Anyone like to have a guess at this one...what has caused the damage to this dogwood, Cornus sanguinea, leaf? The line is about 5cm long and 2mm wide and can be seen on dogwood trees on the public footpath. Look out for the answer soon....
An interesting one here, this is a 'leaf mine' caused by the larva of an insect feeding between the two outer layers of the leaf. The insect in question is in fact a species of true fly (Diptera) called Phytomyza agromyzina, which leaf mines in dogwood, Cornus sanguinea.
The mine of this species occurs towards the upper surface of the leaf and is formed as the larva feeds on the soft tissues inside. The black coloured line inside the mine that appears on the photo is frass (the larva's droppings), that is left inside the mine behind the larva as it moves along feeding.
The mines are active from July - November, however interestingly, it is unknown when the adult insects fly. On pupation, the pupa usually drops to the ground, but can sometimes remain on the leaf, just inside the opening made by the larva for exiting the mine.
It is thought to be quite a common and widespread species throughout England and also occurs commonly in Europe and into western Asia.