Thuja spider mite

Systematic position. The Thuja spider mite (Platytetranychus thujae) belongs to the class Arachnida, order Trombidiformes, and family Tetranychidae. It is a specialized pest that thrives on coniferous plants, often causing severe damage before the infestation is even visually detected by the grower.

Host plants. The primary hosts include species of the genus Thuja. However, this mite also frequently attacks junipers (Juniperus) and false cypresses (Chamaecyparis). Plants growing in dry, dusty, or poorly ventilated conditions are significantly more susceptible to attacks and experience more rapid population growth of the mites.

Biology and life cycle. These mites overwinter as fertilized females hidden within bark crevices or at the base of the scales. As temperatures rise in spring, they resume activity and begin laying eggs on the host needles. In hot and dry summer weather, the life cycle is shortened significantly, allowing for multiple overlapping generations within a single growing season.

Nature of damage. Mites pierce the cells of the foliage to suck out the sap. This feeding causes the needles to develop small yellow speckles, which eventually turn brown and lead to foliage drop. A fine, delicate webbing is often visible on the branches, which serves as a protective structure for the mite colony and a site for egg deposition.

Control measures. Effective management starts with regular inspection and cultural practices such as overhead irrigation, which increases humidity and disrupts the mite's reproductive cycle. For heavy infestations, specific acaricides are required. It is essential to:

• Use products with different modes of action to prevent resistance.
• Apply treatments thoroughly to cover the inner parts of the foliage.
• Ensure the plants are well-watered and healthy to improve their natural resistance.
• Monitor the plants closely during hot, dry periods of the summer.