The beetles were recognised a sap beetles (Family: Nitidulidae), perhaps Carpophilus sp. from the dimidiatus/maculatus group of species. Another possibility is Haptoncus sp. It depends on the size.
Later, Cook Islands found a key for Nitidulidae in a book regarding stored product pests, and considered it likely that the beetle was C dimidiatus. So far there are three Carpophilus species recorded from the Cook Islands, C dimidiatus, C humeralis, and C maculatus.
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