Aphids are a challenge, especially on a mature tree. Aphids are quite defenseless and there are numerous insects that feed on them (Fact sheet 5.550, Beneficial Insects and Other Arthropods). The best known of these natural enemies are lady beetles, with lady beetle larvae being particularly voracious predators of aphids. Other common aphid predators include the larvae of green lacewings and flower (syrphid) flies. Several species of minute stingless wasps parasitize aphids. These wasps insert their eggs into the body of the aphid and the larvae consume it internally. Aphids that have been killed by parasitic wasps have a conspicuous appearance, turning light brown or black and becoming bloated. Aphids killed by parasitic wasps are known as “aphid mummies.”
Scale is a more difficulty control.
European elm scale has a single generation per and passes the winter months as nymphs in bark crevices. However, crawlers (young nymphs) are generally seen on foliage in June and August. Male nymphs can appear as early as January to March but females do not appear until spring. They migrate to branches where mating occurs and females develop waxy sacs under which they lay eggs. A single female can lay up to 400 eggs. Eggs hatch into crawlers, often within an hour after they are deposited. The crawlers move to the undersides of leaves where they feed near the primary leaf veins. Later they migrate back to branches to spend the winter.
Heavy infestations may require treatment. Several options are available if treatment is necessary. One approach is to spray infested trees with horticultural oils. These are highly refined oils that can be applied without damaging the plant. They are available in most garden shops under trade names such as “dormant oil, supreme oil or spray oil.” The oils cover the scales and smother the insect. Most horticultural oils are applied during the dormant season but several types are available that can be applied safely after leaves have developed.
You did not read my e-mail correctly as I said I LIVED UNDER 5 American Elm Trees. The treatments & methods of control, you suggested, will not work on aphids & scale on enormous trees.
I did understand your question and do understand your challenge. There are very few options.