Have you ever noticed pale creamy white trails through Holly leaves? These are made by the larvae Holly Leaf Miners (Phytomyza ilicis).
These tiny flies lay their eggs in June on the underside of the leaf at the base. The larvae hatch and burrow into the leaf midrib (the large vein that runs down the middle of the leaf) where they feed until autumn. They then leave the midrib and meander around feeding inside the leaf until spring leaving these pale trails. They then pupate and the adult flies emerges in late May.
Often you will find many leaves with these trails, but many won't make it to adulthood. Some will be predated by birds and others will be parasitised by wasps.
The adult parasitic wasp, Chrysocharis gemma, lays its eggs into the body of the leaf miner larva with an ovipositor (a long thin tubular structure that is used for laying eggs). The wasp larva feeds on the leaf miner eventually killing it and pupating in the leaf. Another parasitic wasp,
Sphegigaster flavicornis, parasitises the leaf miner in the same way. Both of these form a black pupa which can be seen in the leaf.
Another, less common, parasite involved in this complex little world is Pleurotropis amyntas. This one can parasitise either the leaf miner or the parasitic wasp Sphegigaster flavicornis.