Pear tree viability
Tree perhaps 30+ years old. Originally 5 variety dwarf. Now 25 feet tall comice only variety left. Tree appears to grow vigorously even with small limbs being heavily attacked by catapillers and with black leaf scabs. Scarce Fruit this year all with black blight. Should I give up on this tree?
Your trees likely suffer from pear scab. To control pear scab remove the infected leaves and fruit from under the tree in the fall after the leaves have been shed. Either compost the leaves and fruit a good distance from the tree or dispose of them. Lime and fertilize underneath the tree to speed decomposition of any leaves left over. In the spring if you have had a severe infection, it's good to spray the tree starting at the time the buds begin to break, then again a month after petal fall with a fungicide, like sulfur, bordeaux mixture, neem oil or copper.
You say you also have caterpillars in your pears. There are several insects that like pears. One good method to find out which ones you have is to place sticky traps in your trees in the spring. The insects that get trapped can be identified, and a control program begun. Control methods are based on the insects you have. Using a dormant oil spray on a sunny day in winter can smother the eggs of many insect pests.
Here is are a couple of publications may find useful.
Hope this helps!