Sempervivum leaf miner is a hoverfly that as a larva mines the leaves of Sempervivum species.

You may see the following symptoms:

• The outer leaves on affected house leek rosettes become greyish or brownish-green where the internal tissues have been eaten 
• Damaged leaves go limp before drying up
• Small rosettes may be killed 
• If a larva has left a leaf to move to another, or to pupate in the soil, there will be an exit hole near to its base
• Damage is mainly seen in mid-May to June and again in August to September

Over-watering or poor drainage can cause similar symptoms to sempervivum leaf miner, however closer examination should reveal the presence of plump whitish maggots, up to 8-10mm (about 3/8in) long, inside some of the damaged leaves.

• Leaf mining insects are difficult to control because they are protected inside the plant tissue
• Damaged leaves and larvae can be removed when they are found
• There are no insecticides that are suitable for controlling leaf mining insects in home gardens

The adult fly is 8-10mm (approx. 3/8in) long and is black with short whitish hairs on the thorax and abdomen. Eggs are laid on the foliage of house leeks in late spring to early summer, with a second generation in late summer.

After hatching, the larvae bore into the outer leaves and eat out the internal tissues. Each larva will mine several leaves before it is fully fed and ready to pupate in the soil.

Adults of the second generation emerge in July to August and further larval feeding occurs in August to September. This pest overwinters as pupae in the soil.