[bigsprouts]

cabbage root fly lay eggs at base of plant usually where the plant stem has rocked from side to side leaving a gap between soil and stem the fly then crawls down and lays its eggs the larvae then feed on the roots the proper collars are impregnated with insectaside and covers the gap between stem and soil if the female lays her eggs on the foliage it would not be any advantage to her what so ever hence they are called cabbage root fly
werks for me  

[gobs]

cabbage root fly lay eggs at base of plant usually where the plant stem has rocked from side to side leaving a gap between soil and stem the fly then crawls down and lays its eggs the larvae then feed on the roots the proper collars are impregnated with insectaside and covers the gap between stem and soil if the female lays her eggs on the foliage it would not be any advantage to her what so ever hence they are called cabbage root fly
werks for me  

Hence they are called cabbage root fly, as you quite rightly say, usually they lay their eggs in the soil ( which can be up to at least 12 cm distance from plant, by the way) and then NOT SO USUALLY  lay them on the plant, they prefer the midribs on cabbage, etc leaves, but some had them maggots in big sprouts, too, whether it works for you or not.