Structure elucidation and synthesis of a putative cuticular contact pheromone of the Braconid Wasp Wroughtonia Occidentalis, a native parasitoid of the Brown Spruce Longhorn Beetle (T. Fuscum)

Abstract

The Brown Spruce Longhorn Beetle (BSLB) made its first appearance in Canada in the late 1980s at Point Pleasant Park, NS, and has since then killed several thousand spruce trees in the area. If gone unchecked, the BSLB could decimate spruce populations across North America and cause tremendous economic losses. The wasp W. occidentalis, a native parasitoid of BSLB, could potentially be of use in curbing the invasive pest’s spread, provided that the chemical ecology of this wasp can be elucidated. GC/MS and GC/EAD analysis of W. occidentalis extracts suggested that contact pheromones produced by females of the species are allenes — long chain hydrocarbons with two consecutive sites of unsaturation — which are extremely rare in insects. Furthermore, the pheromones appeared to consist of a homologous series of odd-numbered carbon units, ranging from 23 to 31 carbon atoms, with the sites of unsaturation suspected of being located at the 10, 11- and 11, 12-carbons. Racemic mixtures of the 27 and 29 carbon allenes were synthesized, each of which contained the 10, 11- and 11, 12-isomers, and subsequent GC/MS analysis showed similarity to what was observed in the W. occidentalis samples. Consequently, a synthetic route toward an optically enriched 29 carbon allene was devised and executed, but it was later discovered that these allenes (as well as the racemic versions) had no contact behavioral activity on live insects. Upon reexamination of the original GC/MS data alternative structures were postulated and 17-octacosynal and various long-chain alkynes were synthesized since alkynes are known to have an intermediate allenic state in the mass spectrometer ion source. Further spectral analyses of these compounds point to 11-nonacosyne potentially being the W. occidentalis contact pheromone.