miner matters

WHEN IS A MINE NOT A MINE?
Colin Plant says: In my host plant list, which is being uploaded to the british leafminers web site "as we speak", I define a mine as being caused when the causer feeds on leaf material between the upper and lower epidermis of the leaf whilst leaving the two epidermal layers intact (other than entry holes as in, for example, Coleophora or adult emergence holes). The question has now arisen - How do we know the mine was caused by feeding? Last week I received for naming some apparently mined Berberis leaves from Kerry Robinson in North Hertfordshire, but had no idea what had caused the mines. The leaves were hollowed, but did not contain any frass (photos attached).
I asked Andrew Halstead (Entomologist at the RHS Gardens in Wisley) for an opinion, as he deals with queries such as this on cultivated plants on a daily basis. His reply is as follows: 'The Berberis leaves have arrived. The mines are not mines, but areas of raised cuticle where a female berberis sawfly has inserted a batch of eggs. These are deposited in groups of up to about 7 eggs. This insect can also develop on Mahonia, so beware of ''mines'' on that plant. Berberis sawfly was first confirmed as being present in Britain in 2002, but had probably been here since at least 2000. It is now widely distributed in the counties around London.' So - egg-laying by the Barberry Sawfly (Arge berberidis) (Symphyta: Argidae) can look confusingly like mining! You have been warned!
Images ©Colin Plant