I think it's not just because of pests. It's also because the humidity you give is lacking, which must be caused by over or underwatering. So, the key to your problem seems to be your poor planting medium. Try using well-drained soil such as perlite and orchid bark in a 1:1 ratio, and do maintenance continuously. My suggestion is to just cut the yellowing part and take extra care of the part that still looks healthy.