INTERESTING INSECTS

By N A Martin (2019)

Titirangi gall mite

This gall mite is named after, Titirangi, the area of west Auckland in which the Holotype specimen was found on a Elaeocarpus dentatus (Elaeocarpaceae) tree.

This endemic gall mite is found on leaves four species of native tree, Hīnau, Elaeocarpus dentatus and Pōkākā, Elaeocarpus hookerianus (Elaeocarpaceae), Rewarewa, Knightia excelsa (Proteaceae) and Tāwheowheo, Quintinia serrata (Paracryphiaceae). Feeding by the mites induces the young leaves to grow dense white hairs, erineum. The mite is found on its host plants in the North & South Islands of New Zealand.

Conservation status: This mite species is not threatened.

This gall mite is very tiny. Adult female mites are about, 0.119-0.135 mm long. The adult mite is like a tiny white worm-like 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 forming the erineum gall

The mites have pointed mouth parts that puncture the surface cells of plant leaves. They suck up the cell contents. During feeding, the mites may inject saliva into the plant. When this happens on young expanding leaves, chemicals in the saliva induce the plant to grow hairs on the leaf surface. These growths form the gall called an 'erineum'. The mites shelter amongst the hairs, and feed and breed there. The erineum protects the gall mites from predators. The gall may also maintain a high humid atmosphere around the mites.

Dispersal to new leaves and plants

When the plant grows new shoots, adult female mites disperse to the new leaves and their feeding causes a new erineum to form. It is presumed some mites walk from the old leaves to the new growths. When this gall mite colonises new plants, it is unlikely that mites walk all the way. It is possible that mites could be transferred on leaves, but 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 an erineum on leaves of its four host plant species, Hīnau, Elaeocarpus dentatus and Pōkākā, Elaeocarpus hookerianus (Elaeocarpaceae), Rewarewa, Knightia excelsa (Proteaceae) and Tāwheowheo, Quintinia serrata (Paracryphiaceae). The erineum is mainly induced on the underside of young leaves. The leaf often bulges upwards in the area above the erineum. Old erineum turns dark brown.

Erineum on other plants is caused by other species of mite.

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

The Titirangi gall mite is found on leaves of four species of endemic trees belonging to three different families. Feeding by the gall mites induces the plant to grow white hairs that are called erineum. The erineum is mainly found in patches on the underside of leaves. On some the host plants the erineum is found where a side vien joins the main vien. When this happens the tissue forms a hollow on the underside and a swelling on the upper side of the leaf. This structure superficially looks like the domatia found on leaves of other trees such as Coprosma species. The older erieneum becomes brown.

Table: Host plants of the Titirangi gall mite, Aceria titirangiensis (Acari: Prostigmata: Eriophyidae) from Plant-SyNZ database (27 January 2019). 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
Hangehange, Hīnau, Pōkākā, Whīnau	Elaeocarpus dentatus (J.R.Forst. & G.Forst.) Vahl	Elaeocarpaceae	10	endemic
Mahimahi, Pōkākā, Puka, Whīnau	Elaeocarpus hookerianus Raoul	Elaeocarpaceae	10	endemic
New Zealand honeysuckle, Rewarewa	Knightia excelsa R.Br.	Proteaceae	10	endemic
Quintinia, Kūmarahou, Tāwheowheo	Quintinia serrata A.Cunn.	Paracryphiaceae	10	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.

Lamb KP. 1953. New plant galls. II. Description of seven new species of gall mites and the galls they cause. Transactions of the Royal Society of New Zealand. 80 (3-4): 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.