That doesn't document anything. Unless you have a study you can cite, that documents the genome of a species of insect losing that resistance, it is easily explained. Ever heard of dominant and recessive traits, or punnet squares? After DDT stopped being sprayed, offspring of insects that had genes that did not receive the DDT resistance from their parents were able to survive once again. Making the resistance much more random once again, like it was before. Its standard genetic variation generation to generation. Example: two parents can have shared traits, like blue eyes, and there children can still born with different color eyes.