(−)-myrtenol and (−)-α-pinene: Aggregation pheromone components of the cypress bark beetle Phloeosinus aubei

Abstract

The cypress bark beetle, Phloeosinus aubei, is of increasing concern in the European market of scale-leafed conifers. Neither the reproductive behaviour nor the chemical ecology have been studied in detail so far, and no specific attractant traps are available to monitor the flight of adults. We established that stridulation by males is crucial for acceptance by calling females. To unravel the chemical communication, hindgut extracts from unmated females were analysed by GC-EAD, using male antennae. Gut extracts and Thuja occidentalis leaf extracts were analysed by GC/MS. Two antennally active compounds from gut extracts were identified as (−)-myrtenol and (−)-α-pinene by enantioselective GC/MS. (−)-Myrtenol was shown to be female-specific, while (−)-α-pinene was found as a 2:1 mixture with the (+)-enantiomer also among the volatiles of the host tree, T. occidentalis. To prove that the production of the above compounds in the beetles is related to reproduction, we treated females with JHIII. The gut extracts from these sexually mature females showed elevated levels of (−)-α-pinene compared to untreated control groups. Synthetic enantiomers of myrtenol and α-pinene were tested separately by electroantennography, and all were found to be electrophysiologically active. The attractiveness of different mixtures of these compounds was compared using 4-arm olfactometer tests: 4:1 mixtures of racemic α-pinene with either (−)- or (+)-myrtenol were significantly attractive to males and females, respectively. In field tests, multifunnel traps baited with a 5:1 mixture of (−)-α-pinene and (−)-myrtenol caught significantly more males and females than unbaited control traps. We concluded that (−)-α-pinene and (−)-myrtenol are components of a female-produced aggregation pheromone. EAD activity and behaviour assays indicated that α-pinene from Thuja is used by the beetles as part of a host plant kairomone. The complex communication system of P. aubei is discussed in a chemoecological context.