Mangold fly
Pegomyia betae curtis

Description
The adult mangold flies are up to 7 mm long with greyish / brown bodies.  The maggots (referred to as `beet leaf miners`) are legless, without clearly defined heads and are 6 - 7 mm long with semi-transparent bodies which are  predominantly white in colour. 

Symptoms
The first signs of beet leaf miner activity are small white elongated eggs, approximately 1 mm long, attached to the undersides of leaves or cotyledons.  These are usually found in groups of 2 - 10 placed side by side.