Apple-galls in the inflorescences of Acacias are caused by wasps which lay there eggs in the young flower buds. These wasps belong to the genus Trichilogaster of the family Pteromalidae. Some species are known to lay up to 400 eggs. When they do so, they also inject chemicals which cause the woody growth known as a gall. The wasp grubs then feed on this plant material from within. Eventually they will pupate and then eat their way out. The adult females are short lived surviving just 2 - 3 days.
As you may imagine, there is potential here for a lot of stress and damage to the tree as well as a reduction in it's ability to set seed. For this reason one species, T. acaciaelongifoliae, has been introduced to South Africa as a biological control agent for Acacia longifolia. So, you may wonder why our own Acacias are not being wiped out. It's because we have something the South Africans don't want. There are a range of parasitic wasps that keep the gall wasp population in check. These lay there eggs in the galls and there grubs feed on the gall wasp grubs. OK so what stops the parasites from wiping out the gall wasps? Simple - we have a range of hyperparasites which keep the parasitic wasps in check. In fact, in one study, "twenty-one insect species were reared from galls formed by Trichilogaster acaciaelongifoliae on Acacia secies in Tasmania".
There are at least 10 Trichilogaster species in Australia. From what I've read, T. trilineata is the most likely species to seek out Acacia dealbata an so it is the most likely candidate for the galls below which I photographed at Latrobe the other day. You'll notice in the photos that the plant is making a pretty good effort at flowering despite the galls but they will not set seed.
(Click on Photos to Enlarge)
- Bashford, R. 2004, The insects associated with galls formed by Trichilogaster acaciaelongifoliae (Froggatt) (Hymenoptera: Pteromalidae) on Acacia species in Tasmania. - Australian Entomologist, 2004 (Vol. 31) (No. 1) 5-12 - Entomological Society of Qld