INTERESTING INSECTS

By N A Martin (2010, revised 2018)

Pohutukawa leaf miner

This endemic weevil lives in the North and South Islands of New Zealand where its main host plants, medium to large-leaved rata (Metrosideros) species, occur. The weevil lives on host plants in city gardens and parks as well as in native ecosystems. The adult chews young leaves, while the larvae tunnel in leaves.

Conservation status: Widespread, not threatened, a minor pest in gardens.

Adults are typical weevils, each with six legs, hard wing covers (elytra) and a long snout (rostrum). They are small, about 3 mm long and 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.

Eggs and larvae

Female weevils start laying eggs when new leaves appear. A single egg is inserted into the blade of a young leaf. After hatching from an egg the larva tunnels into the leaf forming a serpentine mine that gradually gets wider and may end in a blotch. If a larva consumes all of a leaf, it can transfer to another leaf. Larvae can also complete development if the leaf falls from the stem.

Larvae are whitish, with no legs and a brown head capsule that has a strong V-shaped invagination on the upper (dorsal) side. 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 about three weeks.

Pupa

When it is fully grown, the larva cuts a slit in the leaf skin (epidermis) covering the mine, crawls out and drops to the ground. It burrows into the litter or soil and makes a chamber in which it pupates. The pupa is white with all the appendages of the adult weevil visible - long legs, rostrum (snout), 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 the skin (cuticle) hardens and darkens. After leaving the pupal chamber the males and females find one another and mate. If suitable young plant foliage is available, they feed and lay eggs, even in winter. However, most emerging adults wait until spring and the new flush of foliage, before laying eggs. Adults feed at night and hide during the day. They drop to the ground if disturbed.

Feeding

Both adult and larval weevils have chewing mouth parts.

The mandibles of the adult are at the tip of the rostrum. When the adult wants to feed, the rostrum is pushed onto the leaf surface and the mandibles bite the leaf tissue, making a small round or oval hole in the leaf. The weevil does not eat right through the leaf, but leaves the skin (epidermis) on the far side of the leaf intact, so creating a ‘window’. When a young leaf expands, the hole enlarges and the epidermis breaks, creating a see-through hole. When an adult feeds on a fully expanded leaf, the ‘window’ remains intact.

The larva also has mandibles (jaws) at the front of its head. It uses them to create a tunnel in the leaf. It ingests the internal tissue of the leaf and leaves the upper and lower skins of the leaf intact. Weevil larvae can feed in leaves while they are attached to the plant and after they have fallen. The digested leaf tissue is excreted as discrete pellets (frass) that back-fill the mine.

Photo-montage of a dead adult pohutukawa leaf miner weevil, Neomycta rubida (Coleoptera: Curculionidae). Creator: Birgit E Rhode. © Plant & Food Research. [Image: 3GK]

Adult pohutukawa leaf miner weevil, Neomycta rubida (Coleoptera: Curculionidae), about 3 mm long; upper side. Creator: Tim Holmes. © Plant & Food Research. [Image: 3GA]

Adult pohutukawa leaf miner weevil, Neomycta rubida (Coleoptera: Curculionidae), about 3 mm long; underside. Creator: Tim Holmes. © Plant & Food Research. [Image: 3G9]

A mine formed by a larva of the pohutukawa leaf miner, Neomycta rubida (Coleoptera: Curculionidae), in a fallen leaf of Metrosideros fulgens . Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 3FW]

A larva of the pohutukawa leaf miner, Neomycta rubida (Coleoptera: Curculionidae), in a juvenile leaf of pohutukawa, Metrosideros excelsa. Creator: Tim Holmes. © Plant & Food Research. [Image: 3G6]

A larva of the pohutukawa leaf miner, Neomycta rubida (Coleoptera: Curculionidae), dissected from a juvenile leaf of pohutukawa, Metrosideros excelsa; note the faecal pellets (frass) in the open mine. Creator: Tim Holmes. © Plant & Food Research. [Image: 3G4]

A larva of the pohutukawa leaf miner, Neomycta rubida (Coleoptera: Curculionidae), dissected from a juvenile leaf of pohutukawa, Metrosideros excelsa; note the faecal pellets (frass) in the open mine. Creator: Tim Holmes. © Plant & Food Research. [Image: 3G5]

Pupae of the pohutukawa leaf miner, Neomycta rubida (Curculionidae). The weevil pupates in cells made in the soil or litter. Creator: Tim Holmes. © Plant & Food Research. [Image: 3G8]

Pupae of the pohutukawa leaf miner, Neomycta rubida (Coleoptera: Curculionidae); note the transparent rostrum, legs and wings. Creator: Tim Holmes. © Plant & Food Research. [Image: 3G7]

Pupae of the pohutukawa leaf miner, Neomycta rubida (Coleoptera: Curculionidae); note the setae on the body. Creator: Tim Holmes. © Plant & Food Research. [Image: 281C]

Holes in young leaves of mature foliage of pohutukawa, Metrosideros excelsa, made by adult pohutukawa leaf miner weevils, Neomycta rubida (Coleoptera: Curculionidae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 3GE]

There are many kinds of small weevils in New Zealand. They can only be distinguished by an expert. However, those found on the young foliage of pohutukawa and rata (Metrosideros species) trees are most likely to be the pohutukawa leaf miner weevil.

The pohutukawa leaf miner weevil is the only insect forming mines in leaves of pohutukawa and rata (Metrosideros) trees.

The adult damage to leaves can be confused with that caused by adult chrysomelid beetles in early summer after the main period of weevil feeding is over. These beetles are related to bronze beetle (Eucolaspis brunnea) and at least two species feed on pohutukawa leaves. One species (A) feeds on very young leaves and removes large areas of leaf tissue on the upper or lower surface of the leaf, leaving a layer of epidermis. This species is usually found near sandy beaches. The other species (B) makes large ragged holes in young leaves. Small holes or ‘windows’ in leaves are a sign that weevils were present.

Adult pohutukawa leaf miner weevil damage to last seasons (dark green) leaves and the new spring foliage of pohutukawa, Metrosideros excelsa. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 3GF]

Damage to young leaves of pohutukawa, Metrosideros excelsa, by adults of Eucolaspis beetles (Chrysomelidae) associated with this plant growing near sandy beaches, note the large area of leaf tissue removed from one side of the leaf and leaving most of the epidermis (skin of the leaf). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 3FV]

Damage to young leaves of pohutukawa, Metrosideros excelsa, by adults of Eucolaspis beetles (Chrysomelidae) associated with this plant growing near sandy beaches, note the beetle on the leaf and the large area of leaf tissue removed from one side of the leaf and leaving most of the epidermis (skin of the leaf). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 3FU]

Damage to leaves of pohutukawa, Metrosideros excelsa, by adults of one of two species of Eucolaspis beetles (Chrysomelidae) associated with this plant. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 3G3]

Damage to leaves of pohutukawa, Metrosideros excelsa, by adults of one of two species of Eucolaspis beetles (Chrysomelidae) associated with this plant. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 3G2]

Adult weevils are probably preyed on by birds and spiders, but no observations of predation have been reported.

Parasitoids

One or perhaps two un-named wasp parasitoids have been found in pohutukawa leaf miner weevil mines. The larvae live in the weevil larvae. They pupate out of the larvae in the leaf mine. One forms black faecal pillars around the black pupa that help support the walls of the leaf mine. The other appears to be a large wasp. When the adult wasp has emerged from the pupa and its body and wings have hardened, it chews a hole in the leaf mine through which it emerged.

Leaf mine of pohutukawa leaf miner, Neomycta rubida (Coleoptera: Curculionidae) with a dark shadow at the end of the mine made by the dead weevil larva. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 2M2K]

Leaf mine of pohutukawa leaf miner, Neomycta rubida (Coleoptera: Curculionidae) opened to show the larva of the wasp parasitoid that was living in and killed the weevil larva. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 2M2L]

Dead larva of a pohutukawa leaf miner, Neomycta rubida (Coleoptera: Curculionidae) killed by the larva of the wasp parasitoid that was living in it. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 2M2M]

A wasp pupa partly emerged from a dead larva of a pohutukawa leaf miner, Neomycta rubida (Coleoptera: Curculionidae): the wasp usually leaves the dead larva and pupates in the leaf mine. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 2M2N]

Adult wasp that emerged from a pupa of a larva that had killed a larva of a pohutukawa leaf miner, Neomycta rubida (Coleoptera: Curculionidae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 2M2P]

A wasp pupa in a leaf mine of the pohutukawa leaf miner, Neomycta rubida (Coleoptera: Curculionidae): note the round black faecal pillars made by the wasp larva to keep the leaf mine walls apart. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 2M2S]

A wasp pupa in a leaf mine of the pohutukawa leaf miner, Neomycta rubida (Coleoptera: Curculionidae) with the brocken skin of another pupa: the live pupa may be a hyperparasitoid. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 2M2R]

The pohutukawa leaf miner weevil lives on medium and large-leaved rata (Metrosideros) trees and climbers, including a tropical species (Metrosideros kermadecensis) grown in Auckland gardens. Adult beetles were once found feeding on leaves of a small-leaved climber, Metrosideros diffusa. Normally the leaves of this species do not have the characteristic round leaf feeding holes.

Plant damage

The weevil larvae and adults both feed on leaves, but each causes different kinds of damage.

Larvae burrow through leaves forming long serpentine mines that gradually widen. The mine is most obvious on the upper side of leaves. Larval mining may cause leaves to fall prematurely.

Adult weevils feed on young leaves, making round or oval holes in the leaf with their rostrum (snout). If the leaf has reached full size, the holes stay small. However, if the leaf is still growing, the holes become larger as the leaf expands. When many adult beetles feed on small leaves, the leaves may be killed.

Why are the young leaves of some trees heavily damaged?

In native ecosystems, host plants usually only have low levels of damage and weevil populations are probably kept low by natural enemies. However, in Auckland urban areas, some pohutukawa trees and some ornamental tropical rata (Metrosideros polymorpha) plants may have a high proportion of young leaves damaged by adult weevils, while other nearby trees have few signs of weevil feeding. Possible reasons include:

1. a more attractive and nutritious plant,

2. timing of the appearance of young foliage in spring, and

3. more than one flush of young leaves, so that the weevils can breed more than once each year.

Holes in young leaves of mature foliage of pohutukawa, Metrosideros excelsa, made by adult pohutukawa leaf miner weevils, Neomycta rubida (Coleoptera: Curculionidae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 3GE]

Holes in fully expanded leaves of mature foliage of pohutukawa, Metrosideros excelsa, made by adult pohutukawa leaf miners, Neomycta rubida (Coleoptera: Curculionidae); the large holes were made in growing leaves and became enlarged as the leaf expanded. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 3GD]

Mines in young leaves of mature foliage of pohutukawa ,Metrosideros excelsa, made by larvae of the pohutukawa leaf miner, Neomycta rubida (Coleoptera: Curculionidae, leaf tissue turning black and leaf dropping from tree. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 3GG]

Mine in a young leaf of mature foliage of pohutukawa ,Metrosideros excelsa, made by larvae of the pohutukawa leaf miner, Neomycta rubida (Coleoptera: Curculionidae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 3GJ]

Full sized mine in a young leaf of mature foliage of pohutukawa ,Metrosideros excelsa, made by larvae of the pohutukawa leaf miner, Neomycta rubida (Coleoptera: Curculionidae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 3GC]

Holes in fully expanded leaves of Metrosideros robusta made by adult pohutukawa leaf miner weevils, Neomycta rubida (Coleoptera: Curculionidae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 3FX]

Mine in a young leaf of Metrosideros robusta made by larvae of the pohutukawa leaf miner weevil, Neomycta rubida (Coleoptera: Curculionidae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 3FY]

Holes in fully expanded leaves of Metrosideros fulgens made by adult pohutukawa leaf miners, Neomycta rubida (Coleoptera: Curculionidae); note that the large holes were made when leaves were smaller. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 3GH]

Holes in mature leaves of Metrosideros kermadecensis made by adult pohutukawa leaf miners, Neomycta rubida (Coleoptera: Curculionidae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 3G0]

Mines in young leaves of Metrosideros kermadecensis made by larvae of the pohutukawa leaf miner, Neomycta rubida (Coleoptera: Curculionidae); note the small initial mine starting at the edge of the leaf (left) and the large mine (right). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 3G1]

Mine in young leaf of Metrosideros kermadecensis made by larvae of the pohutukawa leaf miner, Neomycta rubida (Coleoptera: Curculionidae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 2M2Q]

Adult pohutukawa leaf miner can cause large amounts of damage to young leaves of some pohutukawa trees in gardens and parks. If this is of concern, plant selections that are less susceptible to damage by the weevil should be grown.

Ornamental tropical rata (Metrosideros kermadecensis) plants may sometimes be heavily damaged. If the plant is small, the adult weevils may be picked off the young shoots at night or the shoots can be tapped over a bowl into which the adult weevils can fall. If this does not work or the plant is too large, an insecticide could be applied to the young foliage in the evening.

Damage to young leaves of pohutukawa and northern rata by the weevil has sometimes caused concern, but is not believed to be a significant factor in the decline of these species.

Research project

Most adult weevils emerge from pupae when host plants have no more young foliage, their preferred food. There is little information about their activities between emergence and their appearance on the young foliage the following spring. However, if host plants have young foliage in summer or autumn that is suitable for breeding, adult weevils may be found feeding on it, laying eggs, and larval leaf mines may also be found. These observations raise several interesting questions:

1. Do newly emerged adult weevils feed?

2. If so, on what do the adult weevils feed in the absence of young leaves?

3. How do they know when plants are producing young foliage?

4. Also, how do adult males and females find one another?

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/products/botanic-labels. The Bug Signs are listed near the bottom of the ‘Fauna Species list’. The signs come in two sizes, 100 x 200 mm, 194 x 294 mm. The signs can be bought ready mounted on a stand that needs to be ‘planted’ in the ground, or they can be bought unmounted with holes for fixing into your own mounts.

The signs for the Pohutukawa leafminer are best placed by trees where the adult beetle has made many small holes in young leaves. The holes may be larger in older fully expanded leaves. It is unlikely that people will see the leaf mines. Leaves with mines, tend to drop off.

Large Bug Sign (5004) for Neomycta rubida, Pohutukawa leafminer, 194 x 294 mm. . Creator: Metal Images Ltd. © Metal Images Ltd & Entomological Society of New Zealand. [Image: 1L13]

Small Bug Sign (5004) for Neomycta rubida, Pohutukawa leafminer, 100 x 200 mm. Creator: Metal Images Ltd. © Metal Images Ltd & Entomological Society of New Zealand. [Image: 1L14]

Kuschel G 2003. Nemonychidae, Belidae, Brentidae (Insecta: Coleoptera: Curculionoidea). Fauna of New Zealand 45: 1-100.

May BM 1993. Larvae of Curculionoidea (Insecta: Coleoptera) a systematic overview. Fauna of New Zealand 28: 1-223.

Plant-SyNZ: Invertebrate herbivore-host plant association database. plant-synz.landcareresearch.co.nz/.

Tim Holmes and Birgit E Rhodes for photographs

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.

1 September 2018. NA Martin. Two photos of a mating pair of weevils were the wrong species and have been removed.

1 April 2018. NA Martin. Bug signs updated

1 September 2017. NA Martin. Added photos of adults mating. Natural Enemies, parasitoids with photos.

5 June 2015. N A Martin. Annual Cycle: reworded and added photograph of adult feeding damage to young leaves and a photo of a pupa. Details of Bug Signs added.