Technical Abstract: In the 1970-80s, vapors of the common conifer tree monoterpenes, myrcene and a-pinene, were shown to serve as precursors of ipsenol, ipsdienol and cis-verbenol, aggregation pheromone components of Ips paraconfusus. A paradigm developed that Ips bark beetles utilize pre-formed monoterpene precursors that are simply hydroxylated in order to minimize biosynthetic costs. In 1990, however, we reported that amounts of ipsenol and ipsdienol produced by male I. paraconfusus feeding in five host pine species were nearly the same even though there were large differences in myrcene precursor amounts among these tree species. Thus, myrcene vapor conversion to pheromone components may be a minor pathway in nature. Subsequent research has indicated ipsenol and ipsdienol are biosynthesized from smaller precursors such as acetate and mevalonate, and this is the major biosynthetic pathway. We report concentrations of myrcene, a-pinene and other major monoterpenes in five pine hosts (Pinus ponderosa, P. lambertiana, P. jeffreyi, P. sabiniana, and P. contorta). A scheme for biosynthesis of ipsdienol and ipsenol from myrcene and possible metabolites such as ipsenone is presented. Mass spectra and quantities of ipsenone are reported and its possible role in biosynthesis of aggregation pheromone. Coevolution of bark beetles and host trees is discussed in relation to pheromone biosynthesis, host plant selection/suitability, and plant resistance.