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.