INTERESTING INSECTS

By N A Martin (2017)

This endemic fly is found in the North and South Island of New Zealand. It is a leaf miner of herbaceous brassicas (Cruciferae or Brassicae) and garden nasturtium, Tropaeolum majus (Tropaeolaceae) which has similar sulphur compounds in its leaves. It occurs on native plants, garden plants, watercress and weeds.

Conservation status: Widespread. Occasionally it causes noticeable damage to garden plants in spring, summer and autumn.

The fly breeds throughout the year. There are probably 4-5 generations per year in Auckland. Populations tend to be low in summer due to the activities of parasitoids.

Adult fly

The flies are small, about 2.5 mm long, smaller than vinegar flies, Drosophila species, that are seen around rotting fruit. The fly is coloured is black and yellow. The head is yellow between the eyes and on the underside. The top of the mesonotum (first visible segment after the head) is mat black with some areas of yellow on the sides. The scutellum (last part of the thorax) has a prominent central yellow area typical of Liriomyza flies. The abdomen has black segments on top and underside, with yellow between the tergites (upper plates) and sternites (underside plates). The underside of the thorax is black. The head is yellow between the red compound eyes. The short antennae are yellow with a black first segment. It is a typical fly, having one pair of wings. The hind pair of wings is reduced to two small knobs, or halteres, which help the fly to balance during flight. These are yellow. The legs are black except for yellow ‘knees’ on the first pair of legs. The male has rounded black external genitalia at the end of the abdomen, while the female has a dark slender end containing an ovipositor. The ovipositor is used to make holes in host plant leaves into which eggs are laid. Females also feed on leaf sap from holes in leaves made with their ovipositor.

Eggs and larvae

Single eggs are inserted into the upper side or underside of leaves. Newly hatched larvae tunnel into leaves making serpentine mines that are visible on one side of the leaf, usually the upper side. The larvae feed on the internal cells of the leaf. They have a single black jaw which is moved from side-to-side, scraping the plant cells at the head of the mine. The plant cells are ingested and the dark green faeces excreted into the mine behind it, usually in the middle of the mine. The larva moults, or changes skin, as it gets larger. There are three larval stages (instars). The last larval instar is white and when fully grown is about 2.5 mm long.

The mine starts where the egg is laid and meanders over the leaf. The mine gradually widens. The mature larva cuts a hole in the epidermis (skin of the leaf) and drops to the ground to pupate in the soil or litter.

Pupa

The larva pupates inside its larval skin, which turns brown and hard. This structure is called a puparium. The puparium is pale brown. It has a pair of stigmata (organs for breathing) at each end of the body. Shortly before the fly emerges its eyes and bristles of the adult fly can be seen through the skin of the puparium.

Fly emergence

When ready to emerge, part of the head, just above the antennae, balloons out. This structure, the ptilinum, pushes the front of the pupa open. There is a line of weakness between the top and bottom halves of the first three-and-a-half segments that splits allowing the top and bottom to open up. After the fly has crawled out, the ptilinum retracts into the head, the wings expand, and the body hardens. Over the next 12 hours the fly acquires its full body colour.

It is not known how the male and females of this species find each other for mating.

Top view of an adult female New Zealand cress leafminer, Liriomyza watti (Diptera: Agromyzidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2HYX]

Underside of an adult female New Zealand cress leafminer, Liriomyza watti (Diptera: Agromyzidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2HYW]

Side view of an adult female New Zealand cress leafminer, Liriomyza watti (Diptera: Agromyzidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2HYV]

Underside of an adult male New Zealand cress leafminer, Liriomyza watti (Diptera: Agromyzidae), note the black genital capsule at the tip of the abdomen and the pale sternites (ventral abdominal plates). Creator: Tim Holmes. © Plant & Food Research. [Image: 2HZ2]

Ovipositor punctures in New Zealand bittercress, Cardamine debile (Cruciferae), made by adult female New Zealand cress leafminer, Liriomyza watti (Diptera: Agromyzidae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 2HZ4]

Leaf mines in New Zealand bittercress, Cardamine debile (Cruciferae), made by larvae of New Zealand cress leafminer, Liriomyza watti (Diptera: Agromyzidae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 2HZ3]

Larva of New Zealand cress leafminer, Liriomyza watti (Diptera: Agromyzidae) in leaf mines in garden nasturtium, Tropaeolum majus (Tropaeolaceae), note the black mandible and mandibular skeleton. Creator: Tim Holmes. © Plant & Food Research. [Image: 2HYN]

Larva of New Zealand cress leafminer, Liriomyza watti (Diptera: Agromyzidae), dissected from mine leaf in garden nasturtium, Tropaeolum majus (Tropaeolaceae), note white colour of the larva and the black mandible and mandibular skeleton. Creator: Tim Holmes. © Plant & Food Research. [Image: 2HYR]

Puparium of New Zealand cress leafminer, Liriomyza watti (Diptera: Agromyzidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2HYT]

Puparia of New Zealand cress leafminer, Liriomyza watti (Diptera: Agromyzidae), after flies have emerged. Creator: Tim Holmes. © Plant & Food Research. [Image: 2HYZ]

These small yellow and black flies require expert knowledge for identification. Larvae of three species of fly form leaf mines in brassicas and plants that have similar sulphur compounds in thier leaves. Flies reared from these mines and larvae found in the leaf mines can be identified.

The adults of the two species of Liriomyza are small and black and yellow. The New Zealand cress leafminer, Liriomyza watti, has a mat black mesonotum (the first visible body segment behind the head) and the legs are all black except for a small pale area on the ‘knees’ of the first pair of legs. The cabbage leafminer, Liriomyza brassicae, has a shiny black mesonotum and the first segments of the legs are yellow.

Adults of the turnip leafminer, Scaptomyza flava (Fallen, 1823) (Diptera: Drosophilidae), are yellow-brown and larger, about the same size as vinegar flies, Drosophila species (Drosophilidae).

The larvae of the two species of Liriomyza form serpentine leaf mines, whereas the turnip leafminer makes a narrow mine that expands into a blotch. The turnip leafminer larva has a compact mandibular skeleton, while the two Liriomyza species have a thin curved armature. The mandibular skeletons can be seen when the leaf mines are examined with transmitted light. To distinguish between the two Liriomyza species, the larvae need to be exposed. The larvae of the New Zealand cress leafminer are white, while those of the cabbage leafminer are yellow.

Top view of an adult female New Zealand cress leafminer, Liriomyza watti (Diptera: Agromyzidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2HYX]

Underside of an adult female New Zealand cress leafminer, Liriomyza watti (Diptera: Agromyzidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2HYW]

Top view of an adult female cabbage leafminer, Liriomyza brassicae (Diptera: Agromyzidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2HXP]

Underside of an adult female cabbage leafminer, Liriomyza brassicae (Diptera: Agromyzidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2HXR]

Serpentine leaf mines in garden nasturtium, Tropaeolum majus (Tropaeolaceae), made by larvae of New Zealand cress leafminer, Liriomyza watti (Diptera: Agromyzidae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 2HZC]

Larva of New Zealand cress leafminer, Liriomyza watti (Diptera: Agromyzidae), dissected from mine leaf in garden nasturtium, Tropaeolum majus (Tropaeolaceae), note white colour of the larva and the black mandible and mandibular skeleton. Creator: Tim Holmes. © Plant & Food Research. [Image: 2HYR]

Puparium of New Zealand cress leafminer, Liriomyza watti (Diptera: Agromyzidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2HYT]

Larvae of cabbage leafminer, Liriomyza brassicae (Diptera: Agromyzidae) dissected from leaf mine in garden nasturtium, Tropaeolum majus (Tropaeolaceae): note the yellow colour and the black mandibular skeleton. Creator: Tim Holmes. © Plant & Food Research. [Image: 2HYA]

Leaf mine in garden nasturtium, Tropaeolum majus (Tropaeolaceae), formed by larvae of the turnip leafminer, Scaptomyza flava (Fallen, 1823) (Diptera: Drosophilidae): note the narrow mine that expands into a blotch. © Plant & Food Research. [Image: 2HYJ]

Larvae of the turnip leafminer, Scaptomyza flava (Fallen, 1823) (Diptera: Drosophilidae), dissected from leaf mine. Creator: Tim Holmes. © Plant & Food Research. [Image: 2HYH]

Mandibles and mandibular skeleton of larvae of the turnip leafminer, Scaptomyza flava (Fallen, 1823) (Diptera: Drosophilidae). Creator: Zheng Qi Zhao. © Landcare Research. [Image: 2HYK]

Puparium of the turnip leafminer, Scaptomyza flava (Fallen, 1823) (Diptera: Drosophilidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2HYG]

Adult of the turnip leafminer, Scaptomyza flava (Fallen, 1823) (Diptera: Drosophilidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2HYI]

Predators

There are no reports of predators of the flies or puparia, but it is likely that they are preyed upon by birds, spiders and predatory insects. Some female wasp parasitoids feed on leaf miner fly larvae as well as parasitising fly larvae.

Parasitoids

Five species of parasitoid wasps from two families have been reared from larvae and puparia of New Zealand cress leafminers. Adult female wasps of the genus Opius (Hymenoptera: Braconidae), lay an egg in a fly larva in the leaf mine. The wasp larva develops in the fly larva but does not kill it until the larva has formed its puparium. The fully developed wasp larva pupates in the fly puparium and emerges from its pupa in the fly puparium and chews a hole in the fly puparium through which it emerges.

The other known wasp parasitoids kill the fly larva and pupate in the leaf mine. Some live in the fly larva and kill it when the wasp larva is fully grown. Other species of wasp, e.g. Diglyphus isaea (Hymenoptera: Eulophidae), kill the fly larva when they lay an egg next to the fly larva. The wasp larva feeds on the fly larva from the outside and pupates in the mine.

An unnamed species, Proacrias n.sp. (Hymenoptera: Eulophidae) kills the fly larva and pupates in the leaf mine.

Adult pupal parastioid, Opius sp. (Hymenoptera: Braconidae), of New Zealand cress leafminer, Liriomyza watti (Diptera: Agromyzidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2HZD]

Underside of adult pupal parastioid, Opius sp. (Hymenoptera: Braconidae), of New Zealand cress leafminer, Liriomyza watti (Diptera: Agromyzidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2HZG]

Dorsal and ventral views of an adult pupal parastioid of New Zealand cress leafminer, Liriomyza watti (Diptera: Agromyzidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2HZ0]

Internal parasitoid larva (dark oval) in larvae of the New Zealand cress leafminer, Liriomyza watti (Diptera: Agromyzidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2HYM]

Internal parasitoid larva (dark oval) in larvae of the New Zealand cress leafminer, Liriomyza watti (Diptera: Agromyzidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2HYL]

Dead larvae of the New Zealand cress leafminer, Liriomyza watti (Diptera: Agromyzidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2HYP]

Adult internal larval parastioid of larvae of New Zealand cress leafminer, Liriomyza watti (Diptera: Agromyzidae): dorsal and ventral views. Creator: Tim Holmes. © Plant & Food Research. [Image: 2HYY]

Two ectoparasitic larvae (arrows) of Diglyphus isaea(Walker, 1838) (Hymenoptera: Eulophidae) feeding on larva of a leafminer, Liriomyza sp. (Diptera: Agromyzidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 154P]

Ectoparasitic larvae (black arrow) of Diglyphus isaea (Walker, 1838) (Hymenoptera: Eulophidae) in leaf mine after feeding on larva of a leafminer, Liriomyza sp. (Diptera: Agromyzidae) (white arrow). Creator: Tim Holmes. © Plant & Food Research. [Image: 154S]

Pupa of Diglyphus isaea (Walker, 1838) (Hymenoptera: Eulophidae) exposed in leaf mine of Liriomyza sp. (Diptera: Agromyzidae), note the red eyes and some of the six black faecal pillars that were on either side of the pupa. Creator: Tim Holmes. © Plant & Food Research. [Image: 154W]

Just before adult emergence, dark pupa of Diglyphus isaea (Walker, 1838) (Hymenoptera: Eulophidae) exposed in leaf mine of Liriomyza sp. (Diptera: Agromyzidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 154X]

Adult Diglyphus isaea (Walker, 1838) (Hymenoptera: Eulophidae) chewing an exit hole from a leaf mine of Liriomyza sp. (Diptera: Agromyzidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 154Y]

Side view of adult Diglyphus isaea (Walker, 1838) (Hymenoptera: Eulophidae), an ectoparasite of larvae of leafminer flies (Diptera: Agromyzidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 1552]

Underside (left) and upper side (right) of an adult Diglyphus isaea (Walker, 1838) (Hymenoptera: Eulophidae), an ectoparasite of larvae of leafminer flies (Diptera: Agromyzidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2HWU]

The New Zealand cress leafminer is a leaf miner of herbaceous brassicas (Cruciferae or Brassicae) and garden nasturtium, Tropaeolum majus (Tropaeolaceae) which has similar sulphur compounds in its leaves. It occurs on native plants, garden plants, watercress and weeds.

The adult female of the New Zealand cress leafminer makes small punctures in young leaves for egg laying and for feeding. The larva burrows through the leaf, making mines that are visible on one or both sides of the leaf. The mine meanders over the leaf gradually widening. On some plants with small leaflets such as Cardamine species, the larvae may tunnel through the leaf stalk to another leaflet or leaf.

Serpentine leaf mines in garden nasturtium, Tropaeolum majus (Tropaeolaceae), made by larvae of New Zealand cress leafminer, Liriomyza watti (Diptera: Agromyzidae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 2HZC]

Serpentine leaf mines in hairy bitter cress, Cardamine hirsuta (Cruciferae), made by larvae of New Zealand cress leafminer, Liriomyza watti (Diptera: Agromyzidae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 2HZ5]

Serpentine leaf mines in watercress, Nasturtium officinale (Cruciferae), made by larvae of New Zealand cress leafminer, Liriomyza watti (Diptera: Agromyzidae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 2HZ7]

Serpentine leaf mines in a native bitter cress, Cardamine debile (Cruciferae), made by larvae of New Zealand cress leafminer, Liriomyza watti (Diptera: Agromyzidae): note the mine extends into the leaf stalk and plant stem. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 2HZA]

The New Zealand cress leafminer and two other species of leaf miner flies have been found in watercress, Nasturtium officinale (Cruciferae), but have not been reported causing problems in commercial crops.

In gardens, three species of fly including the New Zealand cress leafminer form mines in garden nasturtium (Tropaeolum majus (Tropaeolaceae)). They occasionally cause unsightly damage to leaves, but populations are usually kept low by naturaly enemies so that other forms of control are usually not warrented.

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

Spencer KA 1976. The Agromyzidae of New Zealand (Insecta: Diptera). Journal of the Royal Society of New Zealand 6(2): 153-211.

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

Spencer KA 1976. The Agromyzidae of New Zealand (Insecta: Diptera). Journal of the Royal Society of New Zealand 6(2): 153-211.

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.