Source on that if you have one? As I read it, DDT is the ultimate insecticide and nothing ever really became resistant to it despite massive usage.
I believe bedbugs were the exception to the general lack of resistance to DDT, http://www.bioone.org/doi/abs/10.3954/1523-5475-25.1.41 that’s one recent study showing that owing to the high levels of resistance, DDT was the least effective at killing bedbugs of 12 insecticides tested.