They can occur in devastating numbers and are capable of killing mature trees. Damage generally occurs through removal of sap. Excess sap is secreted as honeydew. This can drip down onto leaves below the insect. Molds such as sooty mold can then grow on the sugary excrement.
Some species are beneficial and are used as a source of shellac, dyes and other materials.
They can produce sound and can be very loud. Each species has a specific sound. The sound is produced by a couple of plates called tymbals, which are located near the basal abdominal segment. Below them are hearing organs covered by membrane. These are called tympana.