They lay eggs in holly leaves.  It is one of the few insects that is able to make use of holly leaves as a food source and somewhere to live (when a larva).  The female fly lays eggs in the holly leaf (near the main veins or midrib – on the underside) using a thin tube or ovipositor.  The eggs are usually laid in early Spring when there are young and ‘soft’ leaves.  Older leaves have a thick and tough cuticle that is far more difficult to penetrate.  The larvae or maggots emerge from the eggs and tunnel their way along the midrib / veins emerging some time later into the lamina or blade of the leaf.  Here they feed on the photosynthetic tissues of the leaf.