Pieris cheiranthi from Gran Canaria is a curiousity nowadays. It became extinct a number of years ago on both Gran Canaria and La Gomera and is only surviving on Tenerife and in much better numbers on La Palma. What year are your G.C. specimens from Dunc?
In Tenerife the species also has disappeared from many areas, but in the north you still see individual specimens in downtown Puerto de la Cruz. In the Orotava Valley and further west of there, the species can be common, especially during the winter months and up to about Easter time. Eggs and larvae can be collected in wast numbers on Nasturtium (Tropaelum majus) even along the main hi-way west of Puerto de la Cruz. Around Easter time larvae are often prone to parasitic wasps that spin white/cream colored cocoons along the length of the adult larvae, exactly as we know them from the European large cabbage white.
Females prefer to lay eggs on host plants growing in part shade and on plants that grow under overhanging rocks, mostly north facing. My father grow Nasturtium on his 3 sq.m. north facing balcony and P. cheiranthi females oviposit on a very regular basis on his plants. His potted Nasturtiums are apparently so much better than all the hundreds of wild Nasturtiums in the Barranco next to his apartment! The past few days I papered 2 dozen perfect specimens from pupae he sent me. Its a nice "pest" to have on your balcony!
Some pics of the early stages of Pieris cheiranthi:
Very rare in my collection (non existent in fact), yes I'm not sure about them being separate species to brassicae, from what I've read cheiranthi has been separated on account it doesn't enter diapause, does brassicae in Africa?