I have a potted lemon tree about 5 feet high. My wife has been fighting scale on it.
I was wondering if I could bag it, make a small vent on the top, and add dry ice in it before taping the bag to the planter. The heavier carbon dioxide gas should eventually force out lighter oxygen. After the bag pressurizes, seal the vent. No oxygen = dead bugs .
The closest thing to displacing oxygen I found was an online article about home food preservation using dry ice, so the displacement idea is not new.

to NGA

Insecticidal soap will clear up scale .....(and will "break through" their wax like homes)
it is important to spray under leaves and along stems ....let spray dry, then wash off, as it is no longer effective (and may cause harm to leaves on *hot* days) ...

more than one application may be needed

Insecticidal soap ....either pre-mixed from big box stores ...or ...make your own ....many recipes online

You can use dry ice to kill insects, I'm surprised more people haven't thought about it. We used to kill all sorts of bugs with dry ice, back before all the pesticides now available. But, you can't put the dry ice in the bag with the plant as you will freeze your plant.

Put the plant in the bag with the bag opening at the top. Add the dry ice to a can of water above the plant and let the fog fill the bag. Once the bag is full of fog, seal the top and leave it for the night. Don't leave the dry ice in the bag.