HortFACT
Identification
The two-spotted mite, Tetranychus urticae, has several other common names including two-spotted spider mite, red spider mite, glasshouse red spider mite, and red mite.
The summer form of the adult female is approximately 0.65 mm long, rotund oval in shape, and pale yellow or greenish with two characteristic lateral dark green or black spots on the body. The adult male is slightly smaller than the female and has a narrower, more pointed abdomen. In autumn, females become an orange or brick-red colour and lose their characteristic dark body spots, although they retain the bright red eye-spots. They overwinter in this form (cf. European red mite, which overwinters in the egg stage).
The minute, spherical, translucent eggs, about 0.1 mm in diameter, are usually laid amongst webbing. They age to a pale yellow or green colour, and red eye spots of the young larva become visible just before hatching.
Nymphs (young stages) vary in coloration, depending on their host plant, but are normally pale yellow, pale green, or straw-coloured, with two green spots characteristic of the species. Frequently the end of the abdomen is a dark green. The first nymphal stage (larva) is six-legged, whereas the later protonymph and deutonymph stages are eight-legged.
Two-spotted mites are easily distinguished from the European red mite, which is red and the bryobia mite, which is rather flat in appearance. Several other species of Tetranychus in New Zealand are very similar in appearance and habits, and are difficult to distinguish from T. urticae. They differ only slightly in coloration and host-plant preferences.
Host plants
Two-spotted mite feeds on a wide range of plants. It is an important pest of glasshouse crops, and is commonly found on cucumbers, tomatoes, capsicums, melons, pepinoes, beans, roses, carnations, orchids, chrysanthemums, and many other flowers. It can be a serious pest of outdoor crops, including strawberries, grapes, hops, blackcurrants, raspberries, and most deciduous fruit trees. It may also be found on kiwifruit, sweet corn, maize, peas, and other field crops. It occurs on many common weed species such as clovers, plantains, black nightshade, mallows, Amaranthus, and Convolvuius, and on a variety of shelter plants including willows, poplars, walnut, and elm.
Damage
Two-spotted mites damage plants by piercing leaf cells and sucking out the contents, which causes the cells to collapse and die. This damage is usually most conspicuous as pale-coloured spotting visible on the upper surface of the leaves. In heavy infestations the mites remove nearly all the chlorophyll, and leaves eventually die and fall off. Mites secrete a very fine, silk-like webbing which is usually obvious over the dying leaves and, as these dry out and fall, the mites move away to feed on growing shoots. When large populations have been present for a few weeks webbing may cover the whole plant. Females disperse from a plant of declining food quality on threads of webbing and drift or are blown on to other plants. Loss of leaves from heavily infested plants decreases their vigour and leads to a reduced or damaged crop. Heavily infested annual plants may die prematurely.
Distribution
Two-spotted mite has a worldwide distribution and frequently occurs in glasshouses. Outdoors it occurs throughout New Zealand, and is especially common in hot dry conditions. It often moves from weeds within a crop on to the crop when drought conditions and high mite numbers prevent further weed growth. It may also move from species of shelter-belt trees which host two-spotted mite on to horticultural crops such as kiwifruit and brambles. Wind is an important agent of mite dispersal.
Life cycle
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| Note: Coloured bars indicate periods of peak activity in each of the life cycle stages | |
Female mites overwintering outdoors become active in early spring (usually August-October, but as early as June in some crops such as strawberries) and start to feed and lay eggs on the underside of leaves. The eggs hatch in 2-15 days; however, development is greatly retarded by temperatures below 12°C. There are many overlapping generations throughout the summer, depending on temperatures.
Developing mites pass through egg, larval, protonymph, and deutonymph stages, with periods of quiescence in between. Females begin to lay eggs after 1-2 days; unfertilised eggs produce male mites and fertilised eggs produce female mites. A female mite lays 70-100 eggs and usually lives for 3-4 weeks. Development from egg to adult is from 4 to 24 days, but usually takes 2-3 weeks in summer; breeding is especially rapid in hot, dry conditions.
Colonies of mites occur mainly on the underside of leaves, sheltered by strands of fine webbing. As winter approaches, with shorter days, lower temperatures, and declining leaf condition, reproduction ceases and all stages except the females die. These winter females stop feeding and become orange or deep orange-red . They then begin to cluster together in sheltered situations, where they spend the winter. These dormant female mites also overwinter in straw mulch and dead leaves, in dry crevices in the soil, under loose bark and in cracks, crevices, and pruning damage on trees or on posts in berry and hop gardens. In glasshouses they overwinter in the structure of the house or in litter, and if new crops are grown they may become active again earlier than mites living outdoors. In areas with mild winter conditions a proportion of the population may not become fully dormant and may continue to feed and reproduce, albeit at very much reduced levels.
E. COLLYER