Hello, and thanks for contacting the Adams County Master Gardener program. These small little insects are aphids and the wasps and ants that they attract are after the sticky honeydew that they leave behind. There is a type of aphid that does use the narrow leaf cottonwood as a host. If your tree is small enough that you can hose off the leaves with a strong jet of water it would help, but you may need something stronger if the tree is taller. Aphids generally have at least two hosts, and in your case it could be a choice of two types of aphids. However the treatment is the same. I have included links to two fact sheets on aphids and the chemical treatments you can use, including horticultural oils and soaps and also herbicides. In your particular case it is the honeydew that is causing the problem. Many times Aphids do not cause serious damage to the tree or plant they infest. Please check out these fact sheets and make sure that your choice of treatment will take care of Aphids. Also, remember that aphids or any plant pests will take advantage of any stress that the plant may be experiencing. Watering your trees during drought is very important and providing good "cultural" care. Make sure you have good organic mulch at the base of the tree. I have included links to these subjects as well.
https://extension.colostate.edu/topic-areas/insects/aphids-on-shade-trees-and-ornamentals-5-511/ 
https://planttalk.colostate.edu/topics/insects-diseases/1402-aphids/
https://planttalk.colostate.edu/topics/trees-shrubs-vines/1751-fall-winter-watering-drought/

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