Arborvitae Pests

Arborvitae (Thuja occidentalis) trees are popular throughout the northern United States and Canada. Arborvitae, an evergreen, has soft scale-like leaves that form flat sprays, rather than prickly needles many associate with evergreens. It maintains its shape naturally without pruning. These characteristics make it a favorite among landscapers and gardeners. Unfortunately, arborvitae is also popular with many destructive insects.

Bagworms (Thyridopteryx ephemeraeformis) infest arborvitae May through the middle of June. These pests are identified through their larvae's silken bags that hang from branches. Infused with plant material, the bags are often mistaken as part of the plant. At maturity, the bags can reach 1 inch long. Females will remain in the bag their entire lives, while males will emerge as a black moth with clear wings. Check for and destroy bags throughout the year. Pesticides can be effective when bags are small, although a second application is advised two weeks after initial spray.

Arborvitae leafminer (Thyridopteryx ephemeraeformis) is a silver-gray moth, generally 1/3 inch long. Signs of an infestation include yellow or brown tips on foliage, especially on the south side of the plant. Larvae feed on the tips of arborvitae through the spring and emerge as adults in June and July. Remove tips by hand for light infestations; spray with a systemic insecticide in late fall or early spring for heavy infestations.

The strawberry root weevil (Otiorhynchus ovatus) prefers arborvitae over other evergreens. Wilting and dieback of branches are common symptoms of an infestation. Since the adults feed near leaf margins, notching also indicates weevils. To kill adult beetles before they lay eggs, apply foliar insecticides in July. Larvae overwinter in the soil and feed on roots, and the fungi Beauveria bassiana and entomophagous nematodes are effective controls when applied to the soil in early August. The University of Minnesota Entomology department recommends keeping soil moist to ensure effectiveness.

Though arborvitae is not a favorite of fletcher scale (Parthenolecanium fletcheri), it can fall victim to this pest. Signs of an infestation include yellowing needles, premature needle drop and a black sooty mold growth. Eggs hatch mid to late June, although the round, brown adult females may arrive as early as April, often clustering where leaves join branches. Use horticultural oil to smother exposed eggs, and dormant young adults. Systemic insecticides also protect arborvitae.

Spruce spider mites (Oligonychus ununguis) may be difficult to see with the naked eye, but their effects are clear. Yellow or bronze leaves, foliage drop and branch dieback are symptoms of an infestation. A sheet of paper held under a branch while shaken will confirm spider mites. A dormant oil spray in the fall will remove mites without damaging their natural predators.