Invertebrates (1): The Rhinocerous Beetle (Oryctes monoceros)

Denis Island plays host to some very interesting species of terrestrial invertebrate and insect. During the months of November and December I encountered numerous dead adult specimens of the African rhinocerous beetle (Oryctes monoceros) – so called because of the distinctive horn borne by the male of the species.

Denis was planted from the 1850s onwards as a coconut plantation and it is likely during this time that the beetle which is a pest of coconut trees became established on the island.
Although not as devastating to plantation production as the species that occurs in the Pacific islands, damage by O. Monoceros can be quite severe. Productivity of mature palms can be reduced by adults feeding on the leaf growth points resulting in the characteristic slicing off of leaves. The real damage however is to young palms which can be killed or seriously stunted by continuous attacks. The trunks of dead trees provide the ideal breeding ground for larvae which are very large (See photos in sidebar). On Denis dead logs are often used to demarcate pathways and 10-20 larvae can frequently be found in trunk lengths of only a few feet that exhibit the right degree of decay.

The coconut industry in Seychelles was of significant economic importance in the past and as such the beetle received a lot of attention in terms of measures to control it including attempts in 1949 and 1951 to establish biocontrol in Seychelles through the introduction of the parasitic wasp Scolia ruficornis fabricius. The beetle’s abundance is also related to the abundance of other palm disease agents such as melitomma which influence the amount of dead coconut timber available for rhinoceros beetle larvae. As a consequence one of the best measures to control the beetle is good plantation husbandry.

C. J. Piggot, visiting Denis in 1960 noted:

“Rhinoceros beetle damage is severe but the insect is slowly being brought under control by continued collecting of adults and larvae. It is difficult to discover the main breeding places. There are few fallen palms and no accumulation of decaying organic matter… seems likely they live in manure holes that have been insufficiently covered with sand or soil.”

The economic significance of coconut to Seychelles and the area under plantations today is of course greatly reduced. On Denis despite the recent re-commencement of small scale copra and oil production the coconut coverage is also being reduced as part of the ongoing development of broadleaved woodland habitat for the island’s various conservation programmes such as the introduction of rare endemics like the Seychelles magpie robin and flycatcher. As such I, for one, view the beetle today as an ally in the struggle against the man-made monoculture of coconuts that provides little or no habitat to native species and prevents the re-establishment of native vegetation through the dense undercover generated by sprouting nuts.