INTERESTING INSECTS

By N A Martin (2017)

Mahoe leaf roll mite

This endemic gall mite is found in the North and South Islands of New Zealand on its host plants, Melicytus species (Violaceae). Mites feeding on young expanding leaves induce the leaf-edge roll gall. These galls are particularly common on mahoe, Melicytus ramiflorus, and are usually seen on shrubs in parks and restoration plantings.

Conservation status: Widespread, not threatened.

This gall mite is very tiny. Adult female mites are about 0.159 - 0.174 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.

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. When mites feed on the upper side of the edge of young leaves, the cells on the upper side grow more slowly than the cells on the underside of the leaf. This causes the leaf edge to form a tight roll. The mites live feed and breed within this roll. The leaf edge roll gall protects the gall mites from predators and adverse weather.

Dispersal to new stems and new plants

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

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

This family of mites require special procedures and taxonomic knowledge to identify specimens. However, the presence of this species on a plant can be recognised by plant damage symptoms. Mahoe leaf roll mite is the only species known to induce leaf edge roll galls on its three known host plants, mahoe Melicytus ramiflorus, large-leaved mahoe, M. macrophyllus and swamp mahoe M. micranthus (Violaceae). Leaf edge roll galls on other plants are caused by other species of mite or insects.

No natural enemies of the mahoe leaf roll mite are recorded, but predatory mites may feed on this herbivore.

The mahoe leaf roll mite, Aceria melicyti (Acari: Eriophyidae) is the only species known to induce leaf edge roll galls on its three known host plants, mahoe Melicytus ramiflorus, large-leaved mahoe, M. macrophyllus and swamp mahoe M. micranthus (Violaceae). Mites feed on the upper side of the edge of young leaves. Feeding results in the cells on the upper side grow more slowly than the cells on the underside of the leaf. This causes the leaf edge to form a tight roll. The mites live feed and breed within this roll.

Table: Host plants of the Mahoe leaf roll mite, Aceria melicyti (Acari: Prostigmata: Eriophyidae) from Plant-SyNZ database (22 January 2017). The reliability score shows the quality of evidence for the host association (1-10, 10=high).

Common Name(s)	Scientific Name	Family	Reliability Index	Biostatus
Large-leaved mahoe, Māhoe	Melicytus macrophyllus A.Cunn.	Violaceae	9	endemic
Swamp mahoe, Manakura	Melicytus micranthus (Hook.f.) Hook.f.	Violaceae	10	endemic
Whiteywood, Hinahina, Inaina, Inihina, Māhoe, Moeahu, Kaiweta	Melicytus ramiflorus J.R.Forst. & G.Forst.	Violaceae	10	non-endemic

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.

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.