Mangrove erineum mite - Acalitus avicenniae
By N A Martin (2017)
Biostatus and Distribution
This endemic gall mite is only found in the northern half of the North Island of New Zealand where its host plant grows, the New Zealand subspecies of mangrove, Avicennia marina (Forsk.) Vierh. subspecies australasica (Walp.) J.Everett (Verbenaceae). Feeding by the mite induces the plant to produce erineum galls on the underside of leaves.
Conservation status: Common on mangroves, not threatened.
Life Stages and Annual Cycle
This gall mite is very tiny. Adult mites are about 0.11-0.14 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.
The mite uses its 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 underside of young expanding leaves, leaf cells in the area multiply causing a thickening of the leaf and increase in length of the hairs, which form an erineum. The mites shelter, feed and breed inside the erineum. The erineum 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 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 erineum on leaves of mangrove, Avicennia marina (Verbenaceae) in New Zealand. Erineum on other plants are caused by other mite species. The chlorotic (pale) areas on the upper side of leaves is a good indication that erineum is present on the underside of the leaf.
No natural enemies of this mite have been recorded, but predatory mites may feed on these mites. The white growth was found associated with galls may have been a fungal pathogen or a saprophyte of dead gall tissue.
The mangrove erineum mite, Acalitus avicenniae (Acari: Eriophyidae), lives on leaves of the mangrove subspecies growing in New Zealand, Avicennia marina (Forsk.) Vierh. subspecies australasica (Walp.) J.Everett (Verbenaceae). Mites feeding on the underside of young leaves induces the development of erineum with pale yellow (chlorotic) areas on the upper side of the leaf. The erineum is an area of longer leaf hairs on swollen leaf tissue. High numbers of mites on young leaves result in many patches of erineum and distorted 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.
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.