Sticky Sap Dripping from Oak: Aphids or Scale

Honeydew on Deck Under Oak: Insect Diagnosis

A homeowner fertilized a large old oak last fall. Now, a year later, the deck underneath is covered in sticky sap droplets. The tree looks okay. Is this normal or distress?

Cause: Sucking Insects, Not Tree Sap

The "sap" is honeydew—excrement from aphids or soft scale insects feeding on leaves and twigs. It's high in sugar, sticky, and often grows black sooty mold.

Identifying the Culprit

• Aphids: Small, soft-bodied, often on leaf undersides and new shoots.
• Scale: Bumps on twigs/branches; armored (hard) or soft (cottony/waxy).

Treatment Options

Systemic soil drench (best for large trees): Imidacloprid (Bayer Advanced Tree & Shrub Insect Control) applied to soil in spring. Roots move it to foliage; sucking insects die on contact. Lasts months. Low impact on beneficials.

Foliar sprays (if tree is small enough to cover): Acephate (Orthene), bifenthrin, horticultural oil, insecticidal soap. Requires full canopy coverage.

What About the Fertilization?

High-nitrogen fertilizer can boost succulent growth, attracting more aphids. Use balanced 10-10-10 at recommended rates only if soil test indicates need.

Sooty Mold

Black fungus on honeydew. Harmless to tree; washes off with rain or hose. Disappears when insects are controlled.