Q.Large weeping willows tree treatment of insect infestation
Can our willow tree be treated from the ground (root based treatment) or does its canopy have to be sprayed? What insecticide is recommended?

Certified GKH Gardening Expert
With a tree this size, you'll want to be sure to identify what the infestation actually is. You may want to take a sample to your local agricultural extension service. With that knowledge, you can talk with a garden center to learn about what specific kinds of sprays to use and how.
Here's an article to help you troubleshoot the problem: https://www.gardeningknowhow.com/ornamental/trees/willow/treat-peeling-willow-bark.htm You may also want to contact a certified arborist for advice.
Here's a link for your extension service:
https://www.gardeningknowhow.com/extension-search?zip=49415&radius=50&services_search_action=search&submit=Search