Locust leaf miner will cause significant defoliation, but not tree death except that it weakens the tree and makes it more susceptible to other problems.
I'd suggest some additional investigations, binoculars will help.
- Look for borer exit holes, generally lower on the tree, well below branches that are dying.
- look for fungi fruiting bodies, discoloration, bark peeling, or other signs of root or trunk damage by fungi.
- look at adjacent vegetation for patterns of chemical kill.
- examine the falling leaves for evidence of the leaf miners: They hatch from the eggs and burrow into the leaf tissue, where they start feeding on the plant's cells. As they feed, they create serpentine mines or blotches within the leaf, leaving visible trails.
- These trails should be visible in the fallen leaves that die. After completing their feeding and growth phase, the larvae exit the leaf and pupate on the ground or within the leaf litter. During the pupal stage, the larvae transform into adults inside protective pupal cases.
- Look for adult insect: Once the transformation is complete, the adult Locust Leaf Miner emerges from the pupal case. The adults are small, metallic-colored flies with elongated bodies. They have transparent wings and distinctive black markings on their bodies. The adults feed on nectar and continue the life cycle by mating and laying eggs on the leaves of the black locust tree.