I'll probably be considered mad, but my nasturtiums are covered with large white caterpillars. There are a few small white caterpillars as well.
Netherfield - those are large white caterpillars. Small whites are plain green. Large white buterflies are partially migratory (if I remember correctly) and when the weather is good we get an influx of adults from the Continent. A bit like Painted Ladies.
Sweet Rocket (a native wildflower) is a member of the brassica family and is also a food plant for the 'cabbage' whites (well, it is here, anyway). The plant is a biennial and has done flowering by the time the whites lay most of their eggs. Anyone looking for a plant that emits a sweet, heady evening scent for the wilder parts of the garden might like to consider sweet rocket