Mites I

• Mites II

Biology

The two-spotted spider mite is the most prevalent and most damaging mite affecting strawberries on the prairies. It is a small (about 0.5 mm), oval, usually greenish-yellow mite. It has 2 irregular dark spots on its back. The cyclamen mite is oval, light-green, shiny amber or light-tan coloured, and semi-transparent. When mature, cyclamen mites are about 0.25 mm long. The young nymphs are milky-white. The adult female mite is pale-brown.

Symptoms of Damage

The two-spotted spider mite feeds on the undersurface of strawberry foliage, especially young leaves in the crown. Feeding causes discolouration, leaf crinklage, and (sometimes) browning and curling of the leaves. When populations of two-spotted spider mites are very high, there is a general loss in plant vigour and, in the worst cases, no fruit forms.

Because the small cyclamen mite is impossible to see with the unaided eye, growers may mis-diagnose the cause of damage as mistakes in cultural management, spray injury, winter injury, or viral disease. All aerial parts can be injured by this mite. Young, new growth is very susceptible and will develop abnormally. Flower buds may not open or blossoms may be destroyed and drop early. Leaves often appear crinkled, twisted, and stunted. Leaf yellowing and dieback can also occur. Severe symptoms can be apparent even when mite populations are low.

Monitoring

The undersides of mite-infested leaves may be covered with fine webbing. In severe infestations, the foliage becomes yellow, then grey, then dry, and finally dies. The overall appearance may be of "bronze" foliage. To avoid a build-up of mite populations, the under surfaces of leaves should be closely inspected for mites every 10 days throughout the growing season. Use a hand lens to see the individual mites (they are smaller than the period at the end of this sentence). Check individual leaves that appear stippled, mottled, yellowish, or bronzed.

With the two-spotted spider mite, closely examine any webbing found between or beneath leaflets for the tiny orange-red mites and their white cast skins. A simple but effective diagnostic tool is to breathe on the back of the leaf. Warm, moist air will cause mites to move, making them more visible. Tapping a leaf over a white sheet of paper is also effective in making these tiny pests more visible. With the cyclamen mite, check for these mites along the midrib of leaflets, where they join the petiole.

Economic Thresholds

Two-spotted Spider Mites

If the mite population reaches 20-25 mites per leaflet by late April or early May, the use of a recommended miticide is advised. If populations do not reach these levels until a week or two before harvest, treatment is probably not necessary.

Cyclamen Mites

Monitoring and control should begin early in the growing season. Treatment may be necessary if there are 1 to 15 mites per leaf. Densities of 45-65 mites per leaf can cause a 33 per cent yield reduction.

Comments

To reduce mite problems on strawberries, irrigate regularly to avoid water stress, purchase only "mite-free" plants, eliminate weeds that can act as alternate hosts in and near strawberry plantings, and cultivate in the fall to disturb the over-wintering sites in soil little used by the mites.

For further information, contact your GO representative.