The state of Queensland has got an alien thorny invader: Prickly acacia, or in scientific terms Acacia nilotica subspecies nilotica.
Prickly acacia is a shrub or small tree which belongs to the plant family Leguminosae, subfamily Mimosoideae, a family which also accommodates the sensitive plant Mimosa pudica, well-known as a curiosity house plant. The prickly invader A. nilotica is native to Africa, the Middle East and the Indian subcontinent and was introduced from India into Australia in the late 1890s. Originally imported as an ornamental, A. nilotica ssp. indica was later widely used as a shade and fodder tree for sheep. But what was initially a valued addition to the Australian flora soon became a menace. When the grazing regime in Australia changed from predominantly sheep to cows and the country also experienced a number of successive wet years, the balance swung in the favour of prickly acacia. The thorny shrub spread quickly and has now invaded around six million hectares of arid and semi-arid land in the State of Queensland. Acacia nilotica ssp. indica is also present in the Northern Territory as well as in Western Australia. Due to its substantial impact on the environment as well as on the economy, particularly on the livestock industry, prickly acacia was initially declared a noxious weed in Queensland in 1957. Subsequently the plant has also been listed as a “Weed of National Significance” for the whole of Australia. And Australians have every reason to be worried, as the prickly invader has got the potential to spread throughout the arid regions of the whole of northern Australia.
Mechanical and chemical means are being used to control the spread of prickly acacia; however these methods are often uneconomical, particularly when dealing with large areas of infestation. It was, therefore, decided that biological control must be explored as an additional management strategy as it would provide a low-cost and sustainable additional method of control. Since the early 1980s A. nilotica ssp. indica has been the target of an Australian classical biological control programme. The initial focus of this programme has been on arthropods as natural enemies to control the Australian invasion of the thorny shrub. A number of insects from the native ranges of prickly acacia in Africa and Pakistan have since been tested and released in Australia, however, few have established and without substantial impact on the prickly invader. Field surveys in India revealed that here prickly acacia regularly comes under severe attack from rust pathogens, while no other related plant species growing in the same habitat seems to be susceptible. This observation led to the decision to include also the assessment of fungal pathogens into the overall biocontrol programme. When looking more closely, it was found that in fact two different rust fungi are involved in giving A. nilotica ssp. indica a hard time in India: Ravenelia acaciae-arabicae and Ravenelia evansii. Out of the two, R. acaciae-arabicae was chosen as the first pathogen to be evaluated for its host specificity and suitability as a classical biocontrol agent. However, this rust species was found to sporulate on one non-target Acacia species native to Queensland. Quite clearly this poses an unacceptable risk to the Australian flora for which Acacia species constitute a key component. Hence this particular rust species, however damaging in the native range of prickly acacia it might be, cannot be considered any further for control of prickly acacia in Australia. As yet it is early days for the second species, R. evansii, as its assessment has only just commenced. Should this rust species “behave” and prove to be host specific attacking only the target weed prickly acacia, then small rust spores could possibly make all the difference to the future of the weed in Australia.
Dr. Marion K. Seier
Senior Scientist, Invasive Species Management, CABI