The short-ish answer is, if cabbage white butterflies are evil, then yes. OK, there are wood whites, and marbled whites, but I bet the ones that you have been looking at are either small white or large white butterflies.
What’s the difference I hear you ask – well, one is larger than the other! Other than that, the large white has broader wings which does make them easier to identify if you see them at rest (which is not very often as they range around looking for a mate). Other than that there are a couple of differences that will help identify them and I have provided some photos to help.
Large whites have black tips on their wings, the females have two black spots, the males do not have any. Small white man butterflies have one spot on their forewing, the lady butterflies again have two. The small white wingtips are also dark, but as far as I can tell are much lighter than their larger friends. A word of caution on this last point however, there are two broods of white butterfly each year and the markings on the July / August batch tend to be darker than those of the earlier brood – probably a bit like a sun tan. Oh yes, and, when the wings are closed, even the male large white appears to have spots.
In terms of their cabbage-destroying propensities, then they have different modes of attack. Large white butterflies lay their eggs on the outer leaves of plants of the cabbage family, and their conspicous caterpillars appear in large numbers munching all the leaves in sight. The small whites are more devious – they lay their eggs in smaller numbers in the heart of the cabbage where a single caterpillar will munch away the innards making them much more difficult to deal with.
There used to be a lot more of the whites in the UK, but a combination of pesticides and a virus drastically reduced their numbers (mmm… sounds familiar). Many of the whites seen in our gardens will have come over from the continent.
OK, I am not a farmer or allotmenteer, but I have had some brassicas attacked by caterpillars, but I think you have to agree that the world would be a sorry place if the pretty white butterflies were not perpetually on the move around our gardens. I will make sure I have some verbena and buddleia for them in my garden, I will even contemplate a brassica or two for their caterpillars.