INTERESTING INSECTS

By N A Martin (2017)

Glasswort gall mite

This endemic gall mite has only been found in Auckland. It was discovered in the Waitemata harbour in 1948 and rediscovered in 2013 in the estuary near Kaiua, which is near Miranda on the Firth of Thames. It occurs on its host plant, glasswort, Sarcocornia quinqueflorae (Amaranthaceae) in areas sheltered by mangroves. Feeding by the mite induces pocket galls on young stems.

Conservation status: Only known from Auckland, but probably widespread on its estuarine host plant.

This gall mite is very tiny. Adult mites are about 0.2-0.3 mm long. The adult mite is like a tiny white cow’s horn with two pairs of legs at the wide end of the horn. Adult female mites lay tiny spherical eggs. The larva that hatches from an egg looks like a tiny adult. The mite larva moults (changes skin) into a nymph. There is one nymphal stage which also looks like a small adult. The last juvenile stage moults into an adult mite. There are males and females.

Walking

The mite uses the legs for walking, but it can also hold on to the plant with the tip of its abdomen, which acts as a sucker.

Feeding and inducing the gall

The mites have pointed mouth parts that puncture the surface cells of young stems and gall tissue from which they suck up the cell sap. During feeding, the mites may inject saliva into the plant. When mites feed on young stems, stem cells in the area multiply causing a thickening and expansion of the stem tissue, which forms an invagination with thick pink lips that almost join. The mites can leave and re-enter through the narrow opening in the gall. The mites shelter, feed and breed inside the gall. The gall protects the gall mites from predators and adverse weather.

Dispersal to new stems and new plants

When the plant grows new stems, adult female mites disperse to these and their feeding induces the formation of new galls. It is presumed mites walk from the old galls to the new growths.

When this gall mite colonises new plants, it is unlikely that mites walk all the way. It is believed that most mites are dispersed by wind. Some species of mite climb to prominent places on plants and stand waiting for a gust of wind to take them away.

This mite requires special procedures and taxonomic knowledge to identify specimens. However, its presence on a plant can be recognised by plant damage symptoms. This mite species is the only one known to induce pocket galls on stems of glasswort, Sarcocornia quinqueflorae (Amaranthaceae). Pocket galls on other plants are caused by other mite or insect species.

No natural enemies of this mite have been recorded, but predatory mites and insects may feed on these mites.

Glasswort gall mite, Aceria rubifaciens (Acari: Eriophyidae) is only known to live on shoots of glasswort, Sarcocornia quinqueflorae (Amaranthaceae). Mites feeding on young stems induce pocket galls on both vegetative shoots and flowering shoots.

Eriophyid gall mites belong to the super family Eryiophyoidea. These mites have several unusual features. For example, though most mites have four pairs of legs like spiders, Eriophyoid mites have only two pairs of legs. Many of these mites can induce host plants to form galls, some of which may be very complex. Some species of these mites can transmit plant viruses that may cause plant diseases and plant death.

This gall mite was discovered and described by a New Zealand scientist, Dr Kenneth Lamb, who studied gall forming mites and insects during the 1950s. He discovered many kinds of galls on native plants.

Lamb KP 1953. New plant galls: II, Description of gall mites and the galls which they cause. Transactions of the Royal Society of New Zealand 80: 371-382.

Manson DCM 1984. Eriophyinae (Arachnida: Acari: Eriophyoidea). Fauna of New Zealand 5: 1-123.

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