Red Spider Mite

Pests of Field Crops in Southern Africa

RED SPIDER MITE

(Acari: Tetranychidae)

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Text Box: Spider mite belongs to the class Arachnida and is a common pest of a variety of crops. Spider mites are very tiny, and comprise a huge number of different species, most of which are distinguishable only to experts. The majority of damaging species belongs to the genus Tetranychus, and different species attack different crops, although many of them are fairly polyphagous. The adult mites range from 0.3 to 0.5 mm in length, are more or less oval in shape, usually reddish in colour (although some are straw-coloured or greenish), and have four pairs of legs. They are active creatures and usually associated with very fine webbing on the plant surface. When the population becomes heavy, the webbing is strung from leaf to leaf and may be covered in red mites. The eggs are spherical, semi-translucent or creamy, and about 0.1 mm in diameter. They are laid singly on the leaf amongst the threads of webbing. These hatch into tiny flesh-coloured nymphs, which at first have only three pairs of legs. TOP

Text Box: Tetranychus species are found on an extremely wide range of host plants. Amongst these are cotton, tobacco, tomatoes, potatoes, soyabeans and other beans, sunflowers, groundnuts, green peppers, brinjals, citrus, deciduous fruit, strawberries, cucurbits and even maize. Many ornamentals are also attacked; particularly roses, fuchsia, chrysanthemums, carnations, pansies, violets and many others, and the mites flourish in greenhouse situations. A variety of weeds such as “blackjack” (Bidens pilosa), sorrel (Oxalis spp,) and false gooseberry (Physalis spp.) can also support populations of Tetranychus. TOP

Text Box: The first symptom of red spider mite attack is usually the appearance of minute yellow stippling on the upper surface of leaves that have populations on their undersides. This is a result of sap-sucking by the mites. Infested leaves generally become yellow, then bronzed, and drop off the plant if the infestation is left uncontrolled. In extreme infestations, the plants may become covered with webbing, and the mites cluster at the highest point of the plant to facilitate dispersal onto other plants by human movement or wind. Red spider mite infestations are often in “hot spots” in the crop, whether it is a field crop such as cotton, or a greenhouse crop such as cut-flower roses. This may be as a result of inefficient control measures, or because there is a nearby source of uncontrolled mites (such as infested weeds). Sometimes spider mites are a “resurgence pest” after broad-spectrum insecticides have been applied for another pest, killing off its natural enemies. Damage can be at any time of year, but in field crops, red spider mite infestations usually occur in hot, dry spells of weather, and are checked by heavy rains. TOP

The life cycle of these mites is a short one and this enables a rapid build-up of the pest population if conditions are suitable. Four to seven days after being laid, the eggs hatch into six-legged larvae, then pass through two further nymphal stages (the protonymph and the deutonymph) which have eight legs and resemble the adult stage. The three nymphal stages last about three days each and all feed actively. The adult stage lives from 10 to 30 days, and is able to reproduce eggs very soon after the last moult, laying about 10 eggs a day.

Text Box: In some situations, such as in greenhouse crops, certain measures should be put in place to limit the occurrence of this pest. Among these are the removal of weed hosts from the vicinity, the avoidance of planting downwind of common host crops (cotton, tomato and bean crops particularly) and limiting access to the greenhouse by workers who may bring the pests in from infested areas (vegetable gardens or other crops). Within the crop, be it greenhouse or field crop, scouting or monitoring should be carried out regularly, checking the undersides of leaves, particularly in hot, dry weather. At first signs of the pest, control measures should be taken, since the population can spread so rapidly. Red spider mites are notorious for developing resistance to most of the groups of pesticides, and are totally unaffected by many commonly used insecticides today, so spray programmes for both the mites themselves, and for other pests in the crop need to take this into account. Over-use of any one group of pesticides will encourage development of resistance in the mite population, and most pesticides will destroy their natural enemies, further exacerbating the situation. Indeed some insecticides seem to cause “mite explosions”. To alleviate resistance problems in red spider mites in cotton crops, for many years a very successful “acaricide rotation scheme” was in operation in Zimbabwe. This specified the systemic acaricides that were permissible within any particular area, and the acaricides, which were derived from different chemical groups, were rotated through the cotton growing regions on a two-year basis. The system worked as long as farmers were conscientious in sticking to the “rules”. This became much more difficult to operate as cropping became more diversified and a variety of chemicals was used to control spider mite on other crops within each area. The basis of the system should still be used as far as possible, however, even on individual farms or within areas. There are many acaricides registered for different crops, from a variety of chemical groups, and some of them are relatively new compounds. Amongst them are acaricides that have contact action only (such as dicofol and dienochlor) and those that are mainly translaminar (such as abamectin, diafenthiuron, tetradifon and older organophosphates such as triazophos). Translaminar action is a useful property since spider mites are found mainly on the lower leaf surfaces, and spray application is thus facilitated. Good cover of the foliage is still important, however, as translaminar pesticides are not able to move from leaf to leaf. For contact acaricides, underleaf cover is obviously of vital importance if the population is to be controlled. TOP

Red spider mite on the back of a leaf.

Red spider mite stippling and webbing on tomato.

Egg and nymph.