Managing spider mites

Hot, dry weather is tough on plants. Plants under water stress are highly susceptible to pest damage. A common pest in landscapes and gardens is spider mites.

Spider mites damage plants by sucking juice from the underside of leaves. Unfortunately, they are often not noticed until damage becomes apparent.

Identifying spider mite damage 

On leaves, spider mite damage initially looks like small yellow or brown spots (stippling). Leaves might turn yellow, reddish or pale and drop off. You may see webbing on the underside of infested leaves. On conifers, foliage turns yellow or brown and may eventually drop off.

Identifying spider mites

Spider mites are not insects. They are tiny arachnids (like spiders and ticks) with eight legs and no wings. To the naked eye, they may be invisible or may look like tiny moving dots. They are easily seen with the aid of a magnifying lens. There are a variety of species and colors.

Check the undersides of leaves for mites, their eggs and webbing. You can hold a piece of white paper under a branch and hit the branch sharply. The little dots on the paper that start to walk away are mites.

Host plants

In our area, spider mites most commonly afflict skimmia and dwarf Alberta spruce. They may also affect other plants, such as arborvitae. They are also common on houseplants.

Managing spider mites

In order to successfully control mites, you need to understand their life cycle. Most species overwinter as eggs on host plants. Mites become active in the spring. There may be eight to ten overlapping generations per year.

What you can do to control spider mites 

• If you have skimmia or dwarf Alberta spruce, monitor for spider mite damage so management can begin early.
• Keep plants well watered. This is the cornerstone of limiting mite damage.
• Spray the plants (especially the leaf undersides) with a hard, cold water spray several times a day for several days. This will knock many of the mites off.
• Water sprays also raise the humidity around the plants, reducing the mites’ need for moisture and feeding. With enough moisture, natural predators are often able to maintain spider mite populations at harmless levels.
• Mulch around your plants to reduce drought stress.
• Pay special attention to water needs of plants under eaves. Make sure to water the sides of plants facing the house.
• Spider mites have many natural enemies, including predatory mites, lady beetle adults and larvae, lacewing larvae, thrips and minute pirate bugs. These will help limit their numbers, especially when pesticides are not used in the garden.

Consider replacement plant options

You may want to consider replacing your skimmia and dwarf Alberta spruce with plants that are less prone to problems with spider mites.

• Skimmia replacements: Consider sarcococca, mahonia and salal. All of these have a similar size and interest.
• Dwarf Alberta spruce replacements: Unfortunately, there are no plants that will provide a similar shape and form without constant pruning. You may need to try a plant with a different appearance.

For more information

• Spider mites, University of California IPM Program.
• Landscape pests-Spider mite, Pacific Northwest Insect Management Handbook.