Plant damage is usually caused by removal of plant sap. However, scale insects can also transmit plant pathogens and toxins that may further reduce plant vigor and eventually kill the plant. In addition, scale insects produce large quantities of a sticky substance that can cover the leaves. This sticky substance, which is called honeydew, provides an excellent substrate for the growth of sooty mold fungi. Sooty mold fungi in turn can reduce photosynthesis and plant vigor.
Scales also can be beneficial.
Cochineal scales and mealybugs have been used in the control of noxious weeds.