INTERESTING INSECTS

By N A Martin (2019)

Plantain leafminer

Phytomyza robinaldi Goureau, 1851

Phytomyza plantaginis Goureau, 1851

Phytomyza biseriata Hering, 1936

Phytomyza nannodes Hendel, 1935

Phytomyza plantaginicaulis Hering, 1944

This adventive leaf mining fly is found throughout New Zealand on its host plants, two naturalised species of plantain, Narrow-leaved plantain Plantago lanceolata and Broad-leaved plantain, Plantago major (Plantaginaceae),which are widespread weeds and one is grown for feeding farm animals.

Conservation status: Widespread.

The fly breeds in the spring and summer when host plants are growing new leaves. There is a suggestion that the populations of this fly in New Zealand are parthenogenetic, i.e. the female can lay viable eggs without the need to mate with a male. These eggs are usually only for female flies.

Adult fly

The flies are small, about 3 mm long, similar to the size of vinegar flies, Drosophila species, that are seen around rotting fruit. The body is grey-black with pale brown areas. The three pairs of legs and the pair of knob-like antennae are dark grey. Like all flies, they have one pair of wings behind which are a pair of small knobs, or halteres that 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 more tubular end that contains 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 the Plantain leafminer.

Eggs and larvaeSingle eggs are inserted into young leaves. The newly hatched larvae tunnel into the leaf making a mine that is usually visible on the upper side of the leaf. The larvae feed on the internal cells of the leaf. They have a single black jaw that is moved from side-to-side, scraping the plant cells at the head of the mine. The plant cells are ingested and dark faecal pellets are excreted into the mine behind it. The larva moults, or changes skin, as it gets larger. There are three larval stages (instars). A fully grown larva is almost 3 mm long. The larva forms a narrow linear mine that can extend up and down the leaf. It is narrow mine gradually widens. The mines are mainly on the upper side of the leaf.

Pupa

When fully grown the larva makes a chamber at the end of the mine in which it pupates. The larva pupates inside its larval skin, which is white and hard. This structure is called a puparium. The puparium has a pair of stigma (organs for breathing) at each end of the body. The stigma at the front end of the body are pushed through the upper skin of the leaf. 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 to be pushed through the skin of the leaf. The fly crawls out of the puparium onto the surface of the leaf. 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.

Top and underside of a female Plantain leafminer, Phytomyza plantaginis (Diptera: Agromyzidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 3014]

Top and side views of a female Plantain leafminer, Phytomyza plantaginis (Diptera: Agromyzidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 3015]

Two images of a mature larva of the Plantain leafminer, Phytomyza plantaginis (Diptera: Agromyzidae) that had been dissected from its leaf mine: note the dark jaw (right) that is used to excavate the mine. Creator: Tim Holmes. © Plant & Food Research. [Image: 3010]

A leaf of Broad-leaved plantain, Plantago major (Plantaginaceae), with leaf mines made by larvae of the Plantain leafminer Phytomyza plantaginis (Diptera: Agromyzidae): note the dark shape of a larva in a mine (top right) Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 300U]

A larva of the Plantain leafminer, Phytomyza plantaginis (Diptera: Agromyzidae) in its leaf mine: note the dark abdomen and in the clear area on the right, the small dark hooked jaw. Creator: Tim Holmes. © Plant & Food Research. [Image: 3011]

A puparium of the Plantain leafminer, Phytomyza plantaginis (Diptera: Agromyzidae) in the larval leaf mine: note the anterior pair of stigma (breathing tubes) sticking through the leaf skin. Creator: Tim Holmes. © Plant & Food Research. [Image: 300X]

A puparium of the Plantain leafminer, Phytomyza plantaginis (Diptera: Agromyzidae) exposed in the leaf mine: note the pair of stigma (breathing tubes) at each end of the puparium. Creator: Tim Holmes. © Plant & Food Research. [Image: 300Y]

A puparium of the Plantain leafminer, Phytomyza plantaginis (Diptera: Agromyzidae) exposed in the leaf mine: note the pair of stigma (breathing tubes) at each end of the puparium. Creator: Tim Holmes. © Plant & Food Research. [Image: 300Z]

Two images of a puparium of the Plantain leafminer, Phytomyza plantaginis (Diptera: Agromyzidae) after the adult fly had emerged: note the torn surface of the leaf mine and the upper flap of the front of the puparium that has been pushed open by the adult fly. Creator: Tim Holmes. © Plant & Food Research. [Image: 3013]

These small flies require expert knowledge for identification. However, the presence of the species can be detected by the leaf mines. The Plantain leafminer make narrow mines, mainly in the upperside, of two species of naturalised Plantains, Broad-leaved plantain, Plantago lanceolata and Narrow-leaved plantain, Plantago lanceolata.

There are several native species of plantain in New Zealand. One native plantain, Kopakopa, Plantago raoulii, that mainly lives on coastal cliffs, is the host of a native leaf miner, Liriomyza plantaginella (Diptera: Agromyzidae).

A leaf of Broad-leaved plantain, Plantago major (Plantaginaceae), with leaf mines made by larvae of the Plantain leafminer Phytomyza plantaginis (Diptera: Agromyzidae): note the dark shape of a larva in a mine (top right) Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 300U]

The upper and underside (right) of leaves of Broad-leaved plantain, Plantago major (Plantaginaceae), with leaf mines made by larvae of the Plantain leafminer Phytomyza plantaginis (Diptera: Agromyzidae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 300V]

A leaf of Narrow-leaved plantain, Plantago lanceolata (Plantaginaceae), with leaf mines made by larvae of the Plantain leafminer Phytomyza plantaginis (Diptera: Agromyzidae). Creator: Nicholas A. Martin. © Nicholas A. Martin. [Image: 305D]

The underside of a leaf of Narrow-leaved plantain, Plantago lanceolata (Plantaginaceae), with many leaf mines made by larvae of the Plantain leafminer Phytomyza plantaginis (Diptera: Agromyzidae). Creator: Nicholas A. Martin. © Nicholas A. Martin. [Image: 305E]

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

Five species of parasitoids have been reared from the Plantain leafminer. The adult female wasp usually lays an egg in the fly larva. The wasp larva feeds on the live fly larva. Some wasp larvae, such as Opius species, kill the fly after it has pupated, while others kill the fly larva before pupation. Some of the latter group of wasp larvae pupate within the fly larval skin, while others leave the larval skin before pupating.

The skin of a larval Plantain leafminer, Phytomyza plantaginis (Diptera: Agromyzidae) in its mine that has been killed a parasitic wasp (Hymenoptera) larva that is about to pupate: note the black faecal columns on either side of the wasp larvae. Creator: Tim Holmes. © Plant & Food Research. [Image: 3012]

Two images of an adult wasp (Hymenoptera) that had pupated in the leaf mine made by the Plantain leafminer, Phytomyza plantaginis (Diptera: Agromyzidae). The fly larva had been killed by the wasp larva which then pupated in the leaf mine. Creator: Tim Holmes. © Plant & Food Research. [Image: 3016]

Two images of an adult wasp (Hymenoptera) that had pupated in the leaf mine made by the Plantain leafminer, Phytomyza plantaginis (Diptera: Agromyzidae). The fly larva had been killed by the wasp larva which then pupated in the leaf mine. Creator: Tim Holmes. © Plant & Food Research. [Image: 3017]

An exit hole made by an adult wasp (Hymenoptera) that had pupated in the leaf mine made by the Plantain leafminer, Phytomyza plantaginis (Diptera: Agromyzidae). The fly larva has been killed by the wasp larva which then pupated in the leaf mine. Creator: Tim Holmes. © Plant & Food Research. [Image: 3018]

The Plantain leafminer larvae make narrow mines, mainly in the upperside, of two species of naturalised Plantains.

A leaf of Broad-leaved plantain, Plantago major (Plantaginaceae), with leaf mines made by larvae of the Plantain leafminer Phytomyza plantaginis (Diptera: Agromyzidae): note the dark shape of a larva in a mine (top right) Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 300U]

The upper and underside (right) of leaves of Broad-leaved plantain, Plantago major (Plantaginaceae), with leaf mines made by larvae of the Plantain leafminer Phytomyza plantaginis (Diptera: Agromyzidae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 300V]

A leaf of Narrow-leaved plantain, Plantago lanceolata (Plantaginaceae), with leaf mines made by larvae of the Plantain leafminer Phytomyza plantaginis (Diptera: Agromyzidae). Creator: Nicholas A. Martin. © Nicholas A. Martin. [Image: 305D]

The underside of a leaf of Narrow-leaved plantain, Plantago lanceolata (Plantaginaceae), with many leaf mines made by larvae of the Plantain leafminer Phytomyza plantaginis (Diptera: Agromyzidae). Creator: Nicholas A. Martin. © Nicholas A. Martin. [Image: 305E]

The underside of an old leaf of Narrow-leaved plantain, Plantago lanceolata (Plantaginaceae), with many leaf mines made by larvae of the Plantain leafminer Phytomyza plantaginis (Diptera: Agromyzidae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 301L]

An old leaf of Narrow-leaved plantain, Plantago lanceolata (Plantaginaceae), with leaf mines made by larvae of the Plantain leafminer Phytomyza plantaginis (Diptera: Agromyzidae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 301M]

Potential Reservoir for parasitoids of pest leaf miners of crops

Many countries have fly leaf miners belonging to the family Agromyzidae, that can cause severe damage to crops if not controlled. During the last forty years these flies have been spreading to more countries. Fortunately, there is potential for biological control using parasitic wasps (Hymenoptera). In many of these countries, the parasitic wasps that control these pest leaf mining flies also parasitize non-pest leaf miners such as the Plantain leafminer, Phytomyza plantaginis (Diptera: Agromyzidae). If the most effective of these parasitic wasps are found to be safe for release into New Zealand, the Plantain leafminer could provide a useful reservoir of the parasioids in areas close to vulnerable crops. Their host plants, Broad-leaved plantain, Plantago lanceolata and Narrow-leaved plantain, Plantago lanceolata, often live on road verges and field margins.

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.