I have seldom had spider mites using the bags with organic peat/or coco, either, if careful attention is given to watering. Fungus gnats either. In some hydroponic methods I have used, it has been more of a problem, with both.
Spider mites attack primarily the leaves of stressed under watered or over watered plants. Fungus gnats love constantly wet places, standing puddles, debris/dust mixed with nutrient rich water or pot surfaces or sides that are constantly damp or wet can be inviting to the gnats and fungus diseases. Ph is important to help plants resist pest. Under worked compost can bring mites into your grow room, both good and bad mites(some attack other mites or bugs). Amended coco(or peat moss) is an excellent medium for use in fabric bags or pots of any kind).
Cleanliness in the grow room pays off. Good air flow is vital! Defeating fungus gnats in a constantly wet environment requires covering the grow medium and keeping everything clean and dry as possible, maintaining sticky traps and reacting to any that show up immediately! About the same for spider mites, clean, vigilance and react swiftly. A few spider mites at harvest left from control attempts is not so bad, if you wash the plants/buds very well.
Spider mites can easily be identified by either inspection of the leaves or shaking vigorously a branch over a large sheet of white paper. The specks that show up can be inspected with a magnifier or just watch for a speck to begin crawling! If you see crawling, you better get busy fast! Do not use commercial spider mite chemical controls! I use various organic homemade sprays that tend to offer control- not eradication of mites. Rosemary oil, fels soap , neem oil, pyrethrin (made from flower buds of a daisy type, not permethrin a dangerous chemical!)) cocktails take care of reducing or eliminating most pest I have encountered the past few decades. All of these things readily wash off the plants a couple weeks later at harvest,, leaving no harmful residues or flavors to distract from your good bud.
I sometimes just make shatter out of buds that may of come from plants that got heavily infested before controls(after a good wash, still). I have never used any commercial (even organic ) mite sprays due to either cost or not wanting to use chemicals that may linger in the plant tissues. In general the fabric bags/air pots are better for me in most ways. BTW, if you get mites,etc. in your grow room, throw away or very carefully clean the entire room and all tools and equipment. Don't go for a walk in your garden or landscape then waltz into your grow room! Wash your favorite jacket, shoes and etc. that may enter the space.
Relax, smoke a bowl and keep on growing bud! Anyway that works for you!
