Root and stem rots usually result from a soil mix that does not drain quickly or overly frequent watering. Aphids, spider mites and mealy bugs are the major pests of cacti. Check new plants carefully for pests before purchase. Houseplant insecticide sprays or insecticidal soaps can be used for control. Make sure that the spray you use is labeled for cacti and follow label directions carefully.
Scale can also occur and is more difficult to control.
You may be able to hand-pick off a light infestation, but you might have to discard the plant if it is heavily infested. When most people think of cacti, this type usually comes to mind. Adapted to living in desert conditions, these cacti are often heavily armed with spines. While many have beautiful and delicate flowers, they are mainly grown for their unusual shapes, colors and textures. Their soil must be very well-drained.