spider mites like dry weather and live undersides of leaves. Croton like it humid. Thus, when watering the croton I spray under the leaves with a stream of water. That seems enough to keep them down in numbers. If it has been dry for a week or so, I will spray the leaf. Plants can live with some insects on them. Just keep them under control and keep a hostile environment for the bugs. When you use pesticides you kill the benificial insects as well. A commmercial nursery is of course a whole different beast. But, as a homeowner, I've never seen problems with spider mites on any of my plants. Actually, I've never used pesticide on anything. I don't think it is needed. A healthy grown plant should be able to defend an insect attack on its on. Spider mites accumalate on plants when they are grown in a non-air movement, dry environment like inside the house or greenhouse. Move the plant outside occassionaly and spray the undersides with water.
that doesn't answer your question I know. Growing plants inside a greenhouse is a different equation. If I had a greenhouse, I would try Dial Soap mixed with water in a spray bottle. A systematic miticide probably would work as well like recommended but cost $$$$.