Aculus corynocarpi (Manson, 1984)
Karaka gall mite
Parulops corynocarpi Manson, 1984
Biostatus and distribution
This endemic gall mite is found on its host plant karaka Corynocarpus laevigatusi (Corynocarpaceae) which is present in the North and South Islands of New Zealand. The mite and its typical damage to leaves and flower buds has been found in gardens, parks, restoration plantings and in native habitats. The mite is also a vector for a virus of karaka that causes yellow areas on leaves growing in the shade.
Conservation status: Widespread, not threatened.
Life stages and annual cycle
This gall mite is very tiny. Adult female mites are about 0.165-0.225 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 that also looks like a small adult. The last juvenile stage moults into an adult mite. There are male and females.
The mite uses the legs for walking, but it can also hold on to the plant with the tip of its rear end, which acts as a sucker.
The mites have pointed mouth parts that puncture the surface cells of young leaves and flower buds from which they suck up the cell sap. During feeding, the mites may inject saliva into the plant. Mite feeding on young leaves and causes distortion of the leaf. Mites may feed on both leaf surfaces, but highest numbers have been seen on the underside. Mites also feed on inflorescences causing scaring on the stalks and blackening of flower buds, often leading to death of the flower bud. During feeding mites can also transmit a virus to the plant.
Dispersal to new stems and new plants
When the plant grows new leaves or produces young inflorescences, female mites disperse to these. It is presumed mites walk from old leaves and inflorescences to new ones.
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 cause distortion and scaring of leaves, scaring flower bud stalks and blackening and death of flower buds on karaka, Corynocarpus laevigatusi (Corynocarpaceae).
No natural enemies of this mite have been recorded, but predatory mites and fly larvae may feed on these mites.
The karaka gall mite, Aculus corynocarpi (Acari: Eriophyidae) only lives and breeds on karaka, Corynocarpus laevigatusi (Corynocarpaceae). It feeds on and causes distortion and scaring of young leaves, scaring of flower bud stalks and blackening and death of flower buds.
Yellow patches found on leaves, especially those in the shade are symptoms of a virus transmitted by the mite.
Young plants bought from a nursery may have karaka gall mites. Occasionally large numbers develop on shoots in early spring and cause distortion of young leaves. Keep the plant adequately watered and fertilised and it will grow out of the damage. Use of pesticides should not be needed.
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. Eriophyoidea except Eriophyinae (Arachnida: Acari). Fauna of New Zealand. 4: 1-142.
The New Zealand Plant & Food Research Institute Limited (Plant & Food Research) for permission to use photographs.