That looks like a Pear and Cherry Slugworm - they're actually pale yellow, caterpillar like larvae which secrete a black, slimy mucus over their bodies, which makes them look black and shiny, and they;re the larvae of a sawfly. They don't just affect pear and cherry, they'll also attack plums, japanese quince, hawthorn and some other trees. They're a problem from spring up to autumn, so its probably a bit late to treat now, but any contact insecticide spray will kill them, they're not impervious to insecticides. The difficulty might be being able to spray the entire tree.

Given the leaves won't long be on the tree anyway, just make sure you clear them away once they've fallen, and keep a check in spring next year for evidence of them, and spray then. Further information and advice on spray treatments in the link below, you'll need to copy and paste into your browser

https://www.rhs.org.uk/advice/profile?PID=880

Thanks Bamboo, I'll try that