Cabbage whites under threat

Two butterfly species which for generations were the commonest and most familiar in Britain, the "cabbage whites", have now gone into steep decline, figures show. Pesticides are thought to have slashed the numbers of the large and small whites, whose caterpillars have chomped their way through untold millions of cabbages and cauliflowers to the eternal annoyance of gardeners. They are the only British butterflies to have been considered a serious pest.

Fifty years ago, they were the best-known of our species, partly because they were ever-present in vegetable gardens, having adapted to human habitation almost as well as house sparrows. Their numbers were extraordinarily high, and swarms of them, sometimes huge flocks from the Continent, could run into millions. In August 1911, for example, a Professor Oliver saw a gigantic cloud of large whites in Sutton Fen in Norfolk; he estimated the numbers to be six million insects over two acres. In 1846 there was a record of large swarms of small whites which obscured the sun in Kent.

Now, however, the picture has changed dramatically: the large flocks have vanished, with both butterflies declining all over Europe. New figures from the Butterfly Monitoring Scheme show the large white declined in the UK by one-third between 1976 and 2008, while the small white dropped in numbers by one-fifth.

Martin Warren, chief executive of the charity Butterfly Conservation, blames this largely on pesticides. "Commercial crops of Brussels sprouts, say, would all be sprayed nowadays, and not many people grow cabbages in their gardens – and those that do would probably use pesticides." However, Dr Warren says, it is possible to protect crops from cabbage whites organically. "Fine mesh netting will stop them," he says. "And as they are also fond of nasturtiums, you can plant a row around your crop, and the butterflies will lay their eggs on those first."

The third of our status reports concerns three more early season butterflies, the large white, the small white and the green-veined white, all closely related members of the pierid family, which also contains the two species we covered yesterday, the orange tip and the brimstone.

Readers can find the profiles of individual species written by our Environment Editor, Michael McCarthy, at independent.co.uk/britishbutterflies

7 Large white

Pieris brassicae

Easily identifiable as the largest white species, this striking insect is distinguished from the small white not only by its size but by the heavy black tips to the forewings which continue down the wing sides. Females have two black spots on the forewings. It has two broods a year and its population can be considerably boosted by migrants from the Continent. The black and yellow caterpillars take in toxic mustard oils from cabbages and are poisonous; their striking markings are thought to act as a warning to other creatures. They are very conspicuous on cabbage leaves.

Larval food plants: Brassicas – that is, all members of the cabbage family, such as cabbages, cauliflowers, Brussels sprouts, etc, as well as wild cabbage in coastal areas, and also nasturtiums.

Where seen: Anywhere, but very common in gardens (especially those with vegetable patches), allotments and agricultural land.

Current conservation status: A decline in Britain since 1976 of 33 per cent, probably because of pesticide use, and few migrations from continental Europe.

8 Small white

Pieris rapae

A smaller version of its large white cousin, this butterfly is if anything even more common, and with the meadow brown of late summer may be the commonest species in Britain. The adult insect is distinguished from the large white by less bold markings; the caterpillars are quite different, being green instead of black and yellow, and harder to find, as they start feeding from the inside of a cabbage rather than on the outer leaves.

Larval food plants: As with the large white butterfly, brassicas – all members of the cabbage family – and nasturtiums.

Where seen: Anywhere, but especially gardens with vegetable patches, and allotments.

Current conservation status: The small white is 20 per cent down in numbers since 1976, probably because of increased pesticide use.

9 Green-veined white

Pieris napi

This pretty little butterfly is still very common and is sometimes taken for a cabbage white. In gardeners' terms, it is blameless, for it feeds strictly on wild members of the cabbage family. Sometimes occurs in gardens but more likely to be seen in dampish areas away from human habitation. Can be differentiated from the large and small whites by heavy green veining on the underside of the wings. May be Scotland's commonest butterfly.

Larval food plants: Garlic mustard, hedge mustard, lady's smock, watercress, charlock, wild cabbage, wild radish.

Where seen: Damp meadows, wet fields and ditches, edges of lakes, woodland rides.

Current conservation status: Butterfly Monitoring Scheme data show a 16 per cent decline since 1976, but the population generally is believed to be healthy.

Joining the Butterfly Hunt

Whether as an afternoon diversion from a picnic, or a summer-long quest to track all 58 of Britain’s butterfly species, The Independent’s Great British Butterfly Hunt aims to offer a glimpse of the beauty and fragility of our rich natural heritage.

And for those of our readers with a competitive edge, we offer a competition. The aim: to see as many of our native butterfly types as possible. The prize is an afternoon tracking the most elusive, the brown hairstreak, with expert Dr Martin Warren. It is the very last of UK butterflies to emerge (at the end of August), medium-sized and brown, with distinct “tails” on the hindwings. This hidden wonder of a creature can best be spotted by the “white pinprick” of an egg, laid on blackthorn.

The winner’s rail travel expenses within the UK will be covered and lunch provided.

We encourage entrants of all ages. Simply send us, by Monday 17 August, 12pm, your butterfly diary. Briefly list each native species you see; the date and time; and the exact location. And please add one very brief description (no more than 250 words) of your butterfly hunt.

The judges will take into account the number of species spotted and also the description.

Enter by post (Butterfly Hunt, The Independent, 191 Marsh Wall, London, E14 9RS) or email (britishbutterflies@independent.co.uk). The winner will be announced in the newspaper. For terms and conditions please see independent.co.uk/comprules or send an SAE. Best of British.