Chemical ecology of bark beetles Dendroctonus brevicomis and Ips paraconfusus in California (1973-1987)

In the 1970s it was thought that pheromone components, ipsenol and ipsdienol, of Ips bark beetles were biosynthesized from myrcene, a host-tree monoterpene, since only males exposed to increasing concentrations of myrcene vapor produced increasing amounts of these components. Deuterium labeling demonstrated the unequivocal conversion of a host plant chemical, myrcene, to an insect pheromone. Mated males had a decreased ability to convert myrcene to pheromone components. However, by 1981 the myrcene vapor in male nuptial chambers was found to be too low to account for the large pheromone amounts in feeding males, indicating the importance of feeding in obtaining precursor. However, a major de novo route of ipsenol/ipsdienol synthesis was suggested in 1990 when production of these components was similar in males feeding in five host pine species, even though Pinus sabiniana had no detectable myrcene precursor. The conversion of alpha-pinene vapor to another pheromone component, cis-verbenol, does account for the levels found. Ingestion of diet containing streptomycin inhibited conversion of myrcene to ipsenol and ipsdienol in male Ips paraconfusus while synthesis of cis-verbenol from (-)-alpha-pinene and other metabolites was not inhibited by the antibiotic. Natural and synthetic pheromones of Dendroctonus brevicomis and I. paraconfusus inhibited aggregation of both species, presumably the behaviors evolved in each species for avoiding interspecific competition. Production of various pheromone components by male and female D. brevicomis during the sequence of colonization was correlated with orientation behaviors to explain regulation of colonization densities and avoidance of competition and then termination of attraction and attacks.