Scale insects are a fascinating yet strongly neglected group of plant sap feeders. Beside extreme morphological differences between the genders and developmental stages the taxon features a considerable interspecific variety of forms. The spectrum ranges from free-moving forms up to specialized apodous representatives. Furthermore, scale insects show remarkable adaptations to their host plants. One example is their ability to metabolize their plant food into substances such as wax or lac which are secreted by integumentary glands and offer them effective protection from environmental stress.
Scale insects are formidable pests, especially in tropical fruit plantations, where they cause damages amounting to millions of dollars annually. Biological pest control utilizing exotic parasitoids and predators has partially led to excellent results, as is shown in the present book for the mealybug in Africa, and the San Jose scale in Central Europe.
But scale insects also comprise useful animals. In the Middle Ages the Polish cochineal played an important role in Central Europe as a supplier of a crimson dye used for the coloration of clothing. Nowadays several Physokermes and other species are of importance in beekeeping, because their secretions provide the basis of honeydew honey.