The mangold fly is drawn to the mangold wurzel just as the fruit bat is drawn to fruit or the peacock to peas. The fly lays its eggs in neat rows on the underside of the leaves, then flies off to do something else.
The eggs take 3-7 days to hatch, which is quicker than chicken eggs, and emerging from the eggs we find not chicks but the horrible mangold fly larvae. These are known as Beet Leaf Miners because they use their jaws to cut a path through the mangold leaves, eating the juicy bits and leaving the unjuicy bits.
When they have quite finished, the larvae burrow into the soil and pupate, eventually to hatch into more mangold flies. This is known as the Circle of Life and may be represented in a diagram.