Semiochemistry of Dendroctonus armandi Tsai and Li (Coleoptera: Curculionidae: Scolytinae): both female-produced aggregation pheromone and host tree kairomone are critically important

The Chinese white pine beetle, Dendroctonus armandi Tsai and Li, is one of the most destructive insects in natural and managed forests of Chinese white pine (Pinus armandii Franch.) in China. Female-produced volatiles, as aggregation pheromone candidates, and volatiles released from host pine trees, as possible kairomones, were collected via aeration and hindgut solvent extraction techniques, analyzed by gas chromatography-mass spectrometry (GC-MS), and assayed by field-trapping experiments in Shaanxi and Hubei provinces, China. GC-MS analyses showed that monoterpenes are the major volatile compounds from all aeration samples of P. armandii logs, with or without D. armandi attacks, accounting for > 95 % of total volatiles. alpha-Pinene (63.18 %), Delta 3-carene (14.4 %), beta-pinene (7.9 %), limonene (6.2 %) and beta-myrcene (3.85 %) were the major monoterpenes, while (+)-longifolene (1.1 %) was the most dominant sesquiterpene. Four bark beetle-related volatile compounds, trans-verbenol (tV; major component), exo-brevicomin (EBV), seudenol (SD), and 1-methyl-2-cyclohexen-1-ol (MCOL) were identified from both the aeration extracts of P. armandii logs with D. armandi females and their hindgut extracts. Our field bioassays showed that all four female-produced individual pheromone candidates and their possible combinations (binary, ternary and quaternary) were not significantly attractive to D. armandi. Two pheromone candidate blends [one ternary (tV/SD/EBV) and one quaternary (tV/SD/EBV/MCOL)], and a mixture of host terpene kairomone candidates were also inactive when tested alone. However, when the five major host monoterpenes and one sesquiterpene (mimicking the natural ratio) were combined with the pheromone ternary or quaternary blend, D. armandi trap catches were significantly (4-10 times) higher than captures in the blank control traps or traps baited with the individual blends, indicating a strong synergistic effect. These synergistically attractive semiochemicals are potentially of great utility for monitoring and mass-trapping this serious forest pest of Chinese white pine in western China.