1992, ISBN 1-55048-802-3
Species: Tetranychus urticae Koch
The twospotted spider mite (TSM) is a pest of field, fruit, greenhouse and vegetable crops. It also attacks nursery, ornamental and garden plants as well as weeds.
The TSM is one of the most serious pests of all spider mites. In Canada, it is found throughout the country.
TSM have piercing-sucking mouthparts. Nymphs and adults damage plants by sucking sap, primarily from the undersides of leaves. Infestations usually begin on the center leaves and spread outward. Outdoor infestations usually start at the field margins. Light infestations cause tiny white spots to appear. Later on, leaves become speckled with silver or gray patches and become covered with delicate webbing material made by the mites. Heavy infestations cause the leaves to become brown or bronze and eventually drop off. This retards plant growth and reduces the size and quality of the fruit. In severe cases the entire plant may die.
Predominant TSM damage occurs under hot, dry conditions, usually towards the end of summer.
Adult, orange-colored, female TSM overwinter between cracks in tree bark or under leaf litter. Early in the spring females become active and feed, and make delicate webs on the undersides of weeds and tree leaves. Within the webbing, the female lays up to 200 spherical, clear to pale green eggs, about 0.1 mm in diameter. Unfertilized eggs develop into males. Within 3 to 10 days, depending on the temperature, eggs hatch into 6-legged larvae. Larvae molt into 8-legged protonymph and deutonymph stages before becoming adults in 4 to 24 days (Fig. 1) Summer forms of adult mites are yellow-brown or greenish in colour. The characteristic pair of dark spots, of internal body contents, as seen from above, gives these mites their common name. Adult females are 0.3 to 0.4 mm long. Males are 0.3 mm long with narrower, more pointed abdomens and are more active. Females live for an average of 30 days. There are many generations a year. Hot, dry weather favours rapid development of mites. Peak populations usually occur in late summer.