Gardening and Horticulture - ants after my aspen tree

I purchased six aspen trees from a nursery a year ago. I battled (invisible)leaf caterpillers with Neem last year (3 applications 7-10 days apart). This year, there are still signs of the leaves "folding" along the edges, but that's not my biggest problem. One tree looks very sickly (small off-color leaves) and has ants gathering in bunches where the stems and leaves meet. (Does this description make any sense?) I sprayed it with Neem, but it's only been two days, and there are still a few ants on the tree. Any idea what could be causing this? (The nursery doesn't guarantee its trees and shrubs, but would be happy to sell me more insecticide.)

Do you have aphids?

"Aphids / Ants—Aphids feed on the foliage above ground. The ants move up and down the tree taking advantage of the spots the aphids had been feeding on and actually “farm” large populations of aphids for their own benefit." Why Not Aspens?? Phelan Gardens. Retrieved 07 June 2002. http://www.phelangardens.com/GardeningHints/Summer/why_not_aspens.htm