Mealy bugs 
Several species of mealy bugs attack citrus. They suck sap from tender leaves, petioles and fruit.
Feeding on the fruit results in discolored, bumpy, and scarred fruit, with low market value, or unacceptable for the fresh fruit market. 
Mealy bugs excrete honeydew, which leads to the growth of sooty mould on fruit and leaves. Fruit cover with sooty mould at harvest must be washed. The most important is the citrus mealybug (Planoccous citri).
Ants "farm' some insects like aphids and mealybugs. They protect them from predators, and they get honeydew in return. How? The mealybugs secrete the honeydew from their anuses, and the ants drink it. Yum. There are a few shots of this happening in this video
Ants Farming Mealybugs
Grape Mealybugs feel the wrath of the Mealybug Destroyer (Cryptolaemus montrouzieri) during an Integrated Pest Management project at Ridge Vineyards, Santa Cruz Mountains, California
Mealybug Destroyers!
Explains the control of papaya mealy bugs (Paracoccus marginatus) with the help of Acerophagus wasps. These predators lay their eggs into the mealy bugs which kills them. The film demonstrates how the wasps are bred and multiplied for release in the orchard. Pictures taken on Tropigha Farms in the Volta Region.
How to defeat the mealy bug (Engl.)
Managing Garden Insects Begins with a Question: Friend or Foe?