Hardiness Zone: 5a
Cee from Roscoe, Illinois
Without seeing your leaves it's hard to know exactly what kind of insect you're dealing with, but from what you describe it doesn't sound like aphids. If the little hard bumps on your leaves are red or green they are insect leaf galls. Once they appear there is no way to treat them. The good news is, they won't cause any serious damage to your tree-other than being unsightly. If the nodes are hard and waxy looking, you may be dealing with scale.
In regards to Aphids, Wholly Aphids are common to Elms, but they are soft-bodied and their eggs usually spend winter in cracks or crevices in the bark. Aphid infestation on Elms is typically characterized by curling leaves and the appearance of sticky sap in the spring and early summer while the aphids feed on the leaves.
Before you take any action you need to determine exactly what you're dealing with, and whether or not your tree requires treatment. Take a few leaves or a picture of a few leaves to a local nursery or tree expert to diagnose the problem. Good luck!
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Little bumps sound more like scale than aphids. Scale are small insects like aphids, but they make themselves a little waxy covering that protects them, so you don't see the bug, just the covering.
Horticultural oil is normally used by spraying the foliage or wherever the insects are. The point of using the oil is to suffocate the insects, so pouring it out onto the soil around the tree doesn't sound like it would be effective. Most horticutural oils have directions on the packaging to tell you how and when to use it. You'll also most likely need a hose-end sprayer, to help mix the oil with the water into a fine mist.
For your purposes (assuming your problem really is scale), a horticultural soap might work better than oil. A horticultural soap will help to dissolve the waxy covering that the scale insects make, making them vulnerable to predators. Removing their covering also helps kill them by drying them out.
Ask an expert at your local plant nursery for more detailed assistance (and to find out for sure what your pest is!)
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