The sap in the xylem is probably a poorer nutrient source. This is particularly true on Pacific Islands. They are easily dispersed and have few enemies in their new homes. 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.