Two-spotted spider mite

Tetranichus urticae is a dangerous pest of many agricultural crops and flower-ornamental plants. This mite was found on more than 180 species of plants (a wide polyphage) in conditions of outdoor planting and glass-covered ground. It is especially active in buildings for cultivation (greenhouses, tropical houses, hotbeds).

Biology. Life cycle.

The summer female is greenish, its body is egg-shaped, convex from above and below, 0.45–0.5 in length. Dark spots are seen through the body envelope. Dependently on food compound their size and form vary. There is a rhombic figure on the back side which is formed by skin folds. The winter diapause female is orange-red. The tip department of peritremae is not branched, 4-canalled. The male is 0.3 mm in length. He has an extended body sharply narrowed to its hind end. The male is lighter than the female. The egg has exact spherical form. Freshly laid one is pallid. During the development of a germ it becomes turbid and assumes a pearl tint. The larva has semispherical form and 3 pairs of legs. It is white. The nymph has 4 pairs of legs, chaetae (hairs) on the abdominal side of the body. The number of hairs increases after alternate molting.

Life mode

Under unfavorable hydrothermal conditions the female enters diapause and ceases the process of feeding. After the arrival of convenient conditions the pest migrates to plants, begins to consume sap and to lay eggs. In the short period of time the color of females is changed. They become less visible on the plants. In summer populations under moderate temperatures the sex ratio of males and females is about 1:3. The female lays the eggs separately on the underside or on the upside of a leaf. The pest is the most active under high temperature and low relative air humidity (spring-summer period). The optimal temperature of development is 30°C. The fertilized female in diapause overwinters in small colonies under plant remains, in cracks, in the splits of greenhouses and hotbeds. In greenhouses a part of population doesn't enter diapause and multiplies all the year reproducing about 20 generations. The development of one generation takes 12–20 days.

Symptoms of injuries

The mite is active as a rule on the undersides of leaves braiding them with a web. They suck out plant sap causing the disturbance of metabolism and the exhaustion of nutriment storage, the depression of assimilatory leaf function. Herewith the deformation and the decolorization of injured leaves are observed. The leaves assume a marble tint, become dark-brown and wither. The plants perish gradually. The decorative effect, the quality and the yield capacity of ornamental crops are reduced.