subsequent experiments revealed that male pine engravers can control the timing of flight muscle regeneration. In the first experiment, the flight muscles of males that were denied mates degenerated within 5 days of gallery initiation, but then showed complete regeneration 5 days later. In the second experiment, mated males that were removed from their breeding galleries (to simulate breeding failure) also showed extensive muscle degeneration 5 days after gallery initiation, but then regenerated their flight muscles to functional capacity by the tenth day. The ability of males to regenerate their flight muscles in response to conditions at the gallery is probably adaptive because it allows them to fly in search of new breeding opportunities when they are unable to attract mates or when breeding attempts fail. [ABSTRACT FROM AUTHOR]