What's Eating Your Emerald Green Arborvitae?

Thuja occidentalis 'Emerald Green'

Reviewed by Kiersten Rankel M.S.

Quick Answer

For Emerald Green Arborvitae, the most likely culprits are spruce spider mites (speckled bronze patches across whole sections of foliage in summer drought) and bagworms (small brown cone-shaped bags hanging from twigs). Deer browse is the biggest winter threat and can ruin a young hedge in weeks. Fletcher scale clusters as small brown bumps on twigs.

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What does the damage look like?

Tap the closest match to jump straight to the fix.

Bronze patches on whole sections

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Small brown cones hanging from twigs

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Foliage stripped from the bottom up

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Small brown bumps stuck to twigs

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Pests, ranked by impact

Spider mites

Damage

High

Removal

Moderate

What it looks like

Spruce spider mites (Oligonychus ununguis) are almost invisible to the eye, less than 0.5 mm. They cluster on the inner side of the scale-like overlapping needles, especially on south and west-facing sections of the pyramidal canopy. Hot dry summer weather drives population booms.

What the damage looks like

Tiny pale dots across the scale-like needles, then bronze and rust-colored patches that spread to whole sections of the canopy. Damaged foliage doesn't recover and stays brown. Heavy infestations defoliate entire 2 to 4 foot sections within a few weeks of summer drought.

How to get rid of them

Option 1

Hose blast the canopy weekly through hot dry stretches

Run a strong jet of water from a hose nozzle through the inside of the canopy from bottom to top. Mites get knocked off and rarely make it back to the foliage. Hit the south and west-facing sides hardest. Repeat weekly during any dry stretch over 85 degrees. Costs nothing and keeps populations from exploding.

Option 2

Horticultural oil at dusk, every 7 days for 3 rounds

Mix horticultural oil (Bonide All Seasons, ~$15) per the label rate, usually 2 to 4 tablespoons per gallon.

Spray the inner foliage of affected sections at dusk to avoid sun-scorch on the oil-coated needles.

Repeat every 7 days for 3 rounds to catch eggs hatching in the dense scale-needle layers.

Option 3

Deep water the root zone through summer drought

Stressed arborvitae lose moisture through the foliage faster than the roots replace it, and that dry tissue is exactly what spider mites prefer. Soak the root zone with a hose at the dripline for 20 minutes once a week through any drought stretch. Mulch 2 to 3 inches deep over the root zone to hold moisture.

Macro photo of a caterpillar resting on a green leaf

Bagworms

Damage

High

Removal

Moderate

What it looks like

The caterpillar of Thyridopteryx ephemeraeformis. Builds a 1 to 2 inch cone-shaped bag of woven silk and bits of arborvitae foliage. The bag hangs from the underside of twigs and looks like a small dry pinecone. The caterpillar lives inside and pokes its head out to feed on the scale-like needles.

What the damage looks like

Small brown cones of woven foliage hanging from twigs, often missed because they look like the plant. Affected branches turn brown as the caterpillars strip the scale-like needles. Untreated populations defoliate entire arborvitae over a season and can kill a young tree the next year.

How to get rid of them

Option 1

Hand-pick the bags from October through May

Walk the hedge slowly and look for the cone-shaped bags hanging from the twigs.

Snip each bag off with pruners and drop it into a bucket of soapy water. Each bag holds hundreds of overwintering eggs.

Bag and dispose of in the trash, not the compost. Repeat every couple of weeks through winter and early spring.

Option 2

Bt spray in mid to late June, every 7 days for 3 rounds

Mix Bt (Monterey or Safer Caterpillar Killer, ~$15) per the label rate, usually 1 to 2 teaspoons per gallon.

Spray the entire canopy when the young caterpillars are small (mid to late June in most zones), focusing on the soft top whorl where new growth emerges.

Repeat every 7 days for 3 rounds. Bt only works on actively feeding small caterpillars and breaks down in sunlight.

Option 3

Spinosad spray for older caterpillars in July

Once caterpillars get larger and Bt stops working, switch to spinosad (Captain Jack's Deadbug Brew or Monterey Garden Insect Spray, ~$12 to $15). Spray the whole canopy at dusk and reach into the inner branches where the bags hang. Repeat every 7 to 10 days through July.

Deer

Damage

High

Removal

Hard

What it looks like

White-tailed deer treat arborvitae as a favorite winter food. Most active at dawn and dusk. The foliage is one of the few green things available when other forage is scarce, so even deer that ignore arborvitae in summer will browse heavily once snow covers the ground.

What the damage looks like

Foliage stripped from the ground up to about 5 feet, with a clean browse-line where the deer can no longer reach. Twigs and even the soft top whorl ripped off, leaving ragged ends rather than clean cuts. Repeat winter browsing kills young arborvitae outright and permanently disfigures established hedges.

How to get rid of them

Option 1

Build a deer fence, 8 feet tall around the hedge

A physical fence is the only reliable defense once deer have found the hedge. Use 8 foot welded wire or polypropylene deer netting on T-posts. Lower fences just teach deer to jump. Set the fence at least 3 feet away from the foliage so they can't reach through. Cost runs $200 to $400 for a 50 foot run. The only treatment that works through a hard winter.

Option 2

Rotate two repellents on a weekly schedule, fall through spring

Pick two different repellents with different active ingredients. Liquid Fence (egg-based, ~$25) and Plantskydd (blood-based, ~$40) work well as a pair.

Spray the foliage from the ground to 6 feet up. Cover the soft top whorl and the new growth that deer prefer.

Alternate between the two products weekly through fall, winter, and early spring. Deer learn to ignore a single repellent within a few weeks, especially when winter forage runs short.

Option 3

Plant deer-tolerant evergreens for new screens

If deer pressure is high in your area, replace failed arborvitae with evergreens deer mostly leave alone. Norway spruce, Japanese cedar (Cryptomeria), and 'Green Giant' arborvitae (browsed less than 'Emerald Green') all hold up better. Saves repeated replacement costs and the heartbreak of watching a hedge get destroyed every winter.

Common myth

Soap bars and human hair on stakes keep deer out.

Hungry winter deer ignore both. The only reliable defense is a physical fence tall enough that deer can't jump it, plus rotating sprayed repellents while the foliage is exposed. Spend the money on a real fence the first year rather than losing the hedge to a hungry herd.

Brown soft scale (Coccus hesperidum) clustered on a plant stem

Scale insects

Damage

Medium

Removal

Hard

What it looks like

Fletcher scale (Parthenolecanium fletcheri) are 2 to 4 mm brown rounded bumps glued to the twigs and branches. Don't move because they're locked in place. Cluster along the smaller twigs and at twig forks rather than out on the scale-like needles themselves. Often missed because they blend with the bark color.

What the damage looks like

Small brown bumps stuck along the twigs, sometimes in dense clusters at branch joints. Yellowing or browning of foliage on the affected branches as the scale drains sap. Sticky shiny film on lower foliage with black sooty mold growing on the residue. Heavy infestations slowly weaken whole sections of the hedge over a couple of seasons.

How to get rid of them

Option 1

Horticultural oil in early spring, full coverage

Wait for a calm day in early spring before bud break, with temperatures between 40 and 80 degrees.

Mix horticultural oil (Bonide All Seasons, ~$15) at the dormant rate per the label, usually 4 tablespoons per gallon.

Spray every twig and branch surface to soaking. The oil suffocates overwintering scale before they hatch crawlers in late spring.

Option 2

Target the crawler stage in late May to June

Crawlers are the mobile young stage and the only point in the life cycle when scale are vulnerable to contact sprays. Watch for them in late May or June, usually right after lilacs bloom. Spray insecticidal soap or a follow-up round of summer-rate horticultural oil at that window. Repeat once 7 to 10 days later.

Option 3

Prune out heavily infested branches

If a branch is densely covered and the foliage is already browning, cut it out at the next healthy junction. Remove and dispose of the prunings rather than composting. Reduces the population pressure for next season and gives the hedge a chance to fill in from the remaining healthy branches.

Stay ahead of all of them

Four habits that keep arborvitae pests rare and your hedge full and green.

Walk the hedge in winter and snip every bag

Bagworm bags overwinter on the twigs and each one holds hundreds of eggs for next summer. A 10 minute walk through the hedge every couple of weeks from October to May removes them before they hatch. The single highest-leverage habit for keeping bagworm populations from exploding.

Deep water the root zone through summer drought

Spider mites thrive on water-stressed arborvitae. A deep weekly soak at the dripline through any dry stretch over 85 degrees keeps the foliage hydrated and the mite population low. Mulch 2 to 3 inches over the root zone to hold the moisture longer between waterings.

Inspect the inner canopy on the south side every month

Most pest pressure builds up on the inner branches of the south and west-facing sides where heat and dryness are highest. Push the foliage aside once a month and look for bronzing, brown bumps, or webbing. Catching pressure on one section keeps it from spreading through the whole hedge.

Fence young arborvitae before the first hard winter

Deer find arborvitae in winter and remember it. The first winter sets the pattern for years. Put up an 8 foot fence around any newly planted arborvitae in fall, before the first heavy snow. Once the herd has browsed the hedge, repellents alone rarely undo the habit.

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About This Article

Kiersten Rankel M.S.

Botanical Data Lead at Greg · Plant Scientist

About the Author

Kiersten Rankel holds an M.S. in Ecology & Evolutionary Biology from Tulane University. A certified Louisiana Master Naturalist, she has over a decade of experience in science communication, with research spanning corals, cypress trees, marsh grasses, and more. At Greg, she curates species data and verifies care recommendations against botanical research.

See Kiersten Rankel's full background on LinkedIn.

Editorial Process

Pest identification and treatment guidance verified against Thuja occidentalis 'Emerald Green' field reports from Greg's botanical database, cross-referenced with university extension sources and published horticultural research.

0+ Greg users growing this plant

USDA hardiness zones 2a–7b

Citations:
Missouri Botanical Garden
Royal Horticultural Society
NC State Extension
Spider mite photo: Clemson University - USDA Cooperative Extension, Bugwood.org (CC BY 3.0 US)
Scale photo: Whitney Cranshaw, Bugwood.org (CC BY 3.0 US)
Caterpillar placeholder: zleng (CC BY 2.0)
Deer photo: Charles J. Sharp (CC BY-SA 4.0)