What has done this on these holly, Ilex aquifolium, leaves? The photos are of the large holly tree growing in the woodland corner near the end of Cottage Lane. Answer to follow next week....
The blotches on these leaves are made by a species of true fly (Diptera) called Phytomyza ilicis, or the holly leaf miner. Indeed it is the only species of insect in the UK known to leaf mine holly.
The female fly lays a single egg on the underside of a leaf petiole (stalk), or mid-rib in May or June. The larva then hatches out shortly afterwards and tunnels into the mid-rib of the leaf. For the remainder of the summer it feeds exclusively within the mid-rib, moving into the leaf blade in December - January. Here it feeds on the soft tissues in between the upper and lower cuticles of the leaf. The blotches in the pictures are where the larva has tunnelled through the leaf, feeding along the way. The larva spends the rest of the winter feeding inside the leaf, before pupating in early spring. Before it pupates, it cuts a semi-circular slit in the epidermis (upper cuticle) of the leaf for the imago insect to emerge from. The imago insect emerges later in the spring and is a small, black fly.
There are many parasites of the holly leaf miner, most being species of parasitic wasp. The adult wasps lay their eggs on or in the vicinity of the fly larva, the wasp's larva then hatches and feeds on the body of the fly larva. If the emergence hole is circular instead of semi-circular, this is a good indicator that the fly larva was parasitised.
The holly leaf miner is common and widespread through most of the UK and indeed Europe and can occur in many habitats including woodland, gardens, hedgerows and parks, in the presence of the host plant species.