Why they are a Pest
They suck sap from plants and, like aphids and scale (which they are related to), they secrete honey dew, attract ants, and can transmit diseases from plant to plant.
They cause the flower buds of fruits and vegetables to drop and leaves to yellow and fall. They have a huge range of plants they attack. They are common on citrus and ornamental trees as well as vegetable plants including passionfruit, pineapple, coffee, avocado, bananas, eggplant, grapes, peppers, sugarcane.
The honeydew itself can grow sooty mould (a fungus), which will not kill the plant but does inhibit photosynthesis. It can be treated with a soap spray, white oil or neem spray.