INTERESTING INSECTS

By N A Martin (2018)

Dicksonia gall mite

This endemic gall mite has been found in the North Island of New Zealand on its host plant, Rough tree fern, Dicksonia squarrosa (Dicksoniaceae). It has been found in the Kaimai Range, Bay of Plenty and Waitakere Ranges, Auckland. It is probably more widespread. The mite is unusual. It spins a web on the underside of the fern frond and lives under the webbing.

Conservation status: Not threatened.

This gall mite is very tiny. Adult female mites are about, 0.156-0.170 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 that also looks like a small adult. The last juvenile stage moults into an adult mite. There are males and females. A special feature of this species of mite is that they spin extensive webs under which they live and breed.

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 leaves and gall tissue from which they suck up the cell sap. During feeding, the mites may inject saliva into the plant.

Dispersal to new stems and new plants

When the plant grows new leaves, adult female mites disperse to these and makes new webbing. It is presumed mites walk from the old webbing shelters to the new fronds.

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 the white webbing on the underside of the fern fronds. I believe that the Dicksonia gall mite is the only member of the superfamily, Eriophyoidea in New Zealand to spin such prominent webbing.

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

The Dicksonia gall mite has only been found on the Rough tree fern, Dicksonia squarrosa (Dicksoniaceae). It may live on other species of Dicksonia. The mite makes webbing shelters on the underside of the fern frond. The mites have pointed mouth parts that puncture the surface cells of young leaves.

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 is named after Dr Uri Gerson, an Israel Entomologist who studied mites and taught at the Hebrew University of Jerusalem. On his visits to New Zealand he discovered many species of mites living on ferns.

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.