I'd give the Avid a shot and follow it up with Forbid, in a rotation attack......Don't start with out having the forbid....
this way the whiteflies will not build a resistance to it. At least that's the idea in using that method. Whiteflies have a strong ability to become resistant to insecticides. A good bet is to use and insecticide soap to start and apply every 3-4 days till you target the remaining ones with the "nukers".
By attacking outside in your area. you might consider treating the area with Ortho systemic insect killer.
3 weeks later. Hit the area with Bayer advanced tree and shrub insect control and rotate into the nukers....
Whitefly parasites are effective in warmer months.
lacewing
Ladybugs
When i worked and ran greenhouses. We would order
Encarsia formosa and
Delphastus pusillus.. These things do a great job in controlling Whiteflies. At least in the greenhouses it did.....If you have them around, they'll stick around outdoors and really help in control...
Here's a public outlet for Whitefly preditors.
http://www.naturescontrol.com/whitefly.html
Good Luck! I hate them buggers too!