Tree of Heaven (Ailanthus), is the preferred host plant for the invasive Spotted Lanternfly, so it is not surprising that the insect is covering it.
You can learn more about this new pest here:
https://extension.umd.edu/resource/spotted-lanternfly-home-gardens/
Now that it has arrived, we are finding that Spotted Lanternfly is not as serious as we once thought it might be. Like the Brown Marmorated Stinkbug years ago, it is expected to be a nuisance pest for a while but we are already seeing our birds, insects and wildlife finding them tasty and helping with control.
The areas where they first arrived in Maryland in large numbers are not seeing near as many now.
For that reason we are not recommending any pesticides be applied in home landscapes.

Perhaps one of the best things you can consider doing is have the Tree of Heaven removed if that is a possibility. You can't just cut them down as they respond by sending roots out in all directions and you may end up with a plantation of them. There is more info on how to get rid of them (from Penn State) at this link here:
https://extension.umd.edu/resource/tree-heaven/
The timeframe to follow those directions through leaf coloring in the fall, which it may be too late now if the leaves have dropped. Being that it is a park property you may have to go through a county approval to have the tree removed. 

Emily