fraxinus leaf caterpillar in Vancouver?

I have a healthy fraxinus, @ 10 years old over our rear deck. A couple weeks ago I noticed lots of "frass" all over the deck and immature & chewed leaves were falling on the deck and table. Today I noticed some small (2 cm) green caterpillars had fallen on the deck. Above, many of the leaves show signs of caterpillar damage, but the tree seems healthy. Any idea what this is?

 

consider Bruce Spanworm (and BtK as a 'cure' if thats the culprit)