INTERESTING INSECTS

By N A Martin (2010, revised 2017)

Leather-leaf gall mite

This endemic gall mite induces witches broom galls on its host plant, leather-leaf fern, Pyrrosia eleagnifolia. In the north of New Zealand, the mite has been found in Auckland, Palmerston North, Manawatu Gorge and Rotorua. It has only been found five times, despite its host plant being common in the North Island. It appears to be restricted to places where there are old colonies the fern. However, the mite occurs on its host plant on old trees and stone walls in city gardens and parks as well as in native ecosystems.

Conservation status: Widespread, not threatened, though rarely found.

This gall mite is very tiny. Adult mites are 0.173-0.191 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. The nymph also looks like a small adult. The last juvenile stage moults into an adult mite. There are males and females.

Walking

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

Feeding and forming the gall

The mites have pointed mouthparts that puncture the surface cells near the growing tips of stems of the fern. They suck up the cell sap. During feeding, the mites may inject saliva into the plant. This induces the plant to produce a cluster of stems that are called a "witches’ broom". The mites live under the scales on the young expanding stems where they shelter, and they feed and breed there. The dense scales on the stems protect the gall mites from predators and adverse weather.

Dispersal to new stems and new plants

When the plant grows new shoots, adult female mites disperse to these and their feeding induces the formation of new galls. It is presumed that some 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 from associated plant damage symptoms. This mite species is the only one known to induce witches’ broom galls on the leather-leaf fern, Pyrrosia eleagnifolia. Witches’ broom galls on other plants are caused by other mite species.

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

The leather-leaf gall mite, Acerimina pyrrosiae (Acari: Eriophyidae) only lives on the leather-leaf fern, Pyrrosia eleagnifolia (Polypodiaceae). Mite feeding induces the proliferation of stems to form a "witches’ broom" gall.

Eriophyid gall mites belong to the superfamily Eryiophyoidea. These mites have several unusual features. For example, though most mites have four pairs of legs like spiders, Eriophyoidea 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.

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

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.

Landcare Research New Zealand Limited (Landcare Research) for permission to use photographs.

1 August 2017, NA Martin. Photo of live mites added

1 Feb 2014. NA Martin. Suggested citation added

11 October 2012. NA Martin. Life cycle: juvenile stages reworded