Tetranychus urticae

Tetranychus urticae Koch

(Formerly known as Tetranychus telarius (L.).

Taxonomic placing: Acari, Prostigmata, Tetranychoidea, Tetranychidae.

Common name: Two-spotted spider mite (TSSM); for the red forms, the carmine spider mite.

Geographical distribution: The TSSM has an almost cosmopolitan distribution, occurring indoors in most regions, outdoors in subtropical areas, relatively rare in the tropics.

Host plants: Over 200 host plants, with a preference for legumes.

Morphology: All stages of the mite, including the females (which are about 0.4-0.5 mm in length), are green-yellow, with two darker internal spots (hence “two-spotted”); however, when feeding on certain host plants the body may be darker. The females of some populations are red, and known as the carmine spider mite (Tetranychus cinnabarinus (Boisduval)).

Life history: At 25-30°C a generation may be raised in about two weeks, each female laying over 100 eggs. The mite, which lives in colonies, develops through five stages: egg, larva, two nymphs and adults. Before its last molt the mite covers itself with a few webs that emit a sex pheromone. The attracted males (which develop slightly faster) wait around the molting female and may fight amongst themselves, the winner pushing the loser(s) away; mating takes place as soon as the female frees itself from the webs. Optimal conditions for reproduction and for webbing are at about 25°C and 40% relative humidity (RH). In the Middle East the TSSM may raise 20-25 annual generations. Females disperse between plants and sites by ballooning, becoming detached from the hosts and blown about by winds. Females undergo a winter diapause, turn pink-orange (due to accumulated lipids), hide in debris in the top layers of the soil or in sheltered sites, and lay no eggs until next spring. The red form has no diapause, reproducing the year around. The webs are used to construct and define the mite colony, to protect it from predators and from unsuitable humidities (the TSSM has only a few progeny at 0% RH and none at 100% RH), to discourage competitors, for dispersal and as a carrier of the sex pheromone.

The recently sequenced and annotated TSSM genome is opening new avenues for understanding, and thus managing, this major pest.

Economic importance: The TSSM is a major pest of many crops, including cotton, ornamentals, strawberries, watermelons, tomatoes and others. Yield losses approach 15% for strawberries in the USA, 14% for corn in France, and 14–44% for cotton. The cost of pesticide use to control this pest annually exceeds USD $1 billion in the European Union. The TSSM usually colonizes the lower side of leaves, where they feed by sucking out cell constituents. Such feeding causes leaf greying or yellowing, followed by necrotic spots, leading to loss of photosynthesis, leaf wilt and even death, reducing the quantity and quality of the crop. Severe infestations of trees results in defoliation during the summer and autumn. Damage to ornamentals is seen as browning and withering of petals. Affected cotton leaves become dark-red. When infesting apples, leaves become mottled and drop. Some damage is caused by the webbing that may cover parts of plants and impedes biological and chemical control.

Until the end of World War II T. urticae was seldom considered to be a major pest, most of its damage seen in late summer and autumn. However, the wide-spread use of synthetic pesticides (such as organochlorides), which killed its natural enemies and competitors, elevated this mite to prominence as a pest. In addition, the introduction of covered crops enabled TSSM to raise large populations the year around. New, more susceptible plant varieties, and newer agricultural practices that improve plant health and growth, such as pruning and fertilizing, also promoted increases in pest numbers due to the greater production of young foliage which TSSM prefers.


Monitoring: In cotton, collecting 100 leaves at random and tallying the rate of leaves infested by mites, their cast skins and webbing, from early in the season on, will supply data on the progress of the infestation. Similar methods are in use on other crops. Early TSSM damage to greenhouse pepper leaves can be detected spectrally by the reflectance of the visible and near-infrared regions of damaged leaves.

Cultural control: Reducing the sources of dust, which encourages mite webbing, near crops. Overhead sprinkling also reduces pest populations due to its susceptibility to very high humidities. Destroying nearby susceptible weeds. Plant nutrition may reduce pest populations; adult longevity was curtailed when the mites were confined to plants grown in nutrient solutions deficient in phosphorus, nitrogen or potassium.

Chemical control: The TSSM develops extensive and intensive resistance to many chemicals; by 2008 there were 92 active pesticide ingredients to which the pest had developed resistance. Selection for resistance is facilitated by the short life cycle, which in warm climates or in heated greenhouses results in many annual generations, by the mite’s high fecundity and by its inbreeding and arrhenotokous reproduction. TSSM is thus considered as ‘most resistant’, in terms of the total number of pesticides to which its populations had become resistant. Nevertheless, some populations in the Middle East are still susceptible to avermectins. Insecticidal soaps are sometimes applied, with variable results, and essential oils that kill the TSSM are widely used in the Middle East. Rotation of pesticides may also be practiced. Pyrethroids encourage TSSM populations by repelling them and thus dispersing them to uninfested growth, by shortening the life cycle, by increasing fecundity, or by being very toxic to natural enemies and competitors.

Plant resistance: Plant breeders have selected many varieties of commercial plants (e.g. apples, melons, tomatoes and strawberries) that are resistant to the pest.

Biological control: In the field TSSM is often controlled by predatory phytoseiids. Neoseiulus californicus (McGregor) and Typhlodromus swirskii Athias-Henriot greatly reduced TSSM numbers on faba beans in Egypt. On covered and low-lying crops, such as strawberries and roses, the pest is often controlled by the (commercially available) Phytoseiulus persimilis Athias-Henriot. Coccinellidae, especially of the genus Stethorus are other important natural enemies. Several thrips also feed on the TSSM, as does the commercially available Feltiella acarisuga (Vallot) (Cecidomyiidae). Several entomopathogenic fungi infect the TSSM and some have been used in classical or IPM control projects, but their controlling effect is dependent on suitably humid environmental conditions.


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Websites: http://entnemdept.ufl.edu/creatures/orn/twospotted_mite.htm