The first willow leaf has now almost been consumed, so the trek to the next leaf will commence very soon. Notice how all of the larvae have migrated to the stalk end of the leaf in preparation for the journey. Larvae are currently about 4-6 mm long.
And here's the bad news as far as the larvae are concerned. This is the primary parasite, a Campodorus sp. Ichneumonid. This is the one that oviposits from under the leaf, curling its long abdomen round the leaf edge. The larvae are currently too small to target, but I suspect egg-laying will commence in a day or two.
Interestingly enough, this specimen was checking out individual leaves and then walking along the willow twig to the next leaf. I suppose it's making sure the trekkers don't escape its attention.
A very common leaf-miner found on Alder is the Sawfly Fenusa dohrnii. This makes a brown blotch mine that wanders between two veins, heading towards the leaf edge.
This close-up of the larva shows the distinctive shape, with wide 'shoulders', which confirms that we are looking at a sawfly rather than a fly, which has simple bullet-shaped larvae.