In just 30 years, one weed has taken over vast stretches of the Northern Territory. Mimosa pigra, a native of tropical America, has consumed more than 800 square kilometres of wetlands and now threatens Kakadu National Park. Full-time search-and-destroy teams patrol the World Heritage area, guarding against an invader that has resisted every chemical and mechanical offensive. Brad Collis asks is an armory of insects the answer to this growing problem?
A distant shot of a vast Mimosa thicket on the Adelaide River, Darwin.
A green forest, stretching to the horizon, draws camera-toting travellers to a tourist stop overlooking the Adelaide River flood plain south-east of Darwin. The sudden green swathe is an alluring contrast to the red northern landscape.
Few, however, realise they are photographing an alien; a tree-like weed that in just 30 years has consumed more than 800 square kilometres of wetlands and presses threateningly against the Kakadu National Park. Full-time search-and-destroy teams now have to patrol the World Heritage area, guarding against an invader that has resisted every chemical and mechanical offensive.
The invading weed is called Mimosa pigra, a native of tropical America, and its capacity to double its territory every year gives it the potential to completely redraw the northern landscape.
The day of the thickets
Mimosa pigra was introduced into the Darwin Botanical Gardens in the late 1800s as a curiosity plant. A few 'escapees' were discovered near the Adelaide River township in the 1950s, but it was several big wet seasons in the 1970s that washed seeds downstream to the Adelaide River floodplain, which gave the weed its big break.
A single Mimosa plant can produce over two hundred thousand seeds each year, and the seed pods float, enabling them to spread rapidly during the wet.
The Mimosa pigra problem is be the subject of an International Scientific Symposium in Darwin on 27 September. Scientists from around the world, including the USA where the weed has also invaded the Florida Everglades, will pool their knowledge.
If Mimosa was to establish itself in Kakadu National Park, some have predicted it could turn this unique natural treasure into an alien landscape in a decade.
The worry for scientists, national parks managers and pastoralists is that if the weed isn't stopped it will inevitably spread across the entire tropical north, from WA to Queensland.
The main strategy, now, is to use insect and fungal control agents to reduce infestations levels that might then be manageable with fire and mechanical clearing.
Weed warfare
Sensitive New Age Weed - the Mimosa leaves close when touched.
In Central America, Mimosa is called "the giant sensitive plant" because the leaves close when touched, but it is clearly no shrinking violet. Its impenetrable forests have resisted chemicals, fire and bulldozers, and even some of the biological weapons deployed against it.
In the mid-1980s a few thousand seed-eating beetles, painstakingly collected in the Americas and bred-up in quarantine, were sent in against the weed-and disappeared. Nonetheless, scientists and wildlife officials believe biological control is still their main hope, especially since the weed is often in areas that are flooded for long periods and therefore inaccessible for ground control.
Experience has also shown that mechanical control rarely kills the plants; healthy Mimosa is difficult to burn and the plants usually re-sprout. The quest, therefore, has been to find more determined or resilient insects.
Insects to the rescue?
In recent years, 12 insect species and two fungal pathogens have been reared and released and according to the researcher leading the ground assault, Dr Quentin Paynter, six of the insect species have managed to establish themselves in the Mimosa infestations and seem to be having an effect.
The insects' initial impact has been on the weed's seed production, which has fallen dramatically, and some trees around the perimeter of Mimosa thickets have started to die.
Dr Paytner examines a stem for evidence of infestation by the stem-mining moth Neurostrota gunniella.
Dr Paynter, a Darwin-based biological control specialist with CSIRO, says one of the difficulties facing researchers is the highly seasonal northern Australian climate. Mimosa doesn't flower during the dry season, which means the flower and seed-feeding insects run out of food and usually expire by the time the Mimosa begins to flower again.
"So, not surprisingly, the most successful insects have been those that can feed on the Mimosa all year round - leaf feeders and stem miners," he said.
"We've deployed two stem miners, one that attacks the growing tips and small green stems, and another that can get into the thicker stems and woody parts of the plant. Working together they can be pretty damaging."
The trouble is, these insects are slow workers: "Even if they can begin to shrink an outer perimeter by, say, five metres a year it would take a century for a 100-hectare infestation to disappear-and we've got 80,000 hectares to tackle," said Dr Paynter.
An experimental campaign
The campaign to stop the spread of Mimosa, and one day hopefully control it, is a joint program between the CSIRO and the Northern Territory Department of Infrastructure, Planning and Environment. The two agencies have been running experiments on a 130 hectare patch of Mimosa within an Aboriginal reserve that has lost more than 8000 hectares of its wetlands to Mimosa.
A curious plant, the Mimosa pigra was planted in the Darwin Botanical Gardens in the late 1800s.
A combination of bio-control, bulldozing and chemical spraying has cleared stands on some sites - but this has raised another problem. The Mimosa thickets are so dense that even where the plant has been killed it leaves behind impenetrable stands of dead wood, preventing mop-up work needed to destroy regenerating seedlings.
The obvious solution is to burn the thickets, except fire is becoming another environmental issue for northern Australia. Other researchers are trying to reduce the amount of annual burning which is damaging ecosystems and significantly adding to Australia's Greenhouse debits.
It is yet another illustration of how complex landscape repair work is, once the damage is done, and while scientists are starting to become more confident they can get on top of Mimosa pigra, their ultimate victory is likely to be far into the future.
Other weedy threats
Mimosa has so far attracted the most attention because of its threat to Kakadu National Park, however, other parts of the north are also under siege from different weeds, such as Mesquite and Parkinsonia.
Mesquite is the generic term for four species of thorny shrubs and trees, and like Mimosa have the capacity to transform the northern landscape - in this case into dense thorn forests. The thorns on some plants are 10cm long.
The plant was introduced into Australia as an ornamental tree around 1900 and now infests all the mainland states, although the most serious infestation is in WA where about 150,000 hectares of the Pilbara has become choked by Mesquite.
As with Mimosa, biological controls appear to offer the only long-term answer, and two insects were released in 1998 after extensive assessment under strict quarantine.
One is a sap-sucking bug called a Psyllid (Prosopidopsylla flava) and a leaf tying moth (Evippe sp.), which ties the plant's leaves together, causing defoliation.
Dr Rieks van Klinken examines the thorns of a Mimosa bush.
The researcher running the Mesquite and Parkinsonia programs, Dr Rieks van Klinken, said Mesquite had managed to establish itself in different climate zones, which made it difficult to find a control which worked everywhere.
"So, for example, the insects we've released so far aren't working that well in northern Queensland, but have been effective in the Pilbara-which is fortunate because that's where the biggest infestation is."
Dr van Klinken said the dense thorn forests formed by Mesquite had both an environmental and a commercial impact. The weed pushed out native vegetation and its deep roots lowered water tables. Once it had formed into thickets it also severely hindered livestock operations in pastoral country.
A Pilbarra mesquite Management Committee has now been formed with the backing of agricultural and mining groups, and the National Heritage Trust.
Dr van Klinken said the intended strategy was to use insects to weaken the weed to the point that thickets were more susceptible to other controls such as fire.
Research into Parkinsonia is in its early stages with Dr van Klinken concentrating on understanding the plant's ecology and how it is able to grow in a wide variety of landscapes and climates. This is the basic information needed before looking for suitable biological controls.
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