Heavy infestation of Onopordum thistles reduces pasture growth and livestock production

Onopordum thistles are serious pasture weeds in south-eastern Australia

Biological control of Onopordum thistles in South-East Australia

CSIRO research on the biological control of Onopordum thistles is leading to the return of many pastures to greater productivity.

7 June 2010 | Updated 14 October 2011

Background
CSIRO research
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Background

Scotch thistles, Onopordum acanthium, and Illyrian thistles, O. illyricum, were probably introduced to Australia from Europe in the early 1800s as ornamental plants.

By 1850 Scotch thistles were recorded as a weed in Victoria and today both species are serious pasture weeds in south-eastern Australia and are declared noxious weeds in many regions.

The Onopordum thistles thrive in high fertility soils, such as improved pastures, and dense stands have established across millions of hectares, smothering desirable vegetation with their rosette leaves.

The spiny nature of the leaves make these plants unpalatable to grazing stock.

The thistles have an annual or bi-annual habit and reproduce by seed. Plants may produce up to 20 000 seeds and, left uncontrolled, can seriously impact on the stocking rates of grazing lands. A proportion of seeds become incorporated in a long-lived soil seed bank and such seeds may remain viable for more than 20 years.

The annual cost to Australian rural industries through lost production and the implementation of control measures against Onopordum thistles, exceeds A$20 million.

The use of biological agents to control these thistles, integrated with more effective pasture and land management, has had a significant impact in reducing the impact of weed growth in areas of New South Wales.

CSIRO research

Biological control of Onopordum thistles has been the focus of a long-term CSIRO research program, which commenced in France in 1987.

Scotch and Illyrian thistles are native to Europe, western and central Asia and Asia Minor. Native thistle stands in those areas provide a source of insects as potential candidates to be agents for biological control of Onopordum thistles in south-eastern Australia.

During the 1990s four insects selected from the European-based screening program were successfully released in selected locations in south-eastern Australia to control Onopordum thistles.

There were three weevil species:

seed-head weevil, Larinus latus
stem-boring weevil, Lixus cardui
crown weevil, Trichosirocalus briesei.

The fourth was a moth species, petiole moth, Eublemma amoena.

These are now well-established and are starting to impact on Onopordum thistle infestations.

The three weevils attack the rosette, the stem and the seed head of Onopordum thistles, while the moth also attacks the rosette stage of growth.

The widespread distribution of biological control agents is likely to have an even greater long-term impact as the accumulated seed bank on Onopordum thistles is gradually diminished.

The seed-head and stem-boring weevils are now widespread and together have reduced the seeding rate at several sites in New South Wales by more than 90 per cent.

A crown fly, Botanophila spinosa, and the seed gall-fly, Urophora terebrans, were released in 2000, but have not yet established.

These fly agents attack the seed heads, complimenting the damage caused by the weevils.

Such diversity amongst biological control agents ensures a multi-pronged attack on Onopordum thistles.

Biological control has already impacted on Onopordum thistle stands, with significantly reduced seeding rates of plants in areas where the biological agents flourish.

The distribution of the seed-head and stem-boring weevils has been extended through active redistribution programs initiated by farmers and land owners.

Infested thistle heads are collected prior to adult emergence and moved to suitable 'nursery' sites that are likely to remain undisturbed for an extended period (for example, three to four years).

The widespread distribution of biological control agents is likely to have an even greater long-term impact as the accumulated seed bank on Onopordum thistles is gradually diminished.

Each plant can produce up to 20 000 seeds that can survive in soil for more than 20 years. Sustained management of Onopordum thistles will lead to a decline in the seed bank and return many areas back to greater productivity.