INTERESTING INSECTS

By N A Martin (2017)

Pseudopanax leafminer weevil

Pseudopanax blotch mine sp. 'lessonii' of Martin 1999

Pactola sp. 'lessonii' of Martin & Kuschel 2010

This endemic weevil has only been found in three places, all in the Auckland region. On one West Coast Auckland beach, leaf mines are regularly seen on host plant trees, Pseudopanax lessonii (Araliaceae), and adult weevils were reared from this site. Leaf mines were found twice at an Auckland reserve of a coastal forest remnant in an urban suburb. Leaf mines were found once at a third site near Leigh. While the endemic host plant is common, this insect is very rare.

Conservation status: Very rare. One locality has a large breeding population; a second locality has a smaller breeding population, and at a third locality the weevil was detected only once.

Larvae have been found in all seasons and adults reared from larvae collected in several months throughout the year. It is presumed that there are several generations per year. The generations do not appear to be synchronised.

Adults are typical weevils, each with six legs, hard wing covers (elytra) and a long snout (rostrum). They are small, about 5-6 mm long and mottled brown. Colour is variable, but is generally mottled grey/brown. Their wings, which are longer than the wing covers, are kept safely folded up under the wing covers, except when needed for flying. 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 the terminal segments form a club. The body is knobbly and cryptically coloured. They probably hide by day on tree bark. The weevil is covered by short flat setae (hairs). The hind legs have a prominent enlarged segment (femur) with a spur. The next segment (tibia) is thin and curved.

After they emerge from the pupal chambers, adult males and females are assumed to find one another and mate. They also locate suitable food plants and egg laying sites. The adults are believed to feed at night and hide during the day.

Eggs and larvae

Female weevils lay eggs in mature leaves. Usually a single egg is laid in a leaflet. After hatching from an egg, the larva tunnels into the leaf, forming a blotch mine, which may be compact or elongated.

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). There are probably three larval stages (instars) like other weevils. Larval development usually takes many weeks.

Pupa

When it is fully grown, the larva makes a cell (chamber) out of black insect faeces (frass) in its mine. The larva changes into a pupa in the cell. The pupa is white with all the appendages of the adult weevil visible - long legs, snout (rostrum), wings and wing cases. If the chamber is opened, the pupa waves its abdomen about. On emergence from the pupal skin, the adult weevil stays in the pupal chamber until its skin (cuticle) hardens and darkens. After leaving the pupal chamber, it is assumed that males and females find each other and mate.

Feeding

Both adult and larval weevils have chewing mouth parts. The jaws (mandibles) of the adult are at the tip of the rostrum. The adult feeds on host plant leaves, especially on the soft tissue exposed when the leaf stalk is broken.

The larva also has jaws (mandibles) at the front of its head. It uses them to create a tunnel (mine) in the leaf. It ingests the internal tissue of the leaf and leaves the upper and lower skins (epidermis) of the leaf intact. Weevil larvae will continue feeding in leaf tissue after the leaf has died. The digested leaf tissue is excreted as discrete pellets (frass) that back-fill the mine.

There are many kinds of small weevils in New Zealand. They can only be conclusively identified by an expert. The Pseudopanax leafminer belongs to the subfamily Curculioninae and tribe Eugnomini. Weevils in the genus Pactola have a knobbly appearance, short flat body seta (hairs), an enlarged segment (femur) on each hind leg followed by a narrow, curved segment (tibia). Other weevils in the genus are stem and bark borers in Pseudopanax species.

The leaf mines are distinctive and can be used to determine the presence of the weevil in an area. The Pseudopanax leafminer weevil is one of three insects that form leaf mines in its host plant, Pseudopanax lessonii. The weevil mines can easily be distinguished from the mines made by two species of moth. The weevil mines are in the form of irregular blotches that show on both sides of a leaf, whereas the moth mines are serpentine. The lancewood leafminer, 'Acrocercops' panacivagans (Watt 1920) (Lepidoptera: Gracillariidae) forms a distinctive and easy to see narrow serpentine mine on the upper side of leaves. The mine formed by 'Acrocercops' panacitorsens (Watt, 1920) (Lepidoptera: Gracillariidae), is usually on the underside of the leaf and tends to be wider and transparent.

The weevil blotch mines can be confused with two other forms of leaf damage, dead areas and ‘windows’. When held up to the light, dead areas look dark, whereas mines show up as pale areas. ‘Windows’, where one side of a leaf has been chewed leaving one skin (epidermis) intact, can be distinguished from a mine that has two intact skins (epidermal layers).

An unnamed wasp parasitoid (Hymenoptera: Braconidae: Blacinae) kills weevil larvae after they have formed a pupation cell. The white wasp larva makes a brown silk lined cocoon in which it pupates. The adult wasp emerges in the cocoon and when its skin (cuticle) has hardened it chews its way through the cocoon and the epidermis of the leaf mine.

No observations of predation of adult weevils have been reported, but they are probably preyed on by birds, spiders and predatory insects.

The Pseudopanax leafminer weevil lives in leaves of Pseudopanax lessonii (DC.) K. Koch (Araliaceae). A blotch mine was seen once in lancewood, Pseudopanax crassifolius (Sol. ex A. Cunn.) K. Kocha, a closely related species, but it is not certain if it was caused by a weevil, though lancewood could be a host plant of the Pseudopanax leafminer weevil.

Plant damage

The adult weevils feed on leaves, especially the soft tissue exposed at the broken end of a leaf stalk. Larvae burrow through leaves forming blotch mines that vary in shape and position on the leaf. The larva chews the internal tissues of the leaf and the upper and lower skins of the leaf are left intact. Weevil larvae will feed in leaves after that part of the leaf has died. The digested leaf tissue is excreted as discrete pellets (frass) that back-fill the mine.

Dr Willy Kuschel, who identified the adult weevils and provided information about the genus Pactola.

Dr Willy Kuschel for identification of the adult weevils.

The New Zealand Institute for Plant & Food Research Limited (Plant & Food Research) for permission to use photographs.