You may be interested in the following, copied from my website
http://www.learnaboutbutterflies.com, which represents my own views on this controversial subject :
If wildlife habitats were contiguous, butterflies would be able to naturally recolonise sites from which they had temporarily been lost. Unfortunately, habitats are severely fragmented, and most butterfly species are very sedentary in nature, so natural recolonisations are rare.
Because of this, conservation organisations sometimes capture females from strong and healthy populations, and transfer them to former sites so that artificial recolonisation can occur. Increasing fragmentation of habitats and isolation of colonies means that such "re-introductions" will become a vital conservation tool in the future.
It is essential to realise however that re-introductions MUST be carried out professionally, with a full understanding of the affect on donor populations, and suitable long-term habitat management in place at the receiving site, which must be analysed in great detail to assess it's suitability. Transects, mark and recapture programs, and continual monitoring of the density and condition of larval foodplants and adult nectar sources must be in place, so that conservationists and land managers can understand the reasons why any given re-introduction attempt succeeds or fails.
From the above it is very clear that amateur re-introductions, no matter how well-intentioned, are unwise and not beneficial to the long-term future of butterfly diversity or abundance. The common practice amongst amateur breeders of dumping surplus livestock is positively irresponsible, as bred stock will be genetically weaker, will emerge out of synchronisation with wild populations, will attract artificially high numbers of parasitoids and avian predators to the release site, and may introduce disease.
Adrian Hoskins
http://www.learnaboutbutterflies.com