Arborvitae Browning in Drought: Mites, Leafminers, and Recovery

Globe arborvitae (Thuja occidentalis cultivars) suffering sudden browning in Missouri's extreme heat likely face a dual assault: environmental stress compounded by pest infestations that exploit weakened plants.

Diagnosing the Pest

Arborvitae leafminer:

• Larvae mine inside scale-like leaves, hollowing them out
• Break a brown needle: if hollow/translucent, leafminer active
• Treatment: systemic insecticide (acephate/Orthene) in May-June when larvae feed

Spider mites:

• Microscopic; suck chlorophyll from foliage
• Tap branch over white paper: tiny moving red/black specks = mites
• Treatment: miticide (bifenthrin, spiromesifen) or horticultural oil; repeat weekly x3

Recovery Protocol

1. Water correctly: Soaker hose at base, 45-60 minutes every other day during heat. Deep, infrequent > shallow, frequent.
2. Mulch: 3 inches shredded bark to drip line; keeps roots cool/moist
3. Fertilize: Fall (Oct) and spring (Apr) with slow-release 10-10-10 or evergreen formula
4. Anti-desiccant: Spray Wilt-Pruf in late fall to reduce winter burn
5. Monitor: Check weekly for pest resurgence

Prevention for Next Year

• Pre-emptive mite spray in early June if history of problems
• Improve soil with compost top-dressing annually
• Consider shade cloth during extreme heat waves
• Plant in well-drained soil; arborvitae hate wet feet as much as drought

With aggressive watering and targeted pest control, even severely browned arborvitae often push new growth from surviving green tissue.