Scale insect
Coccoidea
73 host plants
Last updated
Scale insects appear as small, brown, rounded lumps on plant stems, around leaf joints, and undersides of leaves; colors range from brown to white, tan, or orange. Heavy infestations result in poor growth, reduced vigor, yellowed leaves, and eventual decline in heavily infested hosts. Black sooty mold growth indicates honeydew excretion.
For light infestations, remove scales with a soft brush dipped in soapy water. Horticultural oil with thorough coverage suppresses crawlers; repeat every 6-7 days for control. Target immatures rather than armored adults. Maintain vigor to boost natural predators.
Quick Reference
Monitoring & Action
Dormant season (Nov-Feb): Scout bark for overwintering scale; flip bumps to check for live insects. Crawler detection (Apr-June): Wrap double-sided sticky tape around infested branches; check weekly with hand lens for tiny crawlers (appear as colored specs). GDD tracking: Use species-specific GDD₃₂ thresholds (not calendar dates) to time control - pine needle scale ~2,200-2,600 GDD₃₂, oystershell ~2,500-2,800, euonymus ~2,600-3,000, lecanium ~3,400-3,600, cottony maple ~3,600-3,700. White paper tap test: tap branch over white sheet; crawlers fall and become visible. Ant trails running deliberately up trunk = soft scale honeydew source.
Dormant stage (Nov-Feb): Low threshold - apply dormant oil on plants with visible bark encrustation from prior year. Crawler stage (Apr-June): High threshold for action - crawlers are the only vulnerable, mobile life stage; all control efforts target this window. Settled stage (June-Sept): Treatment ineffective; contact sprays cannot penetrate settled scales. Soft scales with heavy sooty mold: Treatment justified when mold reduces photosynthesis on majority of canopy.
Soft scales: honeydew excretion, sooty mold, cosmetic and photosynthetic damage. Armored scales: direct cell-content feeding, branch dieback, plant decline. Both types can kill branches or entire plants in severe infestations.
Cultural Controls
- Protect natural enemy complex. Parasitic wasps (Aphytis, Encarsia, Metaphycus, Coccophagus, Blastothrix), lady beetles (twicestabbed lady beetle Chilocorus stigma is scale specialist), green lacewing larvae, hover fly larvae. When undisturbed, achieve 59-92% parasitism rates on scale nymphs. Single broad-spectrum spray can trigger scale outbreak worse than original infestation by killing predators while mites survive.
- Break the ant bridge on soft-scale hosts. Apply sticky bands (Tanglefoot) around trunks. Ants farm honeydew and aggressively attack approaching lacewing and parasitoid larvae. Ant exclusion reduced surviving scale nymphs by 54% after one year, 69% after two years; on magnolia, 82% within single year. For rough bark, wrap with fabric first, then apply sticky material. Refresh when filled with debris.
- Prune heavily infested branches. On small plants, scrub scale off with soft brush or alcohol-dipped cloth. Physical removal immediately effective.