Whitefly on mint plant
Sorry for the delay in reply to your e-mail.
Encarsia formosa do not work outside very well, as they are specific to glasshouse whitefly. However I think the Mint might be under attack from Mint aphid (they do leave skins on the plant that look like whitefly). I would suggest you use the pyrethrum based spray; Insect Killer spray from Harrod Horticultural.
This has a wide knock down effect on many soft bodied insect pests. It is also safe to use on edible plants, just leave the mint 24 hours after spraying before eating. The pyrethrum breaks down naturally and is used in organic gardening. Spray once a week to break the life cycle and make sure you get a good coverage over the mint.
Hope this helps.