Identification of Components of the Aggregation Pheromone of the Guam Strain of Coconut Rhinoceros Beetle, Oryctes rhinoceros, and Determination of Stereochemistry

Abstract

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.

Potential of Omics to Control Diseases and Pests in the Coconut Tree

“…Purification of this pheromone showed that only males produce the "R" enantiomer, the active compound, while females produce the "S" enantiomer. The pure ethyl (R)-4-methyloctanoate compound was evaluated in the Solomon Islands and it was a success as a strong attractant for males and females of rhinoceros beetles [57]. This result is hopeful and opens the possibility of using this metabolomic strategy to search for biological control methods against other coconut pests.…”

“…Interesting metabolomic studies exist that focus on volatile molecules which are powerful insect attractants. Regarding coconut pests, the aggregation pheromones of the O. rhinoceros beetle haplotype G, ethyl 4-methyloctanoate and 4-methyloctanoic acid were isolated in a metabolomics investigation [57]. Purification of this pheromone showed that only males produce the "R" enantiomer, the active compound, while females produce the "S" enantiomer.…”

Evaluation of packaging design for pheromone product to control Oryctes rhinoceros in oil palm plantation

“…Ethyl 4-methyl octanoate is the aggregate pheromone of O. rhinoceros and has been produced commercially [10]. The application of aggregated pheromones has been reported to trap beetles en masse in areas with very high beetle population density [9].…”