Diversity, endemism and origins of scale insects on a tropical oceanic island: implications for management of an invasive ant.
Detailed assessment of scale insect (Hemiptera: Coccoidea) faunas on islands may help predict impacts of invasive ants (Hymenoptera: Formicidae) and inform options for their management, including biological control. Mutualism between scale insects and the invasive ant Anoplolepis gracilipes on Christmas Island, Indian Ocean, threatens the conservation of the island's endemic land crab fauna, alters rainforest structure and composition, and disrupts ecosystem processes. Diversity and endemism of the scale insect fauna were assessed through broad survey across rainforest, targeted search on endemic plant species, and inspection of ornamental and horticultural plants in settled areas. Emphasis was placed on honeydew-producing species that sustain ant supercolonies and detection of endemic scale insects that could be non-target species in a biological control programme for honeydew-producing scale insects. Origins of the fauna were inferred using scale insect databases and interception records at Ports-of-Entry for the United States and Korea. Twenty-eight scale insect species in seven families are identified for the island. Four honeydew-producing species, the lac scale Tachardina aurantiaca (Kerriidae) and three soft scale species (Coccidae), are abundant in rainforest and tended by ants. No endemic species were found. Compositionally, the scale insect fauna resembles that of many other tropical islands: almost all species are biogeographically widespread, host-plant generalists, and routinely intercepted in human-mediated dispersal pathways. The likely source bioregion is Sundaland where 27 of the 28 species on the island have been recorded and which has been the major pathway for movement of plant material to the island for over a century.