Sometimes you can get a whole cluster of tiny caterpillars that move as a gang eating their way through stuff and are then gone again leaving no trace. If they are no longer there, there is not much point spraying with anything.
The tree should be OK, but it has lost leaves, so will struggle a little, It would be worth looking after it a bit. Make sure you keep it watered and sprinkle round some fertiliser. You could also look at putting a mulch of rotted manure or garden compost round the base, if you have it. Just make sure the soil underneath is well watered and don't let the mulch come right up to the trunk of the tree.