Caterpiller in leaves of crabapple tree

[cazp]

I have noticed that some of the top leaves of my crab apple tree have folded in half and are stuck together with a sticky slik like substance - inside I find a tiny bright green caterpillar - anyone know what this is, and if it needs treating?
Thanks

[FB.]

It is unusual for caterpillars to reach damaging numbers on an apple tree.
Only dwarf fruit trees, with their weak vigour, might be at risk of serious damage.

[cazp]

Thank you. The tree looks quite strong and only a few leaves are affected to hopefully no real harm done.

[Acanthus]

I get that on my Red Hornet, oh and I go on a bug hunt, squashing the caterpillars between the leaves