Cook Islands Insect Expedition: Black Twig Borer

The black twig borer Xylosandrus compactus (Eichhoff, 1875). Image courtesy of Ken Walker via PaDIL. Licence: CC: BY.

A timely interview on Radio New Zealand aired this week demonstrates the importance of taxonomic expertise, and the need for my research on the weevils of the Cook Islands. A Radio NZ interview with Cook Islands Minister of Agriculture, Kiriau Turepu, discusses the impact of black twig borer on agriculture in the country.

The black twig borer is the common name of Xylosandrus compactus, an ambrosia beetle belonging to the weevil subfamily Scolytinae. This species has not been previously recorded from the Cook Islands, though a related species, Xylosandrus morigerus was recorded in a 1990 paper on the Scolytinae of the Cook Islands. Scolytine beetles are tricky beasts to identify due to their size, and so distinguishing between these two species is no easy task. Could this be a case of mistaken identity? Without seeing specimens or knowing more details of who identified it, it's hard to say.

Although it is suspected that the beetle may have been introduced from New Zealand or Australia, Xylosandrus compactus is not known from either country. However, it is known from other South Pacific countries, including Hawaii, Fiji, American Samoa and New Caledonia. The timing of establishment also seems to be in doubt, which will make investigating the invasion pathway somewhat tricky. Once again, specimens would provide valuable data, if regular collections have been made.

Mention is also made of damage on mango fruits. While the black twig borer has a wide host range (upwards of 200 species of plant), I am doubtful that the damage described can be attributed to it. Xylosandrus compactus bores in twigs and cultivates fungi along the tunnel walls (That's right! They're farmers!). The damage to mangos sounds like the work of a different beetle, the mango seed weevil Sternochetus mangiferae which has been in Rarotonga since around 2000. Knowing the difference is important for effective control and mitigation of damage.

For better or worse, it sounds like the black twig borer is well established on Rarotonga now, and I look forward to collecting specimens and adding it to the list of weevils known from the island. Once published, my research will hopefully allow rapid identification of future invaders, leading to more certain recognition of invasion pathways, and fast implementation of eradication or control measures.

References:
Beaver RA, Maddison PA. 1990. The bark and ambrosia beetles of the Cook Islands and Niue (Coleoptera: Scolytidae and Platypodidae). Journal of Natural History 24: 1365–1375.

Cook Islands Insect Expedition: Funding Granted!

Satellite image of Rarotonga, Cook Islands. Image via Wikimedia Commons. Licence: Public Domain.

As the tagline of this blog suggests, I have a profound interest in the biota and people of the South Pacific. I was fortunate to spend an influential part of my childhood in the Solomon Islands, then managed to work on beetles across the South Pacific for my MSc research. Since I've moved on to studying weevils, I've been very keen to contribute to a greater knowledge of the weevil fauna of the Pacific; but until now I haven't had the financial support, nor a topic that is simultaneously ambitious enough to be useful but also achievable.

The Cook Islands caught my eye about a year or so ago, when I realised that the weevil fauna of the group was essentially unknown. What little was published usually didn't get beyond a mention of "Rarotonga" or "Cook Islands" in the Distribution section of a species account in papers that had a different geographic focus. The superb Cook Islands Biodiversity Database lists 35 species of weevil from the Islands, which included several undescribed species and unidentified taxa. Further investigation of the literature revealed that it wasn't just the weevils that were barely known from the Cooks—few collecting expeditions have been undertaken, and not many insect taxa have published data available.

Holotype of Ptilopodius aitutakii Beaver and Maddison. One of two weevils described from the Cook Islands. Image courtesy of Auckland Museum. Licence: CC: BY.

This lack of knowledge surprised me, given the Cook Islands' strategic location in the South Pacific. Being approximately halfway between Samoa and Tahiti, it's in a key position to test hypotheses of the cause of the eastward attenuation of species richness and diversity that is such a feature of the biogeography of the South Pacific. Much has also been made of the fact that several taxa find their eastward limit around Samoa and Tonga, without having data from Niue or the Cook Islands to ascertain if this eastward limit is truly the case or not. A comprehensive work on the weevils of the Cook Islands would be able to confirm or refute some of these ideas of distribution of species in the South Pacific.

The small size of the islands made them attractive too. Rarotonga, the largest and highest of the group, is still only 62 km² in area and 650 m at its highest point. Not too much time (in theory at least!) would be required to make comprehensive collection in the available habitats.

The combination of the geographic location of the Cook Islands, and their small size lets me predict that weevil fauna will be fairly small and depauparate. I'm expecting that the total number of species will be around 200 or so. This is also beneficial, as it gives me hope that I may be able to write up the results of any collecting done in a timely and efficient manner. Additionally, although I expect a lot of the species to be endemic to the Cooks, the genera they represent are likely to be widely distributed, which would result in this work giving me a good foundation for further, future work on the weevils of other Pacific archipelagos

So there were the Cook Islands: small enough to collect everywhere in the space of a few weeks, an unknown fauna with a low number of species that could be written up in a short period of time and opportunity for first-hand experience with Pacific weevil taxa. In my opinion at least, this potential project held a lot of promise.

I am honoured that the board of the New Zealand Winston Churchill Memorial Trust considered it to have a lot of merit also. The trust funds overseas travel that bring New Zealanders into contact with other cultures, learn from their experiences and contribute to the community through their research and knowledge. Over 850 fellowships have been granted since the establishment of the Trust in 1965 and includes artists, educators, economists, academics and musicians. It's an impressive list of highly talented people and it's very humbling to be included among them.

The Trust has offered funding to cover two months in the Cook Islands, collecting weevils and other insects on Rarotonga, Atiu, Mangaia and Mitiaro. I expect it to be a great adventure, and one that I hope will prove productive and useful. I plan to go in March 2017, and intend to post semi-regular updates on aspects of the preparation for the trip, the expedition as it happens, and the aftermath of specimen sorting, preparation and identification. I very grateful that the Winston Churchill Memorial Trust has provided support for this expedition, and I'm excited about being able to make a significant contribution to our knowledge of the insects of the Cook Islands.