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.