Tetranychus urticae

Scientific Name Tetranychus urticae Koch
Synonyms Tetranychus telarius, T. bimaculatus , T. altheae, T. fragariae, T. manihotis and others.
The taxonomy of the genus Tetranychus is controversial. Numerous species described from different host plants are probably in fact synonyms.
Common Names English: Two-spotted spider mite; Spanish: Aranita de las legumbres, Araña roja; French: Tétranyque tisserand, Acarien jaune; German: Gemeine Spinnmilbe, Rote Spinne; Italian: Ragnetto giallo dei giardini; Dutch: Rode Plantenspin
Description The oval body of the female is about 0.5 mm, the more slender and pointed one of the male 0.3 mm long. Color varies from pale green or light yellow to dark green or brown, but overwintering females are nearly always reddish-orange. The two dark spots on the sides of the body which have given this species its common name can be absent in some specimens.

Biology

Damage

T. urticae, like other spider mites, with its mouthparts penetrates plant cells, preferably on the undersides of leaves, and ingests their contents. Each minute 1-2 dozen cells can be destroyed this way. The first visible symptoms are small whitish speckles, mainly around the midrib and larger veins. When these spots merge, the empty cells give areas of the leaf a whitish or silvery-transparent appearance.

With ongoing infestation, damage will not be restricted to the spongy mesophyll but include the palisade parenchyma as well, and the leaf tissue may collapse completely. The function of the stomata is affected and transpiration constrained. The leaf will turn yellow, wilt, and finally be shed; sometimes complete defoliation occurs. Often the whole foliage of attacked plants takes a yellow or brownish cast. The loss of photosynthetically active surface together with reduced transpiration leads to reduced yield, and the plant may be stunted or in severe cases killed. Ornamentals can be rendered unmarketable even by comparatively moderate infestations as they become unsightly due to browning and withering of petals and the copiously produced webbing.

Lifecycle

Each female T. urticae mite lays 10-20 eggs per day, 80-120 altogether during its lifetime of up to 4 weeks. They are mostly attached to the silk webbing. The six-legged larvae hatch after 3-15 days. They molt three times within 4-5 days, towards protonymph, then deutonymph and at last adult. These instars all have eight legs. Before each molt there is a short quiescent stage. At favorable conditions (optimum are 30-32°C and a relative humidity of <50%) the life cycle can be completed in about 1-2 weeks, including a preoviposition period of 1-2 days. Often a change towards hot and dry weather leads to a very rapid increase of population density.

In the glasshouse reproduction can continue all the year round; outdoors 5-10 overlapping generations per season are possible, depending on climatic conditions. For overwintering, mature females diapause in protected places like cracks in bark or under plant litter. In April/May they emerge and begin with oviposition. As the name „spider“ mites suggests, they spin webbing. T. urticae produces so much of it that it may completely cover parts of the plant. It protects the mites to some degree against wind, rain and predators. In addition, attached to strands of silk they can be dispersed by the wind.

Occurrence

Additional Crop Information

T. urticae is extremely polyphagous. It is an economically important pest particularly on bean, cotton, cucumber, hop, peppers, soybean, strawberry, sunflower, tomatoes and vine as well as numerous fruit trees and ornamentals. Besides it is found on aubergine, broad bean, cowpea, groundnut, lettuce, lucerne, melon, okra, papaw, red and black currant, sorghum, sweet potato, tea and many others.

Agricultural Importance

T. urticae is a species of temperate and subtropical regions throughout the world. It is also a pest in glasshouses beyond its natural geographic range.
Due to its huge host range T. urticae is considered the economically most important spider mite.

Control

Integrated Crop Management

Spider mites are easily introduced with materials, tools and people. Early detection of the mites, before noticeable damage occurs, is important for successful control. For monitoring use a good hand lens.
Dust, e.g. from roads, favors mite outbreaks.
Spider mites thrive on plants under stress; vigorous, adequately irrigated plants are far less susceptible. As naturally occuring predators play an important role in regulating spider mite populations, necessary pesticide treatments should be chosen and applied judiciously.

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