I was waiting for a friend in Stanmore when a Banksia tree in someone’s front garden grabbed my attention. It was covered in little knobbly clumps that were not seed pods. I thought might be galls & checked with friends in the know who agreed.
Galls are formed by wasps, beetles, moths & insects who lay their eggs in plant parts. They occur on leaves, stems, buds & roots. This is irritating to the plant who then thickens the tissue around the area, creating many interesting shapes.
These can be small lumps on leaves, great big hanging brown balls, little lumps on stems or curly lumpy things like I spotted on this Banksia.
The inside wall of the gall provides nutritious food for the developing insect & also protection from predators. Sometimes a gall holds one developing insect, sometimes it is like a mini-hotel with different rooms for several insects.
All the advice I have read has said galls are common with Australian trees, especially eucalypts & acacias. The presence of galls does not cause problems or kill the tree. If you want less galls, then increase the biodiversity in your garden. The birds will take care of the insects that cause galls.
Parasitic wasps also help control galls by laying their eggs inside the gall. Once the eggs hatch, they feed on the insect developing in the gall until they eventually kill it. Not such a great hotel room now. Once the wasp matures, it leaves the gall via a small hole. Galls remain on trees after the insect has matured & left.
The bottom line is galls are part of the lifecycle of many native insects & there is no need to take action. You could cut off the offending branch if they really bothered you.
The only time to take action is if your citrus trees are affected because citrus gall will slowly kill your tree. Therefore, it is advised that you remove the branch that has the gall before August, when the insect is likely to leave the gall.