Diagnosing Brown Patches on Emerald Arborvitae: Bagworms vs. Spider Mites

Common Culprits

Brown patches on arborvitae (Thuja occidentalis 'Emerald') often stem from two pests: bagworms or spruce spider mites. Both cause needle browning but require different identification and control.

Identification Guide

Bagworms

• Look for spindle-shaped bags (1–2 inches long) hanging from twigs, made of silk and plant debris.
• Larvae feed inside bags, stripping needles. Heavy infestations can defoliate and kill the plant.

Spruce Spider Mites

• Tiny, reddish mites on needle undersides. Tap a branch over white paper; moving specks confirm mites.
• Feeding causes yellow stippling, then browning, often on lower branches first. Fine webbing may appear.
• Cool-weather mites: populations peak in spring and fall.

Control Strategies

For Bagworms

• Hand-pick bags in fall, winter, or early spring before eggs hatch.
• If many small larvae are present, apply Bacillus thuringiensis (Bt) or conventional insecticides (e.g., spinosad, permethrin) while larvae are under ½ inch long.

For Spider Mites

• Monitor weekly by branch-tapping. Treat if dozens of mites per tap.
• Use insecticidal soap, horticultural oil, or miticides labeled for spruce spider mite (e.g., acephate + fenbutin oxide). Cover foliage thoroughly.

Check neighboring plants; treat them if infested to prevent spread.