Oak Tree Bee Swarms: Identifying Scale Insects and Honeydew

St. Louis homeowner sees thousands of bees, wasps, bumblebees on large oak with small protrusions near leaves. Asks cause and risk.

Diagnosis: Soft Scale Infestation

Protrusions = scale insects (e.g., lecanium scale, oak lecanium). Scales suck phloem sap, excrete honeydew (sugar-rich liquid). Bees, wasps, ants feed on honeydew—not harming tree directly.

Scale Lifecycle & Impact

• Crawlers (mobile nymphs) emerge spring/early summer.
• Settle on twigs/leaves, form protective wax cover.
• Heavy infestations: twig dieback, sooty mold on honeydew, reduced vigor.
• Ants protect scales from predators—worsens infestation.

Management

• Systemic soil drench: Imidacloprid (Bayer Advanced Tree & Shrub) in early spring. Roots translocate to canopy; kills scales as they feed. Season-long control.
• Dormant oil: Late winter spray smothers overwintering nymphs.
• Ant control: Tanglefoot band on trunk; bait stations.

Bee Safety

Imidacloprid is toxic to bees via nectar/pollen. Oaks are wind-pollinated—low bee exposure. Apply after oak bloom (if any) or use dinotefuran (shorter residual) if concerned.

The "bee swarm" is a honeydew feast. Treat the scale, not the bees.