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
