Mites are more closely related to spiders than insects, but some are important plant pests. Mites have two major body parts, four pairs of legs and the plant-feeding mites often have rasping mouthparts. Most mites are very small and difficult to see without magnification. Spider mites include carmine spider mite and two-spotted spider mite and their feeding damage includes stippling of the leaves. Broad mites are found on many plants including papaya and pepper, where they feed on the young, growing leaves, causing distortion and bronzing.
Acarines are extremely diverse. They live in almost all habitats and include aquatic and terrestrial species. Some are plant feeders and damage crops while others acts as predators used to control undesirable arthropods
Red Spider Mites (RSM) are barely detectable with the naked eye. They are pale yellow in colour with two dark spots on the body. When colonies of RSM are in large numbers they spin fine silky webs for protection from predators while they feed and shelter. They also move from leaf to leaf or from one plant to another. During unfavorable or cold weather conditions they may change to an orange/red colour and are then commonly known as red spider mites. RSM eggs are small and round and are laid one at a time near veins on the under surface of leaves.Tetranychus evansi Baker & Pritchard, is the most common in Africa and was introduced into southern Africa in the 1970s from Brazil, South America, and spread northwards from Zimbabwe, reaching Zambia, Malawi and Kenya. T. urticae is the second-most important spider mite pest.
RSM damage is more common during the hot dry weather associated with the dry season. The mites feed on the underside of the leaves by piercing and sucking the sap from the plant leading to bronzing and dehydration of foliage. Damage is first noticed as pale or bronzed areas along the midrib and veins of the leaves. When infestations are severe, the leaves become mottled taking on a silvery-yellow appearance and can prematurely fall from the plant. There is also discolouration of the lower leaf surface. It poses a threat to host plants by sucking cell contents from the leaves cell by cell, leaving very tiny, pale spots or scars where the green epidermal cells have been destroyed. Feeding causes small yellow patches on the upper side of the leaf especially between the main veins, near the leaf stalk. Latter the affected area spreads: the whole plant turns yellow to bronze coloured, then brown, leaves are dropped and the plant eventually dies. Individual lesions are very small but since they attack in hundreds or thousands they cause thousands of lesions significantly reducing the photosynthetic capability of plants, greatly reducing their production of nutrients. Spider mites may also cause spots on the fruits. Can also spread plant viruses
The crop should be checked at least twice a week using a magnifying glass, especially on the undersurface of the leaf as mite damage or presence is not always noticeable on the top of the leaf. Check for presence of reddish and whitish-yellow particules is an indication of RSM, adult mites and eggs on the underside of leaf surfaces and silken webbing especially during heavy infestations. Mark infested plants with flags and check every 2-3 days. Scouting enables the farmer to make an immediate informed decision on pest management action at a given point and time. For example, is it necessary to spray? Which pesticide to use? How should it be applied and when? Or what should be manipulated to reduce rate of pest infestation?
Management and control
Control of RSM should be started at the sight of first symptoms. The silk protects RSM from natural enemies and pesticides and at this level the mites are very difficult to control. In areas where farmers use overhead irrigation, it should be done early in the day to allow foliage to dry to reduce development of Botrytis and downy mildew.
Several cultural practices that can reduce the mite population have been recommended such as regular scouting of the crop to determine the presence of the pest and the level of infestation in an early stage for Integrated Pest Management (IPM). Burning of infested plants can be successful during early stages of infestation when the mites concentrate on a few plants. The separation of infected crops and newly planted crops or nursery areas and the burning or removal of infected crop residues and weeds, also helps to minimise the problem. Since mites favour dry and hot conditions, influencing the micro-climate by reducing the planting distance and applying overhead irrigation can repress mite populations. Avoidance o water and nutrient stress reduces mite populations. Applying mulch and incorporating organic matter into the soil can improve the water holding capacity and reduce evaporation, thus avoid water stress. Avoiding the hot summer months for tomato cultivation is useful. At the moment there are no resistant tomato varieties available.
The predatory mite Phytoseilus persimilis are being used to control mites. These predatory mites are available from commercial suppliers. The predatory mite feeds exclusively on RSM and under the right circumstances can rapidly reduce their populations to very low levels. Information on pesticide application and their compatibility with predatory mites should be obtained from commercial suppliers. Natural enemies include thrips (predators of eggs and mites), minute pirate bugs (Orius sp.), big- eyed bugs and the entomopathogenic fungus Neozygites floridana.
There are a number of miticides recommended against RSM. They can be controlled with potassium soap, or the miticides, Abamecin, Brigade, Bifenthrin, dicofol. Some chemicals are geared towards the control of eggs, nymphs and adults, whereas, others control only adults and nymphs. Miticides are effective although frequent use can cause a build-up of pesticide resistance in mites. Where possible, spot spraying should be done since mites are usually localised. Botanicals such as Neem (Azadirachta indica) and Tephrosia sp. have been evaluated in Malawi, Zimbabwe and Kenya. Others include tested include chilli-pepper, garlic and soap extracts are used and a mixture of buttermilk and flour.
R. B. Halliday, D. E. Walter, H. C. Proctor, R. A. Norton & M. J. Colloff (2001). Acarology, Proceedings of the 10th International Congress [5–10 July 1998]. Collingwood, Vic.: CSIRO Publ., Melbourne. pp. 960 pp. ISBN 0-643-06658-6.