INTERESTING INSECTS

By N A Martin (2019)

Beech witches broom mite

Eriophyes waltheri Keifer, 1939

Aceria waltheri (Keifer, 1939)

The Beech witches broom mite was first found on a New Zealand Silver beech tree, Lophozonia menziesii (Nothofagaceae) that had been planted in California. At that time the scientific name for Silver beech trees was Nothofagus menziesii. The mites were described and named by a Californian scientist, H.H. Keifer in 1939 as Eriophyes waltheri.

This endemic gall mite is found on silver beech, Lophozonia menziesii (Nothofagaceae) in the North and South Islands. The tree used to be called Nothofagus menziesii. The mite is associated with witches broom galls found on this species of tree. However, mites are sometimes found in other sheltered sites on this and other native beech trees.

Conservation status: The mite is not threatened.

This gall mite is very tiny. Adult female mites are about, 0.130-0.209 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.

There are two forms of the adult female. The form associated with inducing the galls and the main multiplication of the population is the protogyne. The other form, the deutogyne, is associated with dispersal to new branches, trees and survival when no new shoots are being produced by host trees. One difference between the two forms is that the deutogynes of this species have fewer microtubercles on their body.

Walking

The mites use their legs for walking, but can also hold on to the plant with the tip of thier abdomen, which acts as a sucker.

Feeding and forming the galls

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. In the area where the mites are feeding in opening buds, the tree produces shoots with many buds. When multiple buds develop producing a cluster of short stems, this is called a witches broom. The mites shelter in the gall and breed there. The galls protect the gall mites from predators. The gall may also maintain a high humid atmosphere around the mites.

Dispersal to new shoots and trees

In the summer when the tree stops growing new shoots and the witches broom gall is less suitable for breeding, it is presumed that the mite population produces deutogynes. This form of the mite is better able to withstand adverse conditions. It has been found sheltering on leaves with erineum, hairy growths induced by the feeding of another species of gall mite. It is unlikely that the dispersing mites walk all the way to new suitable shoots on their current host tree or another tree. It is believed that many 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 a witch's broom galls on Silver beech, Lophozonia menziesii (Nothofagaceae).

Similar galls on other plants are caused by other species of mite, insect or microorganism.

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

The Beech witches broom mite is found on its host plant, Silver beech, Lophozonia menziesii (Nothofagaceae) where it lives in buds that are part of Witches broom galls which is induced by the mites feeding on opening buds. These galls are a proliferation of shoots. The dispersal and overwintering stage, the deuteronyne is sometimes found in leaf erineum of silver beech and red beech, Fuscospora fusca.

Beech witches broom mites have pointed mouth parts that puncture the surface cells of young buds and leaves. The plant responds to their feeding by producing more buds that are colonised by the mites. This compact kind of growth is commonly called a witch's broom that is named after fungal induced galls on British birch trees.

Table: Host plants of the Beech witches broom mite, Cymoptus waltheri (Acari: Prostigmata: Eriophyidae) from Plant-SyNZ database (7 May 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
Red beech, Hutu, Hututawai, Raunui, Tawai, Tawhai	Fuscospora fusca (Hook.f.) Heenan & Smissen	Nothofagaceae	6	endemic
Silver beech, Tawai, Tawhai	Lophozonia menziesii (Hook.f.) Heenan & Smissen	Nothofagaceae	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.

Manson DCM 1984. Eriophyinae (Arachnida: Acari: Eriophyoidea). Fauna of New Zealand 5: 1-123.

Manson, D. C. M. and G. N. Oldfield. 1996. Biology and Ecology. 1.4.1. Life forms, deuterogyny, diapause and seasonal development, pp. 173-183. In: Lindquist, E. E., M. W. Sabelis and J. Bruin (Eds.). Eriophyoid Mites - Their Biology, Natural Enemies and Control. Elsevier, 790 pp.

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