Are you sure they're whitefly? (Usually in a glasshouse rather than outside). The only reason I question it is that I worked in a glasshouse with severe whitefly infestation and didn't get bitten at all - and I'm one of those people that normally get eaten alive by gnats etc.
Anyway, I use a combination of Anthisan cream on the bite and take an anti-histamine tablet such as Benadryl, I find this does the trick of relieving the itching and swelling.
Best of luck.
