Haloragis weevil: Rhadinosomus acuminatus
By N A Martin (2016)
Biostatus and Distribution
This endemic weevil lives on Shrubby haloragis, Haloragis erecta (Haloragaceae). The larva tunnels through plant stems. The weevil is found where its host plant grows in the North and South Islands and Chatham islands.
Conservation status: Widespread, not threatened.
Life Stages and Annual Cycle
Brenda May, in her study of weevil larvae, found adult Haloragis weevil, Rhadinosomus acuminatus, all through the year on Shrubby haloragis, Haloragis erecta, plants at Huia, Auckland. In early summer they took three months to develop from egg to adults, while an overwintering generation took much longer.
Adults are typical weevils, each one with six legs, hard wing covers (elytra) and a long snout (rostrum). They are about10 mm long and much longer than wide. The weevils are dark coloured with white flecks and on each wing cover a small white patch. Their wings, which are longer than the wing covers, are kept safely folded up under the wing covers, except when needed for flying. Each wing cover ends in a distinctive sharp point. A pair of jaws, or mandibles, is at the end of the rostrum and on either side of the mouth. The antennae are also attached near the tip of the rostrum and terminal segments form a club. The antennae may be folded neatly under the head. At the base of each antenna are two grooves, one for holding the antenna when it is at rest and another for holding the antenna when it is pointing upwards and forwards.
After they emerge from the pupal chambers, adult males and females find one another and mate. Some may fly to new patches of host plants. Adults may be seen on their host plant during the day.
Eggs and larvae
Female weevils lay eggs singly on the underside of a young leaf. The slightly convex eggs are cemented into position with anal mucus and covered with green liquid excreta that when dry makes the eggs almost invisible. After hatching from an egg, most larvae chew a hole into the leaf and tunnel through it to the midrib. It then tunnels down the midrib and petiole (stalk) into the stem. Some larvae after hatching feed on flower buds before tunnelling into the stem. The larvae spend the rest of their life tunnelling in the plant stem. Larvae are whitish, with no legs and a brown head capsule. Larvae have large jaws at the front of the head. As a larva grows, it changes skins (moults). It is not known how many larval stages (instars) this weevil has. Larval development usually takes several months.
When it is fully grown, the larva probably makes a cell (chamber) in the woody stem. The larva changes into a pupa in the cell. Weevil pupa are usually white with all the appendages of the adult weevil visible - long legs, rostrum (snout), wings and wing cases. On emergence from the pupal skin, the adult weevil usually stays in the pupal chamber until its skin (cuticle) hardens and darkens. After leaving the pupal chamber males and females find each other and mate.
Both adult and larval weevils have chewing mouth parts. The mandibles of the adult are at the tip of the rostrum. The adults are assumed to feed on young shrubby haloragis leaves.
The larva also has mandibles (jaws) at the front of its head. It uses them to create a tunnel (mine) in the leaf and stem. First stage (instar) larva ingests the internal tissue of the leaf and stalk leaving the upper and lower skins (epidermis) of the leaf intact. The larvae then tunnel through the stem. The older larvae tunnel down the centre of the stem and can feed on the pith and woody tissue. The digested leaf and stem tissue is excreted as discrete pellets (frass) that back-fill the tunnel.
This is the only large, long weevil known to live on Shrubby haloragis, Haloragis erecta (Haloragaceae). The weevil also has distinctive pointed ends to its wing covers, elytra.
The presence of the weevil in a plant can be recognised by the leaf mine and the tunnelling in the young shoots by the larvae. This is the only beetle known to tunnel through live stems, so the presence of weevil larvae in stems also enables recognition of the insect.
Two species of wasp parasitoids have been reared from larvae of the Haloragis weevil: Rhadinosomus acuminatus. No natural enemies of the adults are known, but they are probably preyed on by birds, spiders and predatory insects.
|Scientific Name||Common Name||Classification||Enemy Type||Reliability Index||Biostatus|
|Pteromalidae sp. (May 1993)||(Wasp)||Hymenoptera: Pteromalidae||parasitoid||5||endemic|
|Xanthocryptus novozealandicus (Dalla Torre, 1902)||(Wasp)||Hymenoptera: Ichneumonidae||parasitoid||10||unknown|
The Haloragis weevil: Rhadinosomus acuminatus (Coleoptera: Curculionidae), lives on Shrubby haloragis, Haloragis erecta (Haloragaceae). This is its only known host plant.
Dr Kuschel observed adult weevils feeding on young leaves and flowers. Brenda May reported that newly hatched larvae may feed on flower buds, but most tunnelled through the young leaf from their egg to the leaf midrib. They then tunnel down the midrib and through the leaf stalk (petiole) into the stem. Up to four larvae may be found in their own tunnels in a single stem.
Metal outdoor signs are available for placement in reserves, Regional and National parks, urban parks and school grounds. They can be bought from Metal Images Ltd, www.metalimage.co.nz/bushbirdandbug.html. They come in two sizes, 100 x 200 mm, 194 x 294 mm. The signs can be bought ready mounted on a stand that need to be ‘planted’ in the ground, or they can be bought unmounted with holes for fixing into your own mounts.
The signs for the Haloragis weevil are best placed by plants of Shrubby haloragis, Haloragis erecta especially plants where adult weevils have been seen.
May BM. 1993. Larvae of Curculionoidea (Insecta: Coleoptera): a systematic overview. Fauna of New Zealand. 28: 1-223.
Kuschel G. 1970. New Zealand Curculionoidea from Captain Cook’s voyages (Coleoptera). New Zealand Journal of Science 133: 191-205.
Chris Winks for helpful comments on the draft factsheet.
The New Zealand Institute for Plant & Food Research Limited (Plant & Food Research) for permission to use photographs.
Landcare Research New Zealand Limited (Landcare Research) for permission to use photographs.