A gall is an abnormal growth of a plant caused by another organism, which is usually an insect but could also be a fungus, bacteria or mite. Australian insects known to induce galls include more than 180 species of bug, 6 species of beetle, 27 species of thrip and numerous species of wasp and fly. The adult insect lays eggs in the plant and the larva live inside the gall and feed on plant tissue, although sometimes adults also live inside the gall. A single gall can have one or more insects living inside it.
Each species of insect induces a distinctive shape and colour of gall, and often the galls occupied by male and female insects of the same species also look very different. Galls can occur in the stems, leaves, buds or roots of the plant, with each insect species specializing in one species of plant and one type of plant tissue.
More information can be found in a little book by Rosalind Blanche, and there is some information about galls on eucalypts in Forest health fact sheet number 23. The gall photos page shows some of the many gall forms that can be seen on plants at Woodlands Historic Park. One day perhaps it will be possible to say what organism lives in them!