Norway Maple with Black Crusty Bark: Borer Damage, Not Lichen

Black Crust on Norway Maple Trunk = Borer Activity

The "black lichen" with earthworms and pillbugs underneath is actually sap flow from borer tunnels that has darkened and colonized by fungi.

What's Happening

• Borers (likely flatheaded or roundheaded) tunnel under bark.
• Tree exudes sap to drown them – sap ferments, turns black.
• Insects and fungi colonize the wet bark.
• Wood beneath is solid because borers are in the inner bark/cambium, not deep wood.

Treatment

1. Spray trunk and scaffold branches with Merit (imidacloprid) or Onyx (bifenthrin): Penetrates bark, kills feeding larvae.
2. Apply now and repeat in 3 weeks.
3. Fertilize: 10-10-10 at 1 lb/inch trunk diameter to boost vigor.

Do Not Drill Inspection Holes

Drilling creates new entry points for decay fungi. The external symptoms are diagnostic enough.

With systemic bark spray and fertilization, the tree can outgrow the damage.