The primary mite infestation on white cedar trees is caused by the spruce spider mite, resulting in mild to severe damage. For continued health of your white cedars in the home garden, learn how to identify spider mites as well as what to do should an infestation occur.
Healthy, vigorous white cedar trees (Thuja occidentalis) are more prepared and capable of fighting off or resisting spider mite infestations, particularly when compared to the vulnerability of stressed trees. Grow white cedars in locations that offer full sun to partial sun, according to the Ohio State University Extension. White cedar trees thrive in moist, well-drained soil.
Spruce spider mites are the most prevalent mite infestation of white cedar trees, according to the University of Guelph Pest Diagnostic Clinic. Spider mites (Oligonychus ununguis) are "sucking" bugs that feed on the sap, or tissue fluid, of cedar foliage. Unfortunately, populations often grow quickly and cause considerable damage before the infestation is apparent. Look for silky webs on leaves; as their name suggests, spider mites spin these webs that protect overwintering eggs. Adults measure approximately 1/50 inch so visual identification is tricky.
Through their feeding and potential for the development of up to eight generations annually, populations grow quickly and damage is consistent. Leaves often appear yellow, bronze or gray with a spotted appearance. Along with this cosmetic damage, extreme cases cause leaves and needles to change to a brown color, leading to early defoliation. Injury usually occurs on interior branches and areas hidden from rainfall, according to the University of Wisconsin Extension.
Before treating the problem, correctly diagnose your white cedar with a spider mite infestation. With a light-colored piece of fabric or paper beneath a tree branch, tap the branch to shake mites loose. If you have an infestation, you will be able to see the mites on the lighter-colored surface whereas viewing them on the tree is difficult. To control this infestation naturally, rain will push mites from the tree. If rain is not expected, you may spray your tree with a powerful source of water. While this is not a cure, it will reduce the mite population, according to the University of Wisconsin Extension.
For chemical control of spider mites on white cedars, one option is to apply a dormant oil like horticultural oil that is gentler on the tree and has a lower toxicity to humans and animals than other chemical options. Saturate the tree as these oils work on contact with the mites. For stronger chemical application, apply a miticide, or a chemical formulated for mite control, with the active ingredient bifenthrin, according to the University of Wisconsin Extension.