When you're looking for a white butterfly, it's amazing how many white butterflies you see. Large whites, small whites, green-veined whites – all three lumped by most people (especially gardeners) under the term "cabbage white" – the outright winner in the contest for Britain's most hated butterfly.

But not all white butterflies ravage cabbages. What about the orange tip – one of the earliest butterflies to appear in spring? Female orange tips look remarkably like a delicate version of the small white, as only the males sport the striking orange wing-patches that give this butterfly its name. There's also the marbled white. Confusingly, this is not a white at all, but a brown butterfly spotted with white rather than the other way around.

And finally, there's the only white British butterfly I have yet to see: the rare, elusive and sadly declining wood white. Once found in woodland habitats throughout much of England and Wales, the species is now confined to a few localities in the south and west of the country, from Herefordshire and Worcestershire via the Chilterns to parts of Devon and Somerset.

So last week I went to one of the wood white's Somerset strongholds, Staple Common between Taunton and Ilchester, in search of this delicate wisp of a creature. I've been here before: last June I tramped through these woodland rides for a couple of hours with my ever more impatient children (who gleefully pointed out every white butterfly except the right one) before we gave up and went to the pub.

It's been another frustrating day. I have somehow ended up at the wrong end of the wood, and tried to walk through an impenetrable thicket of pines before giving up. I've also had a classic false alarm: a delicate white butterfly sitting on a bluebell, its wings folded shut, exactly in the manner of the wood white. I got as far as taking close-up photos before I realise I am looking at a green-veined white – beautiful, but not the target of my quest. By mid-afternoon I was lost, hot, sweaty and wishing I had brought some sandwiches.

Then, a small, insignificant little butterfly floats past on pallid wings. Its flight action is somehow fluttery yet decisive, and for a moment I fear it will continue out of sight without landing. But it does settle, and immediately closes its wings to reveal the distinctive rounded shape and ghostly grey streaks that mark it out as a wood white.

Wood whites are one of nature's paradoxes. There's plenty of suitable habitat for them to live, and climate change should be allowing them to spread northwards. But while other butterflies such as the peacock and speckled wood are extending their ranges, the wood white is not. The reason? Those delicate little wings. Unlike its stronger cousins the wood white seems to be unable to fly between patches of woodland to colonise new areas.

And if it fails to do so, it could face extinction as a British butterfly. If that seems a bit far-fetched, consider the case of the black-veined white. Once so common that butterfly collectors competed to see how many they could catch with a single sweep of their net, this attractive relative of the cabbage whites began to decline in the early 19th century. Despite its food-plants – hawthorn and blackthorn – being abundant, the decline continued, and in 1925, black-veined whites bred here for the very last time.

So if you want to see wood whites fluttering through an English woodland, perhaps you'd better hurry – they may not be there in a year or two. In the meantime, you're not too late: they fly until the end of June, with a second brood on the wing from mid-July to mid-August.

• Send in your photos of butterflies to our Flickr group and we'll feature our favourites on theguardian.com/environment