I donot know about your garden but my is plaged by slugs. So if yours is like mine do not forget slug bait.

Try white sticky traps. I have found these are best for attracting the most of the bad bugs, aphids etc. I use white foam (meat trays from the grocery store work good) 2" by 6" stapled on a garden stake and then take Tangle Foot spread on lightly and place every 10'. I know most of the bugs like my white and pink dalhia or I can just see them better so place the stake right next to the plant. Do this real early in the season when they just start to hatch.

Lucky me, I've never had whiteflies attack my Dahlias but they do love my Hibiscus and lemon trees here in SoCal. For me, what has worked best is a good systemic poison (although I am now having trouble locating these!) or simply blasting them off the leaves using my hose regularly. I've read that they like humidity so I don't do this in the evening. Morning through mid afternoon only.

If I get in the mood and the infestation is large, I get out my sprayer gizmo and use soapy water. A little dishsoap and water does the trick. Probably not very organic but it works.

As for slugs, I have them too. Constant battle with them, but like whiteflies, they leave my Dahlias alone. I do have one variety where the slugs climb up and eat the flowers though. Odd behavior in my book! All of the usual slug baits work well here.