As with antibiotics in humans, one of the problems has been the overuse of insecticides as a prophylactic measure by nurserymen and collectors that has often led to mealies which are now resistant to many insecticides, just as rats became immune to Warfarin. However it is hard to get over to many plant growers that the more insecticide you use and the prolonged use of a single insecticide without repeatedly ringing the changes with different insecticides, or even alternating between systemic and contact insecticides, the more chance of producing resistant strains of the bugs. Use insecticides only when you need to, rather than indiscriminately.
Basically the problem is the more effective the killer the greater chance of producing resistant strains. As I understand it if a reasonably effective insecticide is used a larger percentage of non resistant bugs may be left as well as resistant ones, so it is more likely that the few totally resistant bugs will mate with none resistant ones so fail to produce a totally resistant strain. If a very effective killer is used leaving only resistant bugs they will mate together producing a totally resistant strain to that chemical. Keep switching the types of chemical you use increases the chance of killing the bugs resistant to the previous chemical before they can breed a resistant strain.
http://en.wikipedia.org/wiki/Pesticide_resistance" onclick="window.open(this.href);return false;
http://westernfarmpress.com/management/ ... resistance" onclick="window.open(this.href);return false;
http://pesticidestewardship.org/resista ... tance.aspx" onclick="window.open(this.href);return false;
Use pesticides responsibly in doses and frequency they recommend on the label in order to avoid producing resistant strains or eventually your favourite insecticide may be worse than useless and everybody else will be having to deal with the resistant bugs you have produced with fewer effective chemicals left to do so. As an instance of a formally very effective natural killer producing resistance take Myxomatosis in rabbits. It wiped out the majority of the rabbit population leaving only the few remaining totally resistant ones which then interbred with each other producing a resistant strain that bounced back recolonizing their old habitats but now immune to their former killer.
I felt I had to tell them to protect other customers. I know you are ALL screaming at me at this point that I should ALWAYS unpot cacti as soon as I get them home to inspect them! 
Lesson learned! 
I told the cashier that I was sure they received them already potted from their supplier and that the center really had no idea they were infested. We don't have many decent garden centers/greenhouses in my location; this one happens to be a fairly "ritzy" one and very popular in the area. The employee was genuinely concerned and assured me that she would tell the two people in charge of placing orders to inform their suppliers of this problem. I said good, because I'd like to go back to buying cacti from them. Frankly, I can safely say that I've bought better, healthier and longer lasting cacti at the big box stores. The big "plus" for the garden center is that they often stock some of the less common types that you don't find at the BB stores. Let's hope they literally clean up their act.
I soak and wash out old pots (plastic and clay) with a bleach solution but have never tried a weak bleach soak for potted cacti. Something tells me it still might be too caustic, even in a mild dilution. If you do try it with a cactus, please be sure you buy a very cheap one! 
