Photo 2. Mite infestations cause the leaves of taro to appear light green. Most damage is along the sides of the major veins; this can be seen from the upper surface, but most of the mites are on the under surface of the leaf.
Photo 3. Damage to leaves of cassava by Tetranychus mites. As with the symptom on taro (Photo 2) the damage is along the main vein.
Tetranychus species. The different species are difficult to tell apart; they need to be examined using a high power microscope. The two-spotted mite (Tetranychus urticae), also known as the red spider mite, is common in Pacific island countries, infesting over 200 species of plants. This fact sheet mostly concerns this species.
Worldwide. In the tropics and sub-tropics.
Many crops are host to spider mites in Pacific Island countries, among them cassava, okra, papaya, sweetpotato, tomato, eggplant, beans, taro, bele, cucumber, squash and other cucurbits. Many ornamentals and weeds are also hosts.
Spider mites are common plant pests. They have needle-like mouthparts and use them to suck juice from the leaves. This destroys the cells, and the leaves show a characteristic white to pale yellow speckling, often along the sides of the main veins (Photos 1-3). When infestations are severe, the speckling is seen all over the leaf.
Two-spotted mites make webs (like spiders) on the under surface of leaves, and on the upper surface when infestations occur in screenhouses where infestations are often high, and they are less likely to be washed off (Photo 4). As the infestation advances, the leaves turn yellow and die prematurely.
The eggs are round and relatively large in comparison to the size of the adult; they are laid in the webbing near the veins, on the underside of leaves.
Each female lays about 100 eggs udring a lifetime. The eggs hatch in about 3 days, producing larvae that have six legs and are colourless. From these, nymphs develop, which have eight legs; they moult once and within a few days become adult. The adults are about 0.5 mm long, with males smaller than females, and narrower towards the back end (Photo 5). Two-spotted mites vary from light yellow to dark green or brown.
Under tropical conditions the life cycle takes only 7-10 days depending on temperatures. Populations develop rapidly, especially during periods of drought when damage can be considerable (Photos 6&7). The adults live for 2-4 weeks.
Mites produce fine silken webs - from a pair of glands near the mouth. When infestations are high, this webbing covers all or part of the leaf and becomes very noticeable (Photo 9). The webs allow the mites to travel from infested to non-infested leaves; also, the webs are caught by the wind and help the mites disperse.
The extent of the damage caused by mites often depends on rainfall. When rainfall is low, mite populations are high and reduce crop yields. On taro, for instance, yellowing and early maturity of plants occurs and corm size is reduced. Damage is particularly severe during droughts and, presumably, outbreaks will increase with climate change.
Look at the underside of leaves, particularly near the veins for the presence of mites, using a hand lens and/or a microscope. Look for webbing, which can be seen when mites are present in large numbers. The white spots on the upper leaf surface and the presence of webs below are signs of their presence. Look for spots on the mites; the spots are reddish-brown to yellowish-green, depending on the species.
A good way to detect if mites are present is to place a sheet of white paper beneath the leaves and strike the leaves sharply. The mites fall onto the paper and can be more easily seen than on the green leaves.
Predatory mites keep populations of spider mites in check, as do ladybird beetles, lacewing larvae, pirate bugs, big-eyed bugs and predatory thrips. Managing mites requires preserving natural enemies; in most cases this means doing nothing to harm them. It also means not using pesticides.
AUTHORS Helen Tsatsia & Grahame Jackson
Photo 5 Mites field crops. DAF, Queensland government.
Produced with support from the Australian Centre for International Agricultural Research under project PC/2010/090: Strengthening integrated crop management research in the Pacific Islands in support of sustainable intensification of high-value crop production, implemented by the University of Queensland and the Secretariat of the Pacific Community.
This fact sheet is a part of the app Pacific Pests and Pathogens
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