Scale insects feed on plant juices and can easily be transported to new countries on live plants. They sometimes become invasive pests, costing billions of dollars in damage to crops worldwide annually, and farmers try to control them with toxic pesticides, risking environmental damage. Fortunately, scale insects are highly susceptible to control by natural enemies so biological control is possible. They have unique genetic systems, unusual metamorphosis, a broad spectrum of essential symbionts, and some are sources of commercial products like red dyes, shellac and wax. There is, therefore, wide interest in these unusual, destructive, beneficial, and abundant insects.