INTERESTING INSECTS

By N A Martin (2010, revised 2018)

Lacebark gall mite

This endemic gall mite has been found in the North and South Islands of New Zealand. It induces galls on lace bark trees and shrubs (Hoheria species) and occurs in city gardens and parks as well as native ecosystems.

Conservation status: Widespread, not threatened, sometimes a pest in gardens.

This gall mite is very tiny. Adult mites are 0.156-0.201 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 look a tiny 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 mouth parts that puncture the surface cells of young plant stems, leaves, flowers and stem galls. They suck up the cell sap. During feeding, the mites may inject saliva into the plant. The cells around the site of feeding multiply and form fleshy outgrowths with coralline internal tissues, with cavities lined with hairs and nutritive tissues. These growths form the gall, which looks like a knobbly growth on the plant. The galls usually form on young stems, but small galls may be found on leaves and flowers. The mites shelter amongst the hollows in the gall and they feed and breed there. The gall protects 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 knobbly stem galls on Hoheria (lace barks). Knobbly galls on other plants are caused by other mite species.

The only natural enemy known to be associated with this gall mite is an unnamed species of predatory gall fly (Diptera: Cecidomyiidae). Many larvae were found on the outside of a green actively growing gall that had gall mites coming out onto the outer surface of the gall.

Predatory mites may also be seen on galls and they may feed on the gall mites.

Lace bark gall mite is found on most species of Hoheria (lace bark). The galls usually form on stems, but may form on leaves or other young tissues. The form and size of galls varies among the species of Hoheria and within a species. The largest galls are on Hoheria populnea (houhere, lacebark) and may be 10-20 mm in diameter.

Table: Host plants of the Lacebark gall mite, Eriophyes hoheriae (Acari: Prostigmata: Eriophyidae) from Plant-SyNZ database (1 June 2015). 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
Mountain lacebark, Narrow-leaved houhere, Houhi, Houhi-puruhi, Puruhi	Hoheria angustifolia Raoul	Malvaceae	10	endemic
Deciduous lacebark, Mountain lacebark, Hoihere, Houhere, Houhi, Houi, Whauwhi	Hoheria glabrata Sprague & Summerh.	Malvaceae	8	endemic
Mountain lacebark, Hoihere, Houhere, Houhi, Houi, Whauwhi	Hoheria lyallii Hook.f.	Malvaceae	8	endemic
Lacebark, Hohere, Hoihere, Houhere, Houhi, Houhi ongaonga, Houi, Ongaonga, Whauahi, Wheuhi	Hoheria populnea A.Cunn	Malvaceae	10	endemic
Graceful lacebark, Lacebark, Houhere, Houhiongaonga	Hoheria sexstylosa Colenso	Malvaceae	8	endemic

The knobbly stem gall on lacebark can make specimen trees unsightly. If plants only have a few galls, these can be cut off and destroyed, which should prevent new infestations. New infestations are likely to occur when the plant is producing new growth in spring or autumn. New galls can be rubbed off while they are still small.

If a plant is heavily infested, natural enemies may eventually bring this under control.

Most miticides currently available are designed for spider mites and are not suitable for control of gall mites. If one is available for gall mites, it will only control this gall mite before it initiates a new gall. It is unlikely to kill the mites inside a 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 cause 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.

Metal outdoor signs are available for placement in reserves, Regional and National parks, urban parks and school grounds. They can be bought from Metal Images Ltd, www.metalimage.co.nz/products/botanic-labels. The Bug Signs are listed near the bottom of the ‘Fauna Species list’. The signs come in two sizes, 100 x 200 mm, 194 x 294 mm. The signs can be bought ready mounted on a stand that needs to be ‘planted’ in the ground, or they can be bought unmounted with holes for fixing into your own mounts.

The signs for the Lacebark gall mite are best placed by a shrub or tree with these distinctive galls. Old galls may survive for many years and new galls a likely to be formed every year while the plant continues to grow.

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.

1 April 2018. NA Martin. Bug signs updated

5 June 2015. NA Martin. Biostatus and Distribution: reworded. Life stage: juvenile stages revised. Recognition: photos of galls added. Natural Enemies: revised, photo of gall fly larvae added. Host plants: photograph of gall on H. populnea added. Details of Bug Signs added.