Spider mites on arborvitae

Arborvitae browning is one of the most common diagnostic questions from homeowners -- and spider mites are one of the most commonly overlooked causes. The browning that mites cause blends into the overall stressed appearance of arborvitae in summer, and by the time the damage is obvious, mite.

Arborvitae browning is one of the most common diagnostic questions from homeowners — and spider mites are one of the most commonly overlooked causes. The browning that mites cause blends into the overall stressed appearance of arborvitae in summer, and by the time the damage is obvious, mite populations have been building for weeks.

The complicating factor for arborvitae is that two very different mites can be involved, and they are active at different seasons. Getting the species right determines whether you spray in spring or late summer.

I don't grow arborvitae at my Long Island property, so this guide draws on Cornell Cooperative Extension, Penn State Extension, and NC State Extension research.

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The pests

Spruce spider mite (Oligonychus ununguis)

Per Penn State Extension, the spruce spider mite is the most important mite pest of arborvitae, junipers, spruce, and other conifers in the northeastern US. Unlike most spider mites, it is a cool-season species:

• Most active in spring (April–May) and fall (September–October)
• Population drops dramatically when temperatures exceed 85°F (29°C)
• Overwinters as eggs on foliage

Two-spotted spider mite (Tetranychus urticae)

Per UC IPM, the two-spotted spider mite can also infest arborvitae, particularly on drought-stressed plants. It is a warm-season species, most active in summer (July–August) and suppressed by cool fall temperatures.