The citrus gall wasp (Bruchophagus fellis) is native to Australia and lays eggs on all types of citrus but prefers grapefruit and lemon trees. Although I also have them on my orange and lime trees.
The adults can be black or brown in colour and reach a length of 2-3mm. Although citrus gall wasp doesn’t kill citrus trees, the damage it causes is quite ugly, and repeated attacks can weaken and make the tree spindly and unproductive.
When trying to manage your garden to organic principles you need to be vigilant and constantly check you trees for signs of infestation. As soon as you see the lumps, prune off any affected branches preferably before August and burn.
If there are tiny holes in the gall, then it is too late the wasps have already hatched. Or you can use a small knife to lift the bark where the abnormal swelling is and expose the nest to allow air to dry out the eggs and stop them developing any further.
If the tree looks like it’s really suffering – the branches are brittle and there is little fruit – then it might be best to cut it back hard and fertilise to encourage new growth.