Last September a wander along the shore of Te Whanga discovered what seems to be the first recorded occurrence of Coprosma mite (Acalitus cottieri) for the Chatham Islands. The mites are microscopic but their feeding activity distorts and damages the flower buds of Coprosma species so forming conspicuous spherical galls. In the case of the new find the afflicted tree was the endemic Coprosma chathamica known on the islands as ‘karamu’. Searches of other nearby karamu and mingimingi (Coprosma propinqua var. martinii), the only other indigenous Coprosma known from the islands did not find further galls. There are also at least two other Coprosma present on the islands, C. robusta – which may or may not be indigenous, and taupata (C. repens), an aggressive introduction from New Zealand. I have not seen Coprosma mite galls on these two species on the islands, though they are attacked in New Zealand.
The mite discovery poses a quandary, as the galls they form are conspicuous it seems unlikely that they had hitherto been overlooked, so this could imply a recent New Zealand to Chatham introduction. The problem is that the tiny mites are flightless. So were they introduced accidentally by people? The find was made about two kilometres from the Chatham Airport, so perhaps they came to the island from there. For that to have happened though would require the rather fortuitous arrival of infected host material, say as Coprosma garden stock, or rather less likely, someone who happened to be carrying fresh mite infected galls elected to discard them close to or on a suitable Coprosma host.
This seems unlikely, after all I failed to find mites on any of the planted or wild Coprosma near the airport or in nearby gardens. Further the infected tree though two kilometres from the airport plantings, is possibly too far for flightless mites to selectively colonise it and yet ignore other stands of the same host growing closer to the airport. The alternative is that these mites are a new species that may be endemic to the islands. With this view in mind last November I collected specimens and forwarded these to Landcare Research for identification. Very few people in New Zealand work on mites so formal identification may take some time. In the interim, if you see galls on your karamu, mingimingi or taupata it would be useful to report the finds (and if you can take images of them) to the Department of Conservation or to me at email@example.com.