INTERESTING INSECTS

By N A Martin (2018)

Hebe leaf blister gallfly, Hebe midleaf gall midge

This endemic fly induces leaf blister galls in two species of large-leaved Hebe, Veronica species (Plantaginaceae). Based on the presence of galls, it has been found in Auckland, Waikato and Wellington Regions and presumably it is present throughout the North Island. The galls have mainly been found in native ecosystems.

Conservation status: Present in the North Island, not threatened.

Little in known about the fly’s annual cycle. There is evidence indicating two generations per year, with adults emerging from galls in spring and autumn.

The adult flies are small and delicate. They are about 3 mm long (body and wings folded. The body is brown with a darker brown dorsal (top) side of the thorax (middle part of body). The head has large black compound eyes. The body and wings are covered in dense setae (hairs). The fly has three pairs of long dark legs. Like a typical fly it has 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 dark grey. The male has hairier antennae that presumably assist it in finding a female. The male has external genitalia at the end of the abdomen that protrude upwards, while the female has a swollen abdomen with a tapered end. After mating the female uses her ovipositor to insert eggs into host plant leaves. They are usually laid in a row along the midrib of a leaf.

Eggs are inserted along the midrib of a young leaf. The newly hatched larvae form a chamber under the midrib. The leaf tissue on either side of the midrib is stimulated by the presence of the eggs and larvae to grow and form the gall. The larva is white at first, but as it enlarges it becomes orange coloured. The larva moults, or changes skin, as it gets larger. There are three larval instars (stages). Larvae have a special structure near their head on the ventral (under) side. This dark structure varies in shape, but is present in all gallfly (Cecidomyiidae) larvae. It is believed to be involved with feeding.

The last larval instar is yellow and when fully grown is 3-4 mm long. Prior to pupation the mature larva makes a tunnel to the surface of the gall, but leaves the surface skin of the leaf intact. It also constructs a cell in the cavity of the gall. The pupa is yellow and the legs, antennae and wing buds are visible. There are two spikes on the head.

Fly emergence

When the adult fly is ready to emerge, the pupa wriggles up to the surface of the leaf and with the spikes on its head, it pierces the epidermis (skin) of the leaf covering the tunnel. The pupal skin splits and the adult pulls itself out, its wings expand the wings and body harden before it flies away.

Side view of an adult male Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae): note the long setae (hairs) on the antennae and the external genitalia at the end of the abdomen. Creator: Tim Holmes. © Plant & Food Research. [Image: 2R2U]

Top view of an adult male Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2R2W]

Front view of an adult female Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2R2N]

Underside of an adult female Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae): note the fat abdomen full of eggs. Creator: Tim Holmes. © Plant & Food Research. [Image: 2R2Q]

Section through a leaf of a Hebe, Veronica macrocarpa (Plantaginaceae), showing cells (holes) in two leaf blister galls induced by Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae). One gall has a pale larva (right). Creator: Tim Holmes. © Plant & Food Research. [Image: 2R2K]

Underside of a leaf blister gall in Hebe, Veronica macrocarpa (Plantaginaceae), made by Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 2R2X]

Leaf blister gall in Hebe, Veronica macrocarpa (Plantaginaceae), dissected to show insect larva (white) and two pupa of Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae), one pupa in cell and the other removed. Creator: Tim Holmes. © Plant & Food Research. [Image: 2R2H]

Pupa of Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae), Creator: Tim Holmes. © Plant & Food Research. [Image: 2R2F]

Pupa of Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae), Creator: Tim Holmes. © Plant & Food Research. [Image: 2R2E]

Pupa of Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae), shortly before adult emergence. Creator: Tim Holmes. © Plant & Food Research. [Image: 2R2I]

Pupa of Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae), shortly before adult emergence. Creator: Tim Holmes. © Plant & Food Research. [Image: 2R2J]

Side view of an adult female Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae): note the fat abdomen full of eggs. Creator: Tim Holmes. © Plant & Food Research. [Image: 2R2M]

The identification of these gall flies requires expert knowledge. However, the occurrence of the flies in a habitat can be recognised by the presence the leaf blister galls on its host plants, two species of Hebe, Veronica macrocarpa and Veronica stricta (Plantaginaceae).

Leaf blister galls are found in other species of Hebe (Veronica) in other parts of New Zealand and with different leaf form. The species of fly causing these galls are not known.

Section through a leaf of a Hebe, Veronica macrocarpa (Plantaginaceae), showing cells in two leaf blister galls induced by Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2R2L]

Leaf blister gall in Hebe, Veronica macrocarpa (Plantaginaceae), made by Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 2R2Y]

Top view of an adult male Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2R2W]

Predators

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

Parasitoids

One species of wasp parasitoid of is known from Hebe leaf blister gallfly, though it has not been described and named.

Three species parasitoid wasps (Hymenoptera) were reared and photographed. Two kinds of parasitoid wasp pupae were found. One kind has a naked pupa that show the legs, antennae and wing buds, whereas the other kind has the pupa in an oval skin.

Some parasitoids of gall flies lay their eggs in eggs of the gallfly. The wasp larva usually does not kill the gallfly larva until the fly larva is fully grown.

White larva of a parasitoid (Hymenoptera) of Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae), in a leaf blister gall in a Hebe, Veronica macrocarpa (Plantaginaceae), Creator: Tim Holmes. © Plant & Food Research. [Image: 2R2B]

Pupae of two parasitoids (Hymenoptera) of Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae), in a leaf blister gall in a Hebe, Veronica macrocarpa (Plantaginaceae), Creator: Tim Holmes. © Plant & Food Research. [Image: 2R2A]

Row of pupae of a parasitoid (Hymenoptera) of Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae), in a leaf blister gall in a Hebe, Veronica macrocarpa (Plantaginaceae), Creator: Tim Holmes. © Plant & Food Research. [Image: 2R29]

Top view of adult female parasitoid (Hymenoptera) of Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae): note dark patch on wings. Creator: Tim Holmes. © Plant & Food Research. [Image: 2R22]

Underside of adult female parasitoid (Hymenoptera) of Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae): note dark patch on wings. Creator: Tim Holmes. © Plant & Food Research. [Image: 2R21]

Underside of adult male and female parasitoids (Hymenoptera) of Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae): note dark patch on wings. Creator: Tim Holmes. © Plant & Food Research. [Image: 2R24]

Top view of adult parasitoid (Hymenoptera) of Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae): note dark patch on wings. Creator: Tim Holmes. © Plant & Food Research. [Image: 2R25]

Top view of adult parasitoid (Hymenoptera) of Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae): note dark patch on wings. Creator: Tim Holmes. © Plant & Food Research. [Image: 2R26]

Top view of adult parasitoid (Hymenoptera) of Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2R28]

Underside of adult female parasitoid (Hymenoptera) of Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2R27]

Top view of adult parasitoid (Hymenoptera) of Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae), Creator: Tim Holmes. © Plant & Food Research. [Image: 2R2S]

Underside of adult parasitoid (Hymenoptera) of Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae), Creator: Tim Holmes. © Plant & Food Research. [Image: 2R2R]

This species of Hebe gallfly causes leaf blister galls in two large leaved shrubby Veronica species that are found in the North Island. The name of the Veronica shrubs on which the gallfly was first discovered were given as Hebe (=Veronica) salicifolia. The plant genus has since been revised and the plants in that area are now called Veronica stricta and Veronica macrocarpa. Hence, Veronica salicifolia which is only found in the South Island, is not a valid host plant. Leaf blister galls are found in other species of Hebe (Veronica), but are probably caused by unnamed species of gallfly.

The adult female fly lays it eggs in a young leaf by the midrib, and sometimes by a side vein. After the egg hatch, the plant tissue in the vicinity of the feeding larvae multiplies to form a swelling on either side of the midrib or vein.

Leaf blister gall in Hebe, Veronica macrocarpa (Plantaginaceae), made by Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 2R2Y]

Underside of a leaf blister gall in Hebe, Veronica macrocarpa (Plantaginaceae), made by Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 2R2X]

Section through a leaf of a Hebe, Veronica macrocarpa (Plantaginaceae), showing cells in two leaf blister galls induced by Hebe leaf blister gallfly, Dasineura hebefolia (Diptera: Cecidomyiidae). Creator: Tim Holmes. © Plant & Food Research. [Image: 2R2L]

Diversity of gallfly (Cecidomyiidae) life styles

The first formally described gall flies, Cecidomyiidae, were as their name suggests associated with galls on plants of interest to humans. Since their original discovery and naming as Cecidomyiidae, it was found that this family of flies has members with wider range of life styles that include beneficial species and important pests. Others are potentially useful biodiversity indicators.

Among the beneficial species are gall flies that are predators! These species have larvae that feed other insects such as aphids and mealybugs, and mites such as spidermites (Tetranychidae) and gall mites (Eriophyoidea). These predatory gall flies contribute to the biological control of pests of crops. Some species are reared in large numbers for release into greenhouse crops.

Other species of Cecidomyiidae feed on fungi. These include species that may be seen on the surface of plant leaves feeding on the spores of rust fungi. Other gall flies feed on fungi that we call toadstools and mushrooms. These include species that are pests of commercially grown mushrooms. One group has an amazing life cycle, where larvae give birth to more larvae (paedogenesis).

Of the species living on plants, many induce galls though some do not. Those not inducing galls include species living in flowers or seed heads of plants. Others feed on young leaves in expanding buds. Most plant feeding Cecidomyiidae induce galls. Galls are growths on plants formed of plant tissue, but caused (induced) by other organisms. Galls associated with Cecidomyiidae are usually induced by feeding of larvae. Galls may be formed in any actively growing tissue. There is a huge diversity of galls on New Zealand native plants. There are also pest species form other countries that affect crops and ornamental plants such as Hessian fly (Mayetiola destructor) on grasses and barley, Apple leafcurling midge (Dasineura mali) on apple trees and Chrysanthemum gall midge (Rhopalomyia chrysanthemi) on Chrysanthemum leaves. Some gall inducing species have been released into New Zealand to assist the biological control of weeds, e.g. St John’s wort gall midge (Zeuxidiplosis giardia) to control St John’s wort (Hypericum perforarum).

Diversity gall forming Cecidomyiidae in New Zealand

Seven species of gall forming Cecidomyiidae in New Zealand have been described and formally named. However, recent studies have shown that there probably more than 150 species and perhaps more than 200. The seven named and described species include those that form leaf blister galls, bud galls, stem galls and root galls. Some groups of plants have a large diversity of galls, for example many species of small-leaved Coprosma (Rubiaceae) have galls, some species have up to five types of gall. There are usually two types of bud gall, one or two kinds of stem gall, a shoot tip gall and a leaf blister gall. Olearia (Compositae) species may have one or more of three kinds of leaf blister gall and a bud gall. While Hebe, Veronica species (Plantaginaceae) may also have up to five kinds of galls.

Plant galls are distinctive and easily recognised. At present it is assumed most of them are specific to a particular plant species or plant genus. Plant galls are thus potentially useful biodiversity indicators that could be used in surveys by non-experts. This could be done by providing surveyors with pictorial guides for each species of plant in habitats to be surveyed. How many galls can you find on native plants?

Hebe stem galls caused by an unnamed species of gallfly (Diptera: Cecidomyiidae) on two shoots of Veronica stricta (Plantaginaceae): note the swellings on the stems of both shoots. © All rights reserved. [Image: 2R3H]

Hebe stem galls caused by an unnamed species of gallfly (Diptera: Cecidomyiidae) on shoot of Veronica stricta (Plantaginaceae): note the exit holes in the galls made by the flies or parasitoids. © All rights reserved. [Image: 2R3I]

Hebe terminal bud gall caused by an unnamed species of gallfly (Diptera: Cecidomyiidae) in Veronica stricta (Plantaginaceae): note the swollen base of the bud. © All rights reserved. [Image: 2R3J]

Hebe terminal bud gall caused by an unnamed species of gallfly (Diptera: Cecidomyiidae) in Veronica stricta (Plantaginaceae): note the swollen base of the bud. © All rights reserved. [Image: 2R3K]

A terminal bud gall on Veronica stricta (Plantaginaceae) cut open to show the white larvae of the unnamed species of gallfly (Diptera: Cecidomyiidae) that induced the gall. © All rights reserved. [Image: 2R3L]

Small flower bud galls (left) and Large flower bud gall caused by two unnamed species of gallfly (Diptera: Cecidomyiidae) on the shrub Coprosma spathulata (Rubiaceae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 2R3F]

Large flower bud galls (left) and Small flower bud gall caused by two unnamed species of gallfly (Diptera: Cecidomyiidae) on the shrub Coprosma spathulata (Rubiaceae). Creator: Nicholas A. Martin. © Plant & Food Research. [Image: 2R3G]

Stem gall caused by an unnamed species of gallfly (Diptera: Cecidomyiidae) on Red mapou, Myrsine australis (Primulaceae). © All rights reserved. [Image: 2R3P]

Shoot tip gall caused by an unnamed species of gallfly (Diptera: Cecidomyiidae) on Red mapou, Myrsine australis (Primulaceae). Creator: Nicholas A. Martin. © Nicholas A. Martin. [Image: 2R3N]

Shoot tip gall on Red mapou, Myrsine australis (Primulaceae) cut open to show the larvae of an unnamed species of gallfly (Diptera: Cecidomyiidae) that induced the gall. Creator: Tim Holmes. © Plant & Food Research. [Image: 2R3M]

Stem gall caused by an unnamed species of gallfly (Diptera: Cecidomyiidae) on Red mapou, Myrsine australis (Primulaceae): note the exit holes in the gall made by the flies or parasitoids. © All rights reserved. [Image: 2R3Q]

Lamb KP. 1951. A new species of gall midge (Cecidomyiidae) from Hebe salicifolia Forst. leaf galls. Transactions of the Royal Society of New Zealand. 79 (2): 210-212.

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

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