In 1992 infestations of hibiscus erinose mite were reported on the Gold Coast and in northern New South Wales. The problem was so widespread that eradication was not feasible. Unconfirmed reports suggest the mite may have become established in New South Wales as early as 1978.
The problem is now widespread in South East Queensland. In wider Queensland, infestations have been detected at Hervey Bay and in Cairns and surrounding areas.
Aceria hibisci (Syn. Eriophyes hibisci)
Damage is most clearly seen on the young leaves and the developing vegetative buds of hibiscus shrubs. Symptoms include extreme pimpling of the leaves, leaf axillary buds, petioles and calyces. A common characteristic is the production of pockets of galled tissue, which are most obvious on the lower surface of the leaf as yellow, velvety overgrowth. The leaf axils are particularly distorted, as are the proximal margins of the leaves.
The microscopic erinose (eriophyid) mite is also known as the hibiscus erinose mite, the hibiscus erineum mite or the hibiscus leaf-crumpling mite. It is found in Brazil, Hawaii and South Sea Islands such as Tonga and Fiji. It was probably introduced to Australia on illegally imported hibiscus cuttings.
Erinose mites are worm-like in appearance and have only two pairs of legs. They inject salivary compounds into expanding green tissue. This stimulates a plant reaction far out of proportion to the size of the mite, whose presence is indicated by the velvet-textured galls.
The mites are so small (160-185 micrometres) that they are difficult to detect even on the highest magnification of a stereo microscope.
Erinose galls often have a range of more readily visible mites such as predatory mites (phytoseiids) that feed on the eriophyids. Unfortunately, in this case the predators are apparently not efficient enough to prevent galling.
Hibiscus erinose mite is highly host specific and has only been recorded on ornamental hibiscus and overseas in okra, another member of the Malvaceae plant family.
Upright and weeping forms and Indian and Hawaiian types of Hibiscus rosa-sinensis are affected. Affected okra has not been seen in Australia.
Transmission occurs on cuttings or perhaps via transfer in wind currents.
Neglected plants appear to be at a greater risk of infestation. As a precaution, growers have been advised not to take cuttings, even from apparently healthy plants, from affected to non-affected districts. East coast hibiscus cuttings from affected districts and areas adjacent to them are subject to interstate quarantine regulations in Western Australia.
As active plant growth is needed for the mites to establish, hardened older growth will not develop symptoms when exposed to the mite. Colonies of mites move onto new growth as the galls age. In severe cases, whole branches and plants are affected.
Generally, pruning of affected tissue does not provide control without an appropriate spray program. Growers who have cut back their infested plants and allowed them to regrow have reported that the new leaves were still infested.
Affected plants should be pruned to improve their appearance prior to spraying. Existing galls will persist on the plant until the affected tissue dies, making it difficult to judge the success of any spray program. Spraying is carried out to protect expanding leaves and new buds. Sprays at monthly intervals throughout the year have afforded protection in high-risk areas. More frequent sprays may be needed during the main growth period.
There are no registered chemical controls for A. hibisci. Miticides with a generic registration against mites, such as wettable sulphur or maldison, may be of assistance.
To avoid spreading this problem, hibiscus enthusiasts are reminded not to move cuttings from infested areas into other districts. Badly affected shrubs and prunings should be removed and either burnt, buried or taken to the dump in an enclosed plastic bag.