Pemphigus spp.
See:
Pest description and crop damage Aphids cause flask-shape galls on leaf petioles. Galls are not hazardous to trees.
Management-chemical control
See:
Chemical Control of Nursery Pests
For more information
Johnson, W.T. and H.H. Lyon (1991), Insects That Feed on Trees and Shrubs, 2nd ed., Cornell University Press (p. 460).