Unlike many other wasps that assist pollination or are pest predators, the citrus gall wasp is a true pest in coastal gardens of New South Wales and Queensland. It is an extremely tiny black wasp that lays its eggs in young stems of citrus trees, particularly lemon and grapefruit varieties, and the native finger lime. The larvae remain within the stem, stimulating the growth of cells, and causing a gall or swelling to form on the infested stem by early summer. Because these wasps are poor fliers, they tend to reproduce on the same tree unless blown by wind to a new host. Trees that are repeatedly attacked will become weaker and produce less fruit.
During winter, after the Full Moon, prune off all galls and burn them, or dispose of them in a sealed plastic bag. Don’t add galls to the compost heap. The wasps emerge from the galls to lay a new batch of eggs in September or October, so it is necessary to remove all galls before the end of August. Failure to do so can result in fines being imposed in some areas.
It is very likely that the gall in the photo missed last year’s pruning because it is unusual for galls to reach that size in one season. As you can see, the tree in the photo is also affected by scale, and it is more common for citrus gall wasp to attack stressed trees. After pruning, water the tree thoroughly, and feed it with a complete organic fertiliser and as much compost as you can spare. A drink of seaweed extract tea will help it to resist further pest and disease attack.