Role of 3-carene in host location and colonization by Dendroctonus pseudotsugae

The Douglas-fir beetle has been reported to occasionally colonize western larch in addition to its primary host, Douglas-fir when both species are growing in close proximity. In this study we evaluated the toxicity and repellency of 3-carene to Douglas-fir beetle in a series of laboratory and field tests. In a laboratory bioassay, 3-carene and limonene were equally toxic to male Douglas-fir beetles and both were more toxic than myrcene and α-pinene. In a field trial, multiple funnel traps baited with frontalin and 3-carene caught only 8% as many Douglas-fir beetles as traps baited with frontalin and α-pinene. Traps baited with 3-carene in addition to frontalin and α-pinene caught only 30% as many Douglas-fir beetles as traps baited with only frontalin and α-pinene. In another field study, live western larch, felled western larch, live Douglas-fir, felled Douglas-fir, and live Douglas-fir surrounded by 3-carene releasers were baited with Douglas-fir beetle aggregation pheromones. There were significantly fewer Douglas-fir beetle colonization attempts, egg galleries excavated, eggs hatched, and live brood produced on both live western larch and live Douglas-fir surrounded by 3-carene compared to live Douglas-fir. Most egg galleries excavated in live western larch were heavily impregnated with resin and no eggs hatched. There were no significant differences in egg galleries excavated, eggs hatched, or live brood produced between felled western larch and felled Douglas-fir. Collectively, these data support the hypothesis that 3-carene slows the colonization process in live western larch allowing more time for host trees to respond to a colonization attempt.