INTERESTING INSECTS

By N A Martin (2010, revised 2018)

Mahoe leaf miner, Whitey-wood fly

Agromyza flavolateralis Watt, 1923

Phytobia flavolateralis (Watt, 1923)

This endemic leaf mining fly is found throughout New Zealand where its host plant, mahoe, Melicytus ramiflorus (Violaceae), is found. The fly appears to be restricted to native ecosystems. It is present in native reserves in cities, but does not appear to spread to new restoration areas unless the host plant is present in an adjacent remnant of the original forest.

Conservation status: Widespread, not threatened.

The fly breeds all year. There is a winter generation and several generations in the spring and summer.

Adult fly

The flies are small, about 2.5-3.2 mm long, similar to the size of vinegar flies, Drosophila, that are seen around rotting fruit. The body is grey with yellow areas on the sides of the thorax (the middle part of the body to which the legs and wings are attached). They also have short yellow antennae. 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. The male has rounded black external genitalia at the end of the abdomen, while the female has a slender end containing an ovipositor. The ovipositor is used to make holes in young leaves into which eggs are laid. Females of other species of this kind of leaf-mining fly also make holes with their ovipositor and feed on the leaf sap. This has not been observed for this species.

Eggs and larvae

Single eggs are inserted into young leaves, usually near the midrib. The newly hatched larvae tunnel into the leaf making a mine that is visible on the upper side of the leaf. 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, generally to the sides of the mine. The larva moults, or changes skin, as it gets larger. There are three larval stages (instars). A fully grown larva is about 3 mm long.

The mine starts near the midrib of the leaf and meanders over the leaf. The mine is narrow at first, gradually widening.

Pupa

When fully grown, the larva makes a semicircular slit in the thin upper wall of the mine. The larva usually drops to the ground into the soil or litter. The larva pupates inside its larval skin, which turns brown and hard. This structure is called a puparium. The puparium has a pair of stigma (organs for breathing) at each end of the body. After several weeks, the 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 fly 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 find each other for mating.

Side view of an adult male mahoe leaf miner: Liriomyza flavolateralis (Diptera: Agromyzidae), the fly has a large yellow area above the legs on the thorax; the yellow knob near the base of the wing is the haltere, or balance organ, and the black round structure at the end of the abdomen is the external male genitalia. Creator: Tim Holmes. © Plant & Food Research. [Image: 3F2]

Underside of an adult male mahoe leaf miner: Liriomyza flavolateralis (Diptera: Agromyzidae), the black round structure at the end of the abdomen is the external male genitalia. Creator: Tim Holmes. © Plant & Food Research. [Image: 3F3]

Mahoe leaf miner: Liriomyza flavolateralis (Diptera: Agromyzidae), the flies are about 2.5-3.2 mm long. Creator: Tim Holmes. © Plant & Food Research. [Image: 3F1]

Two leaf mines of Liriomyza flavolateralis (Diptera: Agromyzidae), in a leaf of mahoe, Melicytus ramiflorus; note the unusual short blind mines near the place where the larvae first burrowed into the leaf. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 3F5]

Larva of the mahoe leaf miner: Liriomyza flavolateralis (Diptera: Agromyzidae), in a mine in a leaf of mahoe, Melicytus ramiflorus; the toothed mandible rasps the plant cells at the head of the mine. The white arrows point to the tip of the mandible. Creator: Tim Holmes. © Plant & Food Research. [Image: 3F0]

A semicircular slit made in a leaf mine by a mature larva of the mahoe leaf miner: Liriomyza flavolateralis (Diptera: Agromyzidae), in a leaf of mahoe, Melicytus ramiflorus. Creator: Tim Holmes. © Plant & Food Research. [Image: 3EZ]

A puparium of the mahoe leaf miner: Liriomyza flavolateralis (Diptera: Agromyzidae), on a leaf of its host plant, mahoe, Melicytus ramiflorus; normally the fly pupates in the litter under the plant, the arrow points at the rear pair of spiracles, or breathing tubes. Creator: Tim Holmes. © Plant & Food Research. [Image: 3EY]

Upper side of a puparium of the mahoe leaf miner: Liriomyza flavolateralis (Diptera: Agromyzidae), note the anterior and posterior pairs of spiracles, or breathing tubes. Creator: Tim Holmes. © Plant & Food Research. [Image: 15MT]

Underside of a puparium of the mahoe leaf miner: Liriomyza flavolateralis (Diptera: Agromyzidae), note the anterior and posterior pairs of spiracles, or breathing tubes Creator: Tim Holmes. © Plant & Food Research. [Image: 15MS]

Creator: Tim Holmes. © Plant & Food Research. [Image: 3EW]

Puparia of the mahoe leaf miner: Liriomyza flavolateralis (Diptera: Agromyzidae), showing the opening through which the adult fly emerges; the first three and a half segments split halfway down the side and are pushed apart by the emerging fly; note also the anterior and posterior pairs of spiracles, or breathing tubes. Creator: Tim Holmes. © Plant & Food Research. [Image: 3L3]

These small grey and yellow flies require expert knowledge for identification. However, the species can be detected by the leaf mines.

These are the only leaf mines known on mahoe, Melicytus ramiflorus. Leaf mines have been found in other species of Melicytus, but these are believed to be caused by other, closely related species of fly.

Mahoe leaf miner: Liriomyza flavolateralis (Diptera: Agromyzidae), the flies are about 2.5-3.2 mm long. Creator: Tim Holmes. © Plant & Food Research. [Image: 3F1]

Leaf mine of the mahoe leaf miner: Liriomyza flavolateralis (Diptera: Agromyzidae), in a leaf of mahoe, Melicytus ramiflorus. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 3EV]

Leaf mines of the mahoe leaf miner: Liriomyza flavolateralis (Diptera: Agromyzidae), in a leaf of mahoe, Melicytus ramiflorus, normally there are only one or two mines in a leaf. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 3F4]

Predators

There are no reports of predators of the flies, but it is likely that they are preyed upon by birds, spiders and predatory insects.

Parasitoids

The only species of parasitoid that has been reared from the mahoe leaf miner: Liriomyza flavolateralis, is a species of Chrysonotomyia (Hymenoptera: Eulophidae).

Black pupa of wasp (hymenoptera) parasitoid of the mahoe leaf miner: Liriomyza flavolateralis (Diptera: Agromyzidae), in a leaf of mahoe, Melicytus ramiflorus; note the black faecal pillars on either side of the pupa. Creator: Tim Holmes. © Plant & Food Research. [Image: 1IW0]

Round exit hole of wasp (hymenoptera) parasitoid in the leaf mine of the mahoe leaf miner: Liriomyza flavolateralis (Diptera: Agromyzidae), in a leaf of mahoe, Melicytus ramiflorus. Creator: Tim Holmes. © Plant & Food Research. [Image: 1IW5]

Side view of adult wasp (Hymenoptera) parasitoid of the mahoe leaf miner: Liriomyza flavolateralis (Diptera: Agromyzidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 1IW1]

Dorsal (top) view of adult wasp (Hymenoptera) parasitoid of the mahoe leaf miner: Liriomyza flavolateralis (Diptera: Agromyzidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 1IW2]

Underside of adult wasp (Hymenoptera) parasitoid of the mahoe leaf miner: Liriomyza flavolateralis (Diptera: Agromyzidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 1IW4]

The fly only lives on Melicytus ramiflorus J. R. et G. Forst. (Violaceae) (mahoe, whiteywood). The adult female fly makes small punctures in young leaves for egg-laying and possibly for feeding. The larva burrows through the leaf making mines that are visible from the upper side of the leaf. The mine starts near the midrib, gradually getting wider and making a serpentine pattern.

Leaf mine of the mahoe leaf miner: Liriomyza flavolateralis (Diptera: Agromyzidae), in a leaf of mahoe, Melicytus ramiflorus. Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 3EV]

The mahoe leaf miner appears to be restricted to native ecosystems or areas where mahoe has persisted after much of the other original vegetation has been removed. It may be a good indicator of the history of an area. It is not known how readily it will colonise newly restored areas of native vegetation that are not connected to remnants of the original native forest.

It should be easy to survey populations of the fly, though populations tend to be low and mines tend to be found on particular trees that probably had leaves of a suitable age for feeding by the adult female fly and for egg laying.

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 Mahoe leafminer should only be placed in a native reserve where the leaf miner is known to be present. The mines do not occur on the same tree every year, so install the sign near a mahoe tree in a convenient place for people stop.

Large Bug Sign (5006) for Liriomyza flavolateralis, Mahoe leafminer, 194 x 294 mm. Creator: Metal Images Ltd. © Metal Images Ltd & Entomological Society of New Zealand. [Image: 1L12]

Small Bug Sign (5006) for Liriomyza flavolateralis, Mahoe leafminer, 100 x 200 mm. Creator: Metal Images Ltd. © Metal Images Ltd & Entomological Society of New Zealand. [Image: 1L11]

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.

Watt MN 1923. The leaf-mining insects of New Zealand: part III - species belonging to the genus Agromyza (Fallen) and Phytomyza (Fallen). Transactions and Proceedings of the New Zealand Institute 54: 465-489.

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

1 April 2018. NA Martin. Bug signs updated

1 June 2015. NA Martin. Annual Cycle: added two photos of pupae. Recognition: photo of adult fly added. Details of Bug Signs added.