A total of 13 species of Australian acacias (Mimosaceae) have become naturalized in South Africa and are now declared invasive weeds. Two of the most successful invasive plant species in the fynbos are Acacia cyclops (rooikrans) and Acacia saligna (Port Jackson willow). Both these species were introduced originally to stabilise coastal sand dunes and have proven exceptionally well adapted to local conditions, thriving on the nutrient poor soils of the Cape and proliferating after the regular fires typical of fynbos landscapes.
Port Jackson thrives on acidic mountain soils while rooikrans is most at home on alkaline coastal soils.
A few years ago a multivoltine midge (Dasineura dielsi) that induces galls on the ovary of its host , was introduced from Australia for the biological control of Acacia cyclops in South Africa. It spread extremely rapidly and now colonizes virtually all mature Acacia cyclops in the region. By inducing the gall it stops the flower from forming and hence drastically reduces the seeds produced. Extensive studies are undertaken prior to the release of biological control agents in South Africa and all indications are that D. dielsi will be of no significance on any species other than A. cyclops.
Acacia saligna or the Port Jackson willow is a very adaptable and fast growing species, native to Western Australia. It has become one of the most widespread and destructive invasive species in the fynbos region. It is a very difficult species to manage as it is able to resprout from its roots after being cut down and creates massive, long-lived seed banks. The difficulty and cost of eradicating this fast spreading species led to it being targeted for biological control measures.
The gall forming rust fungus Uromycladium tepperianum has proven very successful at controlling reproduction and seed set in Port Jackson. Through wind dispersal this rust fungus is now found wherever Port Jackson is growing in fynbos and it has been found to lower population densities by at least 80% in the absence of fire. The rust fungus reduces the flowering and seed set on Port Jackson and often ends up killing the plants after a few years. It is thought that the rust fungus does not kill the plants by itself but death is rather the result of the high number of galls formed on the plant together with environmental stress such as drought.
These two biological control agents have proven very useful in the battle to control alien invasive species in the fynbos.