Government of New Brunswick

1992, ISBN 1-55048-802-3

Species: Tetranychus urticae Koch

Acari: Tetranychidae

INTRODUCTION

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.

DAMAGE

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.

LIFE HISTORY

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.

Adult male (left) and female (right) twospotted spider mites.

CONTROL

TSM populations are often kept under control when rain, low temperatures, and high humidity occurs. Frequent spraying of plants with a strong stream of water is also effective in controlling TSM, providing plants can withstand the treatment. TSM may be controlled in orchards early in the season by clean cultivation and weed control. Predacious mites1 are very important in keeping TSM populations under control. Ladybird beetles2, thrips3, and other predators4 also attack TSM. Some pesticides are highly toxic to predators, especially predacious mites. In situations where TSM have been kept under control by predators, pesticide application may kill predators and cause TSM populations to increase.

When using pesticides it is important to avoid using the same type of pesticide as strains of mites quickly develop resistence. Before spraying, please consult your provincial agriculture department for recommendations on chemical use. Always read the label for mixing instructions, dosages, and precautions.

1 Genera: Amblyseius, Phytoseiulus, Metaseiulus

2 Stethorus punctum (LeConte) and Stethorus picipes Casey

3 Leptothrips mali Fitch, Haplothrips faurei Hood and Scolothrips sexmaculatus (Pergrande)

4 Chrysopa carnea Stephens, Chrysopa oculata Say, Chrysopa nigricomis (Burmeister), Chrysopa rufilabris and Orius tristicolor (White)

Acknowledgement:

The author would like to thank Mr. Gerald Walker and Mr. Wayne Roberts for their photo contribution.

Christopher Maund,
Revised 1999