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.

—- title: "Spider mites on arborvitae" slug: spider-mites-on-arborvitae hub: problems category: "Disease-by-host" description: "Spider mites on arborvitae cause foliage to bronze and die in hot weather. Identify the species involved, distinguish mite damage from drought browning, and apply control at the right season." date: 2026-06-10 updated: 2026-06-10 author: "Thomas A." reading_time: 8 —-

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.

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.

Identification

Mite size and appearance

Both species are tiny (0.5mm), requiring a hand lens or the paper-tap test to confirm presence. Per Penn State Extension:

• Spruce spider mite: dark greenish to reddish-brown
• Two-spotted spider mite: pale greenish to orange-red with two dark spots

Plant symptoms

Per NC State Extension, mite feeding on arborvitae produces:

1. Bronze, stippled, or grayish foliage — individual feeding punctures produce pale specks that coalesce into a general bronze discoloration
2. Fine webbing — webbing between foliage scales and branches; most visible in early morning when dew condenses on the webbing
3. Foliage browning from the inside out — mite damage typically starts on interior and lower foliage before progressing outward; arborvitae browning that starts inside the shrub is often mite-related
4. Foliage drop — severely affected foliage browns completely and drops; unlike normal inner foliage die-off (which is gradual), mite damage can be rapid and extensive

Distinguishing mite damage from other arborvitae browning

Per Penn State Extension, arborvitae browning has multiple causes:

• Drought stress — uniform browning from outside in; often in late summer; no webbing
• Winter desiccation — bronzing on the wind-exposed or south-facing side; appears in late winter/early spring
• Bagworm feeding — distinct silk webbing at growing tips; see Bagworm on arborvitae
• Spider mite damage — interior browning with fine webbing; confirm with paper-tap test

Seasonal timing: the key to effective control

Per Penn State Extension, the timing of control depends on which species is present:

Spruce spider mite: Target spring (April–May) and fall (September–October) when populations are active. Summer treatments are ineffective — populations collapse in heat and reinvasion from eggs occurs in fall.

Two-spotted spider mite: Target mid-summer (July–August) when populations are building on drought-stressed plants.

Per Cornell Cooperative Extension, conducting the paper-tap test monthly from April through October, especially in years of hot, dry weather or after any stress event, is the most reliable way to catch the right treatment window.

Management

Water sprays

Per Penn State Extension, strong water sprays directed into arborvitae foliage dislodge mites and eggs. Apply every 3–4 days during the active season. This is most practical on small or young plants; large established arborvitae hedges are difficult to spray thoroughly with a hand nozzle.

Horticultural oil

Per NC State Extension, dormant oil (2–3% solution) applied in late winter before bud swell kills overwintering eggs of spruce spider mite. Summer-rate horticultural oil (1%) applied during active populations (spring or fall for spruce spider mite; summer for two-spotted mite) kills active mites and eggs on contact. Do not apply horticultural oil when temperatures exceed 90°F (32°C) or to drought-stressed plants.

insecticidal soap

Per Cornell Cooperative Extension, insecticidal soap (2% solution) kills mites on contact with no residual activity. Thorough coverage of foliage is required. Reapply every 5–7 days during active infestations.

Miticides

Per Penn State Extension, registered miticides (bifenazate, spiromesifen) provide faster knockdown for heavy infestations. Apply when mite populations are confirmed present and active. Follow label directions; some products require 2 applications 7 days apart to address egg stages that hatch after the first application.

Avoid broad-spectrum insecticides

Per Cornell Cooperative Extension, pyrethroids and other broad-spectrum insecticides destroy predatory mite populations without killing spider mites effectively, often causing worse mite outbreaks. Avoid these products on arborvitae.

Common problems table

Frequently Asked Questions

How do I know if the mites on my arborvitae are the cool-season or warm-season type?

Per Penn State Extension, conduct the paper-tap test in spring (April–May), midsummer (July–August), and fall (September–October). If you find mites in spring and fall but not summer, the species is likely spruce spider mite. If mites are present primarily in summer, it is likely two-spotted spider mite.

Can mite damage on arborvitae be reversed?

Per NC State Extension, foliage killed by mite feeding does not regenerate. New growth from buds at the base of affected branches may hide the damage over time, but there is no chemical or treatment that reverses dead foliage. Controlling the mites stops further damage; recovery depends on new growth.

Should I replace my arborvitae after severe mite damage?

Per Cornell Cooperative Extension, arborvitae with dead interior foliage covering more than 30–40% of the canopy may not recover aesthetically. New growth from healthy buds will eventually fill in from the outside, but the process takes several years. For severely damaged plants, especially in exposed sites with recurring drought, replacement with a more stress-tolerant species may be more practical.

Are there arborvitae cultivars resistant to spider mites?

Per Penn State Extension, significant resistance variation exists, but no cultivar is immune. 'Green Giant' arborvitae (a Thuja plicata × standishii hybrid), often cited as a more vigorous substitute for T. occidentalis, is considered slightly less susceptible but still requires monitoring.

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Sources

1. Penn State Extension — Spruce Spider Mite
2. Cornell Cooperative Extension — Landscape Pest Management
3. UC IPM — Spider Mites
4. NC State Extension — Arborvitae Pest Management