I have a 9 year old agave plant living indoors. I have several other plants that have become infested wih scales and mealy bugs, and other pretty common house plants which all have them to. Its been going on for 4 years. I keep treating them and sure eough when i think i have beaten the pests they start to appear again. I have changed the soil, dabbed them with rubbing alcohol, used safers mealy bug and scale killer and they just keep coming back. What do i do other then throw them out. I love my plants.