Hi Mary, 

At first glance, my mind went straight to Eastern tent caterpillars, but on closer inspection of one of the photos, I'm leaning towards a fall webworm, which appears in late summer. The identifying pattern of the black paired tuburcules along the back, in combination with the tent being located at the end of a branch as opposed to a "crotch" likely confirm these are webworms. 

In either case, the advice is the same. These are native species with lots of natural predators, and rarely kill a healthy established tree, although they may defoliate sections of the tree which affects its ornamental value. Recommendations generally include: 

- Leaving them alone and accepting the risk of significant aesthetic damage  

- Removing the webbed "tent" and the caterpillars within with a stick or other implement (something that will do minimal damage to the branches on which the tent is situated)

- Pruning affected branches if the tent is in one place, is small, is within reach, and if location allows you to do so without affecting the desired "look" of the tree 

Please note chemical controls can be used, but are likely to be overkill. If you choose to remove the tent physically, caterpillars can be tossed into a bucket of soapy water or frozen to kill them and ensure they do not return. Additional advise includes doing your best to maintain the health of the tree while the pests are there, to ensure it bounces back from the damage successfully (A good watering/feeding schedule, etc). Branches with egg sacs in the off season if you find them - can be pruned as well. 

Hope this helps! 

Best, 
Kristin

Resources: 

Fall Webworm (psu.edu)

Fall webworms: Should you manage them | Purdue Extension Forestry & Natural Resources

Hi Mary! 

The images you provided have given our specialist in "woody" plants such as this tree a chance to try and ID the species, so I can give you more specific advice, but the photos aren't quite giving him enough to work off of. If you can provide clearer, close up images of leaves, etc that would be very helpful in identification. Does the tree flower or fruit at all? If you can provide some more specific photos for identification and give us more information on whether it has any flowers or fruit on it at any point during the year, it would help us ID the tree and provide you with specific advice or resources. 

The best advice I can give without knowing the species of tree (and therefor its specific requirements), is to continue to check it for signs of distress once you've removed the pests. Often they bounce right back from smaller setbacks, and it sounds like this tree is determined to thrive! 

Hello Mary, 

It appears this could be common buckthorn. There are some others that are common lookalikes (presented in the below link), but if it is buckthorn it is an invasive. 

Do you see berries on the tree at any point? Buckthorn has dark berries (as seen in the below resources) and would provide another identification opportunity. 

https://www.burnsvillemn.gov/DocumentCenter/View/2164/Printable-ID-Guide?bidId=

https://vtinvasives.org/invasive/buckthorn-common