What states can you still get a DUI for just having a trace of marijuana in your system?

Why don't you Google it? If you got arrested for a DUI, we know it isn't in Texas. If you are just doing research, the object is for you to actually do the research.

Don't know of any such states. Normally you have to be "impaired" (and therefore have enough alcohol or drugs in you system to effect or impair your driving ability) in order for you to be charged with driving under the influence.

In theory, any state. MJ DUIs aren't well litigated yet so you have to be extremely careful.

All states. If an officer has reason to believe (by odor, admission, or physical evidence) that you have used marijuana "recently" and determines after investigation that you were operating a motor vehicle while impaired, he can arrest you. Whether the DA can secure a conviction based on that evidence is up to a jury. If the THC levels are very low, the DA may consider reducing the case rather than going forward with trial. These are case-by-case decisions, which are obviously extremely fact-specific.

Unless the officer has a drug test in his possession there is no way they can determine if you smoked pot 3 weeks ago. They can however smell it if you smoked recently.

Wisconsin is such a state, although they have to have legal justification for the blood test.