For a long time I've had trouble with indoor catnip. The plants would grow great for a couple of months and then just shrivel and die. On inspection, there were lots of small white dots on the underside of leaves. Under the microscope, they were flying insects. Whiteflies! It would seem that whiteflies are strongly attracted to mint family plants. I was sure I could knock them out with Sevin, but ... surprise! Sevin doesn't work on Whiteflies. Evidently they can develop immunity to it. I've now switched to Neem, which I should have started using to begin with. Turns out that zeta-cypermethrin (the "new" Sevin) is highly toxic to cats! Neem is well known to do the job. I am hopeful. Curiously, my outdoor catnip plants never have this problem.