Botanic Gardens Trust, Sydney, Australia

Two-spotted Mite - fact sheet

Mites belong to a group of animals called arachnids and are closely related to spiders. Most mites are very small: many are about the size of a pinhead, and some are as small as 200 micrometers (or 0.2 mm) in length. Like spiders, most mites have four pairs of legs. Mites that feed on plants are often mistakenly thought to be insects, but insects, of course, only ever have three pairs of legs.

The two-spotted mite belongs to a group of mites collectively known as the Spider Mites, because of their ability to produce a fine webbing of silk. This group of mites inflicts great economic damage on a huge range of agricultural and horticultural crops. Plant mites feed by first puncturing the surfaces cells of plant tissue with solid fangs or stylets, then sucking up the liberated juices with mouthparts adapted to the purpose. These mites donít have the types of combined piercing and sucking mouthparts that are seen in spiders or some groups of insects.

Symptoms of mite damage include very fine silvery mottling and flecking of upper surfaces of leaves, extreme distortion of flower and leaf buds, and the appearance of webbing covering leaves and stems. By the time that the webbing is noticeable, much damage has generally already been done. Severely damaged plant parts are usually stunted and will fall from the plant. Growth in infested plants is severely reduced, and, if the infestation is bad enough, the plant can die.

Life cycle

Two-spotted mites reproduce sexually, and the females lay eggs on buds, leaves, twigs, stems and trunks. The eggs, which are laid in vast numbers, hatch to produce nymphs which grow through a succession of moults. The first stage nymphs are six-legged; the subsequent stage produces nymphs with a full complement of eight legs. Two-spotted mites overwinter in the soil. Generation time will vary according to temperature, but in warm conditions this time can be as short as four days.

Control

Cultural: Two-spotted mites thrive in dry conditions. Plants kept in humid conditions are therefore less susceptible to attack. This can be achieved by spraying the plants regularly with a mist of water, or hosing them down if the plants are outdoors. This procedure is only useful if the plants are not sensitive to having water on their leaves (e.g. African violets and begonias spot badly when grown indoors if tap water is allowed to fall on the leaves).

Biological: Fortunately, many mites that attack plants, especially two-spotted mites, are most effectively controlled with the use of predatory mites. These actually are mites, just the same as the plant-attacking ones, except that their food consists of other species of mites. They are relatively cheap, are readily available from companies which breed natural enemies, have no toxic or other undesirable side-effects and are active long after pesticides have broken down in the environment. The most commonly used species in biological control is Phytoseiulus persimilis. Predatory mites (and insects) are available from a number of companies in Australia. For your nearest supplier, please contact the Australasian Biological Control Association.

Chemical: Most of the chemical pesticides used to control two-spotted mites are quite toxic, and also kill predatory mites. In fact, using these harmful pesticides can result in an even worse infestation, since the two-spotted mite breeds resistance to the pesticides very quickly (because of the miteís short generation time), and recovers before the predatory mite population can. Predatory mites are so efficient that use of pesticides is often not necessary anyway.


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Plant Disease Diagnostic Unit
Royal Botanic Gardens & Domain Trust
Mrs Macquaries Rd
Sydney NSW 2000
Telephone: (02) 9231 8186
Facsimile: (02) 9241 1135
Email: pddu@rbgsyd.nsw.gov.au