Hi Jak: Thanks for including the photos. While it's a bit difficult to see detail in the close-up of the leaves and insects, it is an elm tree which makes me think the insect may be the European elm flea weevil, European Elm Flea Weevil | Ohioline (osu.edu). The factsheet provides some information about treatment but also recommends contacting your local, Franklin County extension agent. Alternatively, you could contact Curtis Young in Van Wert County who wrote the article or Joe Boggs in Hamilton County who is an entomologist.

You may want to contact Franklin County Urban Forestry since the tree appears to be on public property. Typically, the county urban forestry department is responsible for health and management of trees on public property.

Good luck.

Thanks for the clarification. It's good news that it's not the weevil which can be destructive. Galls are pretty common and in general do not harm the tree. They are usually caused by an insect, wasp, fungi etc. I believe galls can be formed by the tree itself in response to the whatever organism is on the tree. There is a huge range of galls. The elm tree should be fine. 

Jak -- If these are galls -- they could also be an insect cocoon or other developmental stage of an insect -- they generally do not harm the tree. And the insect that they harbor may be beneficial to the tree or ecosystem. 

You might take a leaf or small twig to a nursery or landscaper to get a more specific identification. A good nursery ought to be able to identify and advise on remedies if any are needed.

Regards,

Jane