Answer: It's really difficult to control insects in trees, unless the tree is small and you know what the insect is. It sounds like your hawthorn may have a population of aphids (these are common pests on hawthorns). While the insects may be annoying to people, once the population gets large enough they will attract predators (such as ladybugs) which will reduce or eliminate the population. If you wait patiently, the problem will solve itself. If you decide you need to eliminate the pests, the best course of action is to call a tree service company and have them spray. They'll need to identify the pests before they choose a spray control, but they have the equipment to do a thorough job.
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