SPME collection and GC-MS analysis of volatiles emitted during the attack of male Polygraphus poligraphus (Coleoptera, Curcolionidae) on Norway spruce

Abstract

Tree mortality caused by bark beetles has increased in recent decades in both Europe and North America. In a large recent outbreak in central Sweden the bark beetle Polygraphus poligraphus was often found together with the spruce bark beetle Ips typographus in killed trees. To increase the understanding of the aggregation behavior of P. poligraphus we used solid phase microextraction (SPME) to collect volatile organic compounds (VOCs) released from single P. poligraphus males, with and without added females, colonizing Norway spruce stem sections and analyzed the sampled compounds by combined gas chromatography and mass spectrometry (GC-MS). High amounts of terpinen-4-ol, a substance found in the hindguts of P. poligraphus males in earlier studies, were released by colonizing males. The emission of both enantiomers of terpinen-4-ol was monitored by GC-MS over time as the males aged in the absence and presence of females. Single males emitted (R)-(-)-terpinen-4-ol for up to 60 days in high enantiomeric purity but the enantiomeric excess (ee) varied between males, and also for the same individual, over time from 96.3% to 99.3% ee. In the presence of females, males also emitted terpinen-4-ol for up to 50 days but now in lower amounts and with lower enantiomeric purity varying from 67.7% ee to 99.3% ee. Small quantities of other volatile compounds were emitted from the colonizing beetles including cis- and trans-4-thujanol, both of which were previously shown to be present in the hindguts of males. In earlier studies frontalin was found to attract P. poligraphus, but in our study it was not identified among emitted compounds from colonizing beetles.

Optimizing the attractiveness of pheromone baits used for trapping the four‐eyed spruce bark beetle Polygraphus poligraphus

“…Other authors have used enantiomerically enriched (−)‐terpinen‐4‐ol (52% ee) (Kohnle et al, ), but it is difficult to say if it was attractive on its own since no control traps were used. However, it was suggested that chirality is important since males produce (−)‐terpinen‐4‐ol of high enantiomeric purity (Rahmani et al, ) and since racemic terpinen‐4‐ol does not attract beetles in the field. Male‐infested bark, on the other hand, has been shown to be very attractive (Kohnle et al, ).…”

“…(−)‐Terpinen‐4‐ol of 90% ee was not tested in 2016, but the results from 2015 indicate that it catches nearly as many insects as 99% ee. Since it has been shown that males produce enantiomerically pure (−)‐terpinen‐4‐ol and that the purity decreases after they have mated (Rahmani et al, ), an interpretation could be that (+)‐terpinen‐4‐ol is a signal which tells the beetles to stay away, that is a repellent. The EAG measurements performed with (+)‐terpinen‐4‐ol on male and female P. poligraphus antennae also show activity of this compound on the antennae and, taken together with our field results, the (+)‐enantiomer seems to act as a repellent on this species.…”

“…(+)‐ trans ‐Sabinene hydrate did not significantly affect the catches of P. poligraphus in our field study (Table and Figure e–f). Other stereoisomers of sabinene hydrate would have been interesting to test in the field as it seems that the beetles may produce both cis ‐ and trans ‐sabinene hydrate (Kohnle et al, ; Rahmani et al, ). A previous study has indicated that none of the stereoisomers of sabinene hydrate are attractive on P. poligraphus (Kohnle et al, ).…”

“…They were kept in a cold‐storage room at 8°C for 1–2 weeks before they were used for the experiments. The compounds selected for EAG analyses were chosen as they were either compounds found in solid phase micro extraction (SPME) samples from boring P. poligraphus or compounds found in the spruce tree background (Rahmani et al, ). 3‐Methyl‐3‐buten‐1‐ol was tested as it is the pheromone of a related species, P. rufipennis (Bowers et al, ), and because it was used in our preliminary experiments.…”

“…It is possible that such effects could also exist in P. poligraphus . Several compounds which are emitted from spruce trees when P. poligraphus is boring through the bark have been identified but so far not tested for attraction on the beetles (Rahmani et al, ; Wajs, Pranovich, Reunanen, Willfor, & Holmbom, ; R. Rahmani and L. Viklund, unpublished results). It has also been shown that bark beetles of the same genus often have overlapping pheromone components (Blomquist et al, ), and therefore, 3‐methyl‐3‐buten‐1‐ol, the pheromone of Polygraphus rufipennis (Bowers, Gries, Borden, & Pierce, ), was considered interesting for our studies.…”