There's loads of cabbage white butterflies about on our allotments but also lots of hoverflies, especially round my sprout plants. So far as I can see, I have no butterfly eggs (or caterpillars) on my sprout plants and I wondered if the hoverflies are eating the newly laid eggs, because there's areas on the underside of the sprout leaves which look like there might have been butterfly eggs there.
Many hoverfly larvae go for aphids, though there's a wide range of diets to be found. The adults feed on nectar.