INTERESTING INSECTS

By N A Martin (2017)

Hebe stem gall mite

This endemic gall inducing flat mite has been found in the North and South Islands of New Zealand on its host plants, shrubby Veronica species, previously called Hebe. Though stem galls are common only around Auckland and at the type locality, Rangitoto Island. The Hebe stem gall mite, Dolichotetranychus ancistrus is the only New Zealand species of flat mite (Tenuipalpidae) known to induce galls.

Conservation status: Widespread, not threatened.

This gall inducing flat mite is tiny. Adult mites are about 1 mm long. The adult mite is longer than wide and has four pairs of legs. All life stages are red. Adult female mites lay spherical eggs in the cavity of the gall. The larva that hatches from an egg looks like a tiny adult, but only has three pairs of legs. The mite larva moults (changes skin) into a nymph which has four pairs of legs like the adult. The first nymphal stage moults into a second nymph. The last juvenile stage moults into an adult mite. There are males and females.

Feeding and inducing the gall

The mites have pointed mouth parts that puncture the surface cells of young stems 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 young stems, it is believed that stem cells in the area multiply causing a thickening of the stem and a complex invagination in which the mites live. The mites shelter, feed and breed inside the gall. The gall protects the gall mites from predators and adverse weather. The gall remains active for more than 12 months.

Dispersal to new stems and new plants

When the plant grows new stems, 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 "Hebe" (shrubby Veronica) plant can be recognised by presence of the woody stem galls with red mites inside.

Stem galls may also be formed on shrubby Veronica species by an unnamed species of gall fly (Cecidomyiidae). These have fly maggots inside and large circular exit holes for the adult fly and parasitic wasps.

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

Hebe stem gall mite, Dolichotetranychus ancistrus (Acari: Tenuipalpidae). lives on native species of shrubby Veronica (Plantaginaceae). Mite feeding on young stems induces stem galls. Galls have been found on two species of Veronica and there is a strong association of mites with a third species of plant. The published association with a fourth species, Hebe salicifolia (Veronica salicifolia), is based on North Island specimens of ‘Hebe’ that after a revision of the genus are now called Veronica stricta.

Table: Host plants of the Hebe stem gall mite, Dolichotetranychus ancistrus (Acari: Prostigmata: Tenuipalpidae) from Plant-SyNZ database (23 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
Hebe, Kōkōmuka	Veronica macrocarpa Vahl	Plantaginaceae	10	endemic
Green box, Hebe	Veronica odora Hook.f.	Plantaginaceae	7	endemic
Hebe	Veronica stricta Banks & Sol. ex Benth. var. stricta	Plantaginaceae	10	endemic

Tenuipalidae

The Hebe stem gall mite, Dolichotetranychus ancistrus is the only New Zealand species of flat mite (Tenuipalpidae) known to induce galls. There is another species of mite in the same genus in New Zealand, Dolichotetranychus alpines Collyer, 1973, but its host plants are unknown.

The other New Zealand species of Tenuipalpidae, have the appearance of very flat mites. They are often red or transparent and live on plant leaves or young stems. They feed on plant cells.

What is a type locality?

When a species is described and named in a scientific publication, one specimen is designated as the holotype. It is the specimen on which the description is based. Some of the other specimens, usually from the same locality, that are used to help with the description are designated as paratypes. The place from where they are collected is designated the ‘type locality’.

The Holotype of Dolichotetranychus ancistrus was from galls collected from Rangitoto Island, Auckland, on a ‘Hebe’ bush near the wharf. This area is the type locality for the species. If the holotype and paratypes are lost or destroyed, then if required, fresh specimens of the species can be collected from the type locality.

Baker EW, Pritchard AE. 1956. False spider mites of the genus Dolichotetranychus (Acarina: Tenuipalpidae). Hilgardia. 24 (13): 357-381.

Collyer E. 1973. A new species of the genus Dolichotetranychus (Acari: Tenuipalidae) from New Zealand. New Zealand Journal of Science. 16: 747-749.

Zhang Z-Q, Fan Q-H 2004. Redescription of Dolichotetranychus ancistrus Baker & Pritchard (Acari: Tenuipalidae) from New Zealand. Systematic & Applied Acarology 9: 111-131.

The New Zealand Plant & Food Research Institute Limited (Plant & Food Research) for permission to use photographs.