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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.

Caterpillars have created a nest A view from under the leaf

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Remove the tents - these are not uncommon on fruit trees in the UK, Europe and Northern USA. They will be a moth of some sort, and will not be harmful to you. Had you got these on an oak tree, and it was Oak Processionary Moth, they are harmful and need dealing with professionally.

If some of the caterpillars are more adult and moving round the tree independent of the tents (nests) you may need to use an insecticide spray, but its probably a bit late for that. If you use a spray, make sure it is not a systemic type, but a contact spray suitable for use on edible plants, but in reality, its more effective to pick off the caterpillars by hand (wearing gloves), if you can reach to do so. Methods for removing the tents are discussed here https://www.growveg.co.uk/pests/us-and-canada/tent-caterpillar/ but wear gloves and cover your arms before tackling removal.

Note that spraying the tents with insecticide is ineffective and not recommended.

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This is probably Yponomeuta malinellus or apple ermine. They can negatively affect your fruit harvest, but the tree will usually survive an outbreak. The caterpillars will soon fly away as moths.

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