Spruce Spider Mite 

- The spider mite that infests conifers -

Summary: Conifer trees (trees that have cones) like spruce, fir and Douglas-fir have their very own spider mite pest called spruce spider mite. This spider mite differs from most others in that it  passing the winter as an egg rather than as an adult as most other spider mites do. These eggs can be scouted and controlled in late winter.

Jack DeAngelis, PhD
OSU Ext. Entomologist (ret.)

Spruce spider mite (Oligonychus ununguis) occurs on all conifers. Conifers are plants with evergreen needles and cones such as pine, spruce, and fir, and is the spider mite most likely to cause damage on landscape conifers like arborvitae. See Spider Mite Pests for a description of how spider mites damage plants in general and options for spider mite control.

Spruce spider mite eggs are key to control

Unlike most other types of spider mites, spruce spider mite passes the winter months in the egg stage. Most other types of spider mites spend the winter as adults. This is important information for gardeners because it means that spruce spider mites can be monitored and controlled during the winter by targeting this mite's eggs.

spruce spider mite stippling damage

Spruce spider mite damage on Douglas-fir needles. Damage is often called "stippling".

Predator mites (Phytoseiidae) play an extremely important role in the management of spruce spider mite because they feed mainly on eggs and smaller mites. These mites are highly effective egg predators and in fact outbreaks of the spruce spider mite often follow applications of pesticides that disrupt the predator mite population.

Overhead watering combined with low toxicity insecticidal soap alone are very effective treatments for spruce spider mites during the growing season (see Using Insecticidal Soap To Control Spider Mites). Applications of Insecticidal Soap won't disrupt predator mites. During late winter mite eggs that are found on needles and stems can be controlled with horticultural oils.

Certain synthetic pesticides like carbaryl (Sevin) are known to be particularly harmful to predator mite populations and use of these materials can often guarantee a spider mite outbreak because they destroy the predator mite populations! Therefore use pesticides carefully and always monitor for spider mite buildup following applications especially in susceptible plants.

Related Articles

Spruce Spider Mite Guide

Christmas Tree Pests (Oregon, 2005)

Adelgids on Conifers

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