Reproductive Isolation of Ips nitidus and I. shangrila in Mountain Forests of Western China: Responses to Chiral and Achiral Candidate Pheromone Components.

Eastern Palearctic conifers are subject to frequent bark beetle outbreaks. However, neither the species responsible nor the semiochemicals guiding these attacks are well understood. Two high-mountain Ips species on Qinghai spruce, Picea crassifolia, I. shangrila and I. nitidus, are typical in this regard. Six synthetic candidate pheromone components that we earlier identified from hindguts of unmated males of these two Ips species were tested for field activity in Qinghai province, P. R. China. For I. nitidus, racemic ipsdienol ((±)-Id) could replace the naturally-produced blend of enantiomers containing 74% (-)-(S)-Id (74:26 S:R), in attractive ternary or binary blends. In contrast, sympatric I. shangrila were attracted mainly to blends including Id of opposite chirality, 97%-(+)-(R)-Id. Of the verbenols, (-)-trans-verbenol was inactive for I. nitidus or inhibitory for I. shangrila, but (-)-cis-verbenol (cV) was a key component of the pheromone in both species. Two fully factorial experiments demonstrated that (±)-Id, cV, and 2-methyl-3-buten-2-ol (MB) are components of the aggregation pheromone of I. nitidus, whereas only (+)-Id and cV are essential components of the aggregation pheromone of I. shangrila. While MB is not necessary for attraction of I. shangrila, it is an active antagonist and likely functions in species isolation. A review of the pheromone production and responses in Palearctic Ips and Pseudoips showed that cV is more common than methylbutenols, and both elicit qualitatively variable responses. Ipsdienol is the most common component with variable chirality, and is a necessary, but often not sufficient, factor for determining pheromone specificity.