Apple galls of Acacia longifolia

The flowering Acacia longifolia is a very attractive small tree in Australia. Native to New South Wales, it spread to other states in Australia and to New Zealand, South Africa and Portugal. These countries deem this beautiful plant a weed as it seeds abundantly. Being a hardy plant it thrives under drought conditions. It is a mimosoid bean plant (Family: Fabaceae, Subfamily: Caesalpinioideae) and root nodules house Rhizobium bacteria which manufacture nitrogenous compounds by fixing atmospheric nitrogen. Thus, Acacia longifolia, also known as the Sydney Golden Wattle, can proliferate in poor soils and on sand dunes, serving to stabilise the coastline.

View this Video by Sydney Botanical Gardens that features the Acacia longifolia.

A wasp Trichilogaster acaciaelongifoliae, parasitises the Acacia longifolia in Australia. This wasp is host specific. The female wasp lays eggs in the flower buds of its host in spring. The eggs hatch and the larvae then induce the plant tissues to keep multiplying, forming an apple-shaped gall. The parasitised flower buds do not develop fruits and seeds. The wasp larvae feed and develop inside the gall, protected from predators and the elements of nature. When fully formed, the adult wasps bite their way through the gall and emerge^[1]. The adult females then seek out flower buds to lay their eggs and die within a few days.

The wasp has been introduced in South Africa and resulted in extensive galling in the weed, a very effective biological control of this acacia species.

I visited Victor Harbor on two consecutive years. Image 1 is the acacia plant that I observed in 2023. I noticed a lot of galls when I observed the same plant in 2024. The abundance of galls piqued my interest and I decided to investigate them. Images 2-12 and videos 2-4 where taken while examining the galls on this plant.

The few galls that I examined had one or two chambers, occupied by the white colour grubs. One of the galls housed a white grub and a Lepidopteran caterpillar. I had observed the grubs of Trichilogaster acaciaelongifoliae in the gall chambers and chanced upon a gall shared by the wasp larva of Trichilogaster acaciaelongifoliae and the leaf miner Acrocercops alysidota(?).

I have yet to observe birds feeding on the galls or breaking into the galls to pick off the larvae.

References:

1. The long-leaved wattle bud galling wasp
2. The effect of the gall wasp Trichilogaster acaciaelongifoliae
3. Application to release Sydney golden wattle gall wasp
4. Interactions between a gall-inducing wasp Trichilogaster acaciaelongifoliae and its host plant Acacia longifolia
5. Wattle gall-the quintessential Australian plant disease
6. Biology of gall inducers and evolution of gall induction in Chalcidoidea