Will usage of a mattress cover change which pesticides I can use during bed bug jobs if the mattress is treated, the surface dries, and then is encased? i.e. Can I use products that are not labeled for uncovered mattress use?
I think you are just fine treating a mattress with a product labeled for that use, whether or not you then enclose that mattress (or box spring) within an encasement. And, the use of mattress encasements is a wonderful step in overall management of bed bugs, as this eliminates the mattress as a primary harborage area for bed bugs. Studies routinely show that 70-80% of the bed bugs will hide on and around the mattress, box spring, and bed framing, and these of course are the most sensitive places to deal with. If you can force the bugs to hide further away you can minimize the treatments of the place where people sleep.
I checked a couple of product labels to see if they had any specific instructions on this. The Bedlam label, for example, allows treatment of both mattress and box spring, but also states that you should thoroughly “clean and air out” these items first, then treat them, and then allow them to dry completely before replacing bedding. It does not address the use of encasements specifically. The Phantom label is similar but also very specifically states that you are only to treat seams, folds, and edges and not general surfaces. Temprid has similar wording as well.
I would recommend that you use only insecticides that specifically allow mattress applications on their label, as this protects you by having the site listed. I don’t see any problem with then encasing that mattress once it has dried, and this probably should be done. If you do NOT treat the mattress but only enclose it in a high quality encasement it still should kill all the bugs that may be on that mattress, but this could take weeks or months to occur as bed bugs don’t starve to death easily. Even though they should be trapped within that encasement it could give people the creeps knowing that the live bugs are still in there. Better to wash a filthy mattress, treat it with the insecticide, let it dry, and then add the encasement.