tl;dr; Found caterpillar nests. Should I keep them or destroy them?
I spotted three caterpillar nests(?) on my apple tree. The apple tree produces green eating apples (don't know what variety). There are three other apple trees of a different variety and a pear tree which haven't been infested.
Each nest seems to contain between 20-50 caterpillars. The caterpillars are about 2 inches (5cm) long. Some caterpillars are yellow with black markings. Others are grey with black markings. About 50% of them seem to be sleeping while the others are active during the day.
Each nest about the size of a tennis ball, made by wrapping silk around large leaves. There appears to be a honey like secretion (seen in the first picture) on parts of the "nest". There are also lots of black flecks in the nest (approx 2mm in size), which I am guessing is the caterpillars/ excreta.
Question: Can someone identify the species of the caterpillar? Are they harmful to humans? If so, what is the best way to dispose of them? What is their impact on the ecosystem? If they are useful, I don't mind letting them develop into a moth/butterfly.
Location: London, United Kingdom.