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Identification of components of the aggregation pheromone of the Guam strain of coconut rhinoceros beetle, Oryctes rhinoceros, and determination of stereochemistry
The coconut rhinoceros beetle, Oryctes rhinoceros (Linnaeus 1758) (Coleoptera: Scarabaeidae: Dynastinae) (CRB), is endemic to tropical Asia where it damages both coconut and oil palm. A new invasion by CRB occurred on Guam in 2007 and eradication attempts failed using commonly applied Oryctes rhinoceros nudivirus (OrNV) isolates. This and subsequent invasive outbreaks were found to have been caused by a previously unrecognized haplotype, CRB-G, which appeared to be tolerant to OrNV. The male-produced aggregation pheromone of the endemic, susceptible strain of O. rhinoceros (CRB-S) was previously identified as ethyl 4-methyloctanoate. Following reports from growers that commercial lures containing this compound were not attractive to CRB-G, the aim of this work was to identify the pheromone of CRB-G. Initial collections of volatiles from virgin male and female CRB-G adults from the Solomon Islands failed to show any male- or female-specific compounds as candidate pheromone components. Only after five months were significant quantities of ethyl 4-methyloctanoate and 4-methyloctanoic acid produced by males but not by females. No other male-specific compounds could be detected, in particular methyl 4-methyloctanoate, 4-methyl-1-octanol, or 4-methyl-1-octyl acetate, compounds identified in volatiles from some other species of Oryctes. Ethyl 4-methyloctanoate elicited a strong electroantennogram response from both male and female CRB-G, but these other compounds, including 4-methyloctanoic acid, did not. The enantiomers of ethyl 4-methyloctanoate and 4-methyloctanoic acid were conveniently prepared by enzymatic resolution of the commercially-available acid, and the enantiomers of the acid, but not the ester, could be separated by gas chromatography on an enantioselective cyclodextrin phase. Using this approach, both ethyl 4-methyloctanoate and 4-methyloctanoic acid produced by male CRB-G were shown to be exclusively the (R)-enantiomers whereas previous reports had suggested male O. rhinoceros produced the (S)-enantiomers. However, re-examination of the ester and acid produced by male CRB-S from Papua New Guinea showed that these were also the (R)-enantiomers. In field trapping experiments carried out in the Solomon Islands, both racemic and ethyl (R)-4-methyloctanoate were highly attractive to both male and female CRB-G beetles. The (S)-enantiomer and the corresponding acids were only weakly attractive. The addition of racemic 4-methyloctanoic acid to ethyl 4-methyloctanoate did significantly increase attractiveness, but the addition of (R)- or (S)-4-methyloctanoic acid to the corresponding ethyl esters did not. Possible reasons for the difference in assignment of configuration of the components of the CRB pheromone are discussed along with the practical implications of these results
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Identification of the aggregation pheromone of the invasive guam strain of coconut rhinoceros beetle, Oryctes rhinoceros, and determination of stereochemistry
The coconut rhinoceros beetle, Oryctes rhinoceros (CRB), is a major pest of coconut and oil palm that has been effectively controlled by the Oryctes rhinoceros nudivirus for over 30 years. A new haplotype, CRB-G, that is not controlled by the virus appeared in Guam in 2007 and has since spread to other countries in the Region. There has been much research on alternative methods to control CRB-G, and there have been reports that it does not respond to the well-established aggregation pheromone of CRB. We found that the male CRB-G beetles produce ethyl 4-methyloctanoate and 4-methyloctanoic acid in 4:1 ratio, essentially as reported for CRB previously by Hallett et al. (1995). The enantiomers of these compounds were synthesised by enzymatic resolution and both the male-produced compounds were shown to be (R)-enantiomers. Hallet et al. (1995) reported that CRB produced the (S)-enantiomers on the basis of field studies, but re-examination of the pheromone produced by CRB beetles confirmed that they also produced the (R)-enantiomers. Electroantennogram (EAG) responses to natural volatile collections from Oryctes and to the synthetic compounds indicated that both male and female beetles respond to the ester but not to the acid. EAG responses were recorded to both enantiomers, but responses to the ethyl (R)-4-methyloctanoate were consistently greater than those to the (S)-enantiomer. In field testing in Papua New Guinea, ethyl (R)-4-methyloctanoate was attractive to both male and female CRB-G beetles and significantly more attractive than the (S)-enantiomer. The racemic ester was as attractive as the (R)-enantiomer, and addition of (R)-4-methyloctanoic acid gave a marginal increase in attractiveness of the lure. Thus CRB-G beetles produce the same pheromone as CRB, although the enantiomeric composition of this was previously wrongly assigned. Both male and female CRB-G are attracted by racemic ethyl 4-methyloctanoate in the field, so that the same lures can be used for monitoring and control of CRB-G as for CRB