Western larch resistance to Douglas-fir beetle attack

Abstract

Live western larch, Larix occidentalis Nutt., a tree species resistant to the Douglas-fir beetle, Dendroctonus pseudotsugae Hopkins, produces the monoterpene 3-carene in higher concentrations compared to Douglas-fir, the preferred host of D. pseudotsugae (Reed et al. 1986). The inhibitory effects on attraction to aggregation pheromones and toxicity of 3-carene to D. pseudotsugae were investigated. Inhibition of attraction to aggregation pheromones was demonstrated in natural Douglas-fir beetle populations using baited funnel traps. Toxicity to individual Douglas-fir beetles was confirmed in laboratory bioassays. Douglas-fir beetle aggregation pheromones were used to stimulate attack on mature, live western larch and Douglas-fir, as well as mature, felled western larch and Douglas-fir. Trap catches and bark samples revealed the relative resistance of live western larch to Douglas-fir beetle attack. The monoterpene 3-carene may extend the beetle colonization phase, allowing tree defenses to more effectively wall off associated fungi as well as stop brood development and feeding. This monoterpene may be a factor in the resistance of live western larch to Douglas-fir beetle attack. The impairment of olfactory perception and the prevention of aggregation pheromone production using non-host tree volatiles are potential bark beetle management tactics.