Te Papa curator Colin Miskelly has recently returned from volunteering for the Department of Conservation on the Chatham Islands. His inordinate fondness for weevils led to some new discoveries about one of Rangatira Island’s more cryptic yet spectacular inhabitants.
The coxella weevil Hadramphus spinipennis was one of the 50 or so Chatham Island insects discovered by Thomas Hall, an itinerant shepherd and rabbiter who sacrificed his life on the battlefields of western Europe during the first world war. For a few years around 1906-08, Hall worked for Richard Paynter on Pitt Island in the Chatham Islands, where he collected many insects then unknown to science.
His collection ended up in the hands of the entomologist Major Thomas Broun, a prolific describer of new beetle species. Among Hall’s haul was a large flightless weevil (about 20 mm long) that he found only on a large feathery-looking herb, then known as Coxella (also known as soft speargrass, Aciphylla dieffenbachii).
Coxella is highly vulnerable to browsing from farmstock, and as a result became confined to inaccessible cliffs and outlying islets of the Chatham Islands. And on the two main islands of the group, predation by introduced rats and mice soon extirpated the coxella weevil, resulting in its current highly restricted range.
Working on weevils
By the time naturalists started paying attention to the distribution and ecology of the coxella weevil, it was found only on Mangere and Rangatira Islands, both now protected nature reserves off the coast of Pitt Island. The weevil was found to be entirely dependent on coxella, with the larvae feeding on the large tap root, and the adults emerging at night to browse on the stems, leaves, and flower heads.
The plants and weevils were easily found and studied on Mangere Island, where large patches of big healthy coxella plants (up to a metre high) are common, including near the Department of Conservation hut. But on Rangatira the few coxella plants are short and stunted, and are mainly found on the opposite side of the island to the DOC hut. Mangere Island therefor became the main study site for coxella weevil ecology, including a thesis study by Katrin Schöps in the 1990s.
Mangere and Rangatira Islands hold the entire world population of black robins, a critically endangered bird that was reduced to only five individuals in 1979. An intensive recovery programme has built up their numbers to a current population of about 280 adults. The black robin recovery programme plus the concurrent Chatham petrel recovery programme have meant that scores of naturalists have spent long periods of time on Rangatira.
From 1970 on, field workers started reporting seeing occasional large Hadramphus weevils on the trunks and stems of hoho trees and saplings on Rangatira, many hundreds of metres from known patches of coxella speargrass.
Barking up the wrong tree
Hoho is the Chatham Island lancewood (Pseudopanax chathamicus), and grows into a canopy tree with a trunk up to 40 cm in diameter. It is a minor component of the forest in the interior of Rangatira Island, whereas the speargrass patches are on the coastal fringe. What were these large flightless weevils doing on the wrong species and type of plant in the wrong habitat so far from their known host plants?
Initial suggestions were that adult weevils were searching for new patches of speargrass, and were making do with hoho as an alternative food source on their wanderings. It was assumed that they were climbing up into the forest canopy to browse on the hoho leaves. Katrin Schöps even ran a food-choice experiment, providing 24 captive weevils from Mangere Island with the foliage of both plant species, and seeing which species they preferred to eat.
Unsurprisingly, the weevils preferred the speargrass – which was the plant species they were collected from (the weevils are harder to find on hoho) – and the experiment design was also based on the assumption that the weevils on hoho were seeking the foliage to eat.
I found a few Hadramphus weevils on the stems of hoho saplings during a fortnight-long stay on Rangatira Island in 2001, noting one as browsing on hoho leaves, and another as feeding on the 1 cm thick stem of a hoho sapling.
For the last few years, DOC staff have been monitoring these weevils on a large hoho tree near the hut on Rangatira, assuming that they were on their way up to the canopy to feed on foliage. But while photographing one at night in late March, I noticed that it had gnawed a small dent in the bark of the tree, exposing the yellow-orange cambium underneath. This weevil was feeding on wood or sap, not on leaves.
On the trail of cryptic weevils
A quick search of the trunk and two adjacent smaller trees revealed dozens of these little yellow scars. The weevils had been nibbling the bark all along, but we hadn’t seen the wood for the trees.
Armed with this new information on what Hadramphus weevil feeding sign on hoho looks like, I looked for other trees with the same feeding sign. Our Chatham petrel work took us through forested areas up to 2 km distant from the hut, and I soon had a list of target hoho trees to revisit at night, to see if they had weevils gnawing on their trunks.
We were running out of days on the island, and bad weather was forecast (the weevils are usually only active on fine nights), but on the night of 27 March I returned to the distant scarred trees noted earlier that day, and found Hadramphus weevils on two of them in addition to the tree near the hut.
We now know that Hadramphus weevils are living on and feeding on hoho at sites scattered all over the island. It is almost certain that these animals are using the plant to complete their entire life-cycle (i.e. larval stages as well as adults). But we don’t yet know what these hoho weevils are. They could be a cryptic new species, which look like the coxella weevils but diverged from them eons ago. Or they could be a separate ecomorph of the same species, which have recently learned to live on a different host plant, but have not yet diverged enough to be considered a separate species.
Recognition of the characteristic feeding scars left by weevils feeding on hoho provides a rapid survey method for these secretive beetles, and also a way to find enough animals to include in a study of their genetic and morphological diversity compared to coxella weevils.
With many thanks to the Department of Conservation for the opportunity to assist with Chatham threatened bird recovery programmes and to visit Rangatira Island Nature Reserve.