David Harper and David Streeter BIOLS

We couldn't help noticing the piece in the last Bulletin (13 June) on the state of the landscaping around the underpass. Views on the aesthetic virtues of the 'field of Rape' clearly vary. We went down to make up our own mind, only to be confronted by a charming man who clearly mistook us for members of Estates staff as we were vigorously harangued for ploughing up all those lovely yellow flowers!

Now that the area has been prepared for re-seeding, we feel constrained to point out that the controversial yellow flowers were not Rape at all but Charlock, a common weed of arable fields, especially on calcareous soils.

It is one of those plants that has been associated with cultivation for so long that its original home has been lost in history. Fitzherbert, in his Boke of husbandrie, (1523), called it 'an yll wede, and groweth in al maner of corne'. We spent a happy half-hour making a list of the 43 species of 'wedes' that we could find, among which was just one plant of Rape!

What was more interesting was the appearance of a much less common plant of the same family - the brassicas - Hairy Rocket. This is a native of central Europe and the Pyrenees that most often appears as a casual on roadsides and waste ground, and has apparently been seeded as a stabiliser on steep chalk cuttings.

We were also excited to spot a female Clouded Yellow taking nectar from White Clover flowers behind Park Village. When at rest, this butterfly keeps its wings tightly closed above its back and so only the rather dingy yellow undersides are visible. The upper wings, however, are orange (leading to the old name of Saffron Butterfly) with broad black margins. The latter are spotted with yellow on females, but unmarked on males.

Amazingly, our female was not born here: Clouded Yellows rarely survive the winter north of the Pyrenees or Alps. In southern Europe and North Africa, however, Clouded Yellows breed throughout the year and these populations give rise to mass northward migrations every spring. The number reaching Britain varies: they are usually scarce but in 'invasion years' such as 2000, thousands cross the Channel.

Although Clouded Yellows take nectar from many plants, they usually lay their eggs on clover species in a sunny location. These hatch in about a week and the grey-green caterpillars gorge on clover leaves for about five weeks. They will pupate on their food plant, emerging as adults within three weeks. This rapid life cycle means that the immigrant population can soar in numbers, until cut down by cooler weather. Some newly emerged adults fly south in the autumn, but - as with our commoner migrant butterflies, the Red Admiral and Painted Lady - it is not known if any survive the trip.