Chemical Ecology and Reproductive Isolation in Ambrosia Beetles

Species-specificity in signals that mediate reproductive interactions are of adaptive advantage in avoiding the cost of mismating and potential low fitness through hybridization. Phytophagous insects retain species-specificity in part by multicomponent sex pheromones synergized or otherwise enhanced by host chemicals. For four sympatric species of Trypodendron spp. (Coleoptera: Scolytidae) in British Columbia, coupled gas chromatographic-electroantennographic detection analysis of volatiles from beetles, hosts and non-hosts revealed several antennally active compounds. These were tested in field bioassays for their behavioural significance as reproductive isolating mechanisms in finding hosts, attracting conspecifics and repelling heterospecifics. The ESR and ZRR stereoisomers of tetrahydro-2,2,6-trimethy1-6-vinyl-2H-pyran-3-01 (tirathol) were identified as the aggregation pheromone of T. betulae. Responses were synergistically increased when the pheromone was combined with the birch host volatile conophthorin and ethanol. Use of tirathol isolates T. betulae semiochemically from three other sympatric Trypodendron species, which were shown to use the pheromone (+)-lineatin. Trypodendron retusum responded to a synergistic combination of lineatin, ethanol, and the aspen host volatile salicylaldehyde, with apparent phylogeographic variation between coastal and interior populations. Salicylaldehyde repelled coniferophagus T. lineatum and T. rufitarsus, while T. retusum was repelled by the conifer volatile alpha-pinene. Examination of the symbiotic fungal associations of the four native ambrosia beetles and the established exotic T. domesticum revealed overlapping combinations of fungal isolates among the genera Ophiostoma, Ambrosiella, and Ceratocystiopsis, with no evidence of co-evolution between fungi and their beetle vectors. Isolation techniques, anatomic locations, and insect life stage were crucial in determining fungal partners. The construction of a molecular phylogeny based on COI and a Wingless regions of the North American and European Trypodendron species and two other species in the tribe Xyloterini placed the European angiosperm-infesting T. signatum ancestral to all other Trypodendron spp. North American angiosperm specialists, T. retusum and T. betulae are derived from coniferous generalists. The phylogeny does not allow for conclusions to be drawn about the evolution of pheromones or fungal associates as reproductive isolating mechanisms.