Psyllids and lerps
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Psyllids are a category of insects feeding on leaves of a variety of trees. Their sugary secretions are called lerps. Both are a food source for some species of birds (also e.g. for ants).
Psyllid infestation on the leaves of an Emubush, in this case Eremophila longifolia; the brown spots are the insects, the white bubbles are their sugary secretions which they use as a protective shell
... or crystalline, as this one that has been picked up by a Striped Honeyeater
Eucalypt leaf covered with lerps
Many bird species will take both psyllids and lerps, whereas some miners, escpecially Bell Miners, will take only lerps and leave the psyllids behind, which then live another day to do more damage to their host tree.