There are three biological control agents for Ragwort that have been released and become established in Tasmania. These agents have had a major impact on Ragwort and have reduced infestations by 95% at some sites.
Ragwort Flea Beetle (Longitarsus flavicornis)
Ragwort Flea Beetle is established throughout most of Tasmania and has reduced the severity of Ragwort in many localities.
Ragwort Flea Beetles are a little larger than dog fleas. They jump when disturbed and so are hard to detect.
The adults feed on the Ragwort rosettes in summer but do negligible damage. It is the larvae that feed inside the stems and root system of the plant over winter that do the damage.
Typically, it is sometimes three to four years before there is any noticeable effect on the Ragwort. Then, once flea beetles have reached a critical number all of a sudden Ragwort infestations can be devastated. In some areas, 95% control has been achieved in six years.
Although the beetle continues to play a major role in reducing the density and vigour of ragwort in many locations, it must be emphasised that control does not mean eradication. Furthermore, conditions vary from site to site, which may either prevent establishment of beetles, or affect the extent and rate of control. Factors responsible for reduced efficacy of the beetle include frequent pasture flooding in winter causing high larval mortality through drowning. Heavy stocking with cattle is also thought to have an adverse effect on the beetle by trampling larvae feeding externally on the roots and crowns.
For more information on some simple management practices to increase efficiency of the beetle click on TIA Biological Control Pamphlet No. 2.
Ragwort Stem and Crown Boring Moth (Cochylis atricapitana) and Ragwort Plume Moth (Platyptilia isodactyla)
The larvae of both these species damage the stems and crowns of Ragwort with the feeding activities of their larvae. After hatching, the larvae begin mining into the plant, boring into the leaf petioles, stems and crowns. Larval rosette damage is distinguished by blemished or blackened young central shoots. In severely infested rosettes, larvae destroy the central crown killing the plant. Some plants survive and regrow, but with reduced foliage, and do not flower that season.
This damage will complement those of the root feeding ragwort flea beetles. This combined affect will have a greater impact on ragwort than any of the insects acting alone. A native insect, the Blue Stem Borer, (Patagoniodes farinaria) also attaches the stems of Ragwort.
Larval stem damage is identified by small tunnels visible at the leaf axils. The tunnels often have a small pile of fine faecal material at their entrance and where the moth has emerged, a pupal case protrudes from the tunnel. Attacked flowering plants can be identified by a blackened appearance of the flower buds and/or a multi-stemmed flower crown, which branches out from below where the apical bud was damaged.
Monitoring of sites where the moth is well established indicates it is having a major impact on ragwort vigour. It is therefore expected that this agent will play a major role in complementing the root damage caused by the Ragwort Flea Beetle and facilitate integrated control strategies, (such as the annual Tamar NRM Ragwort Raids), particularly if it establishes well in areas where the impact of the beetle is restricted by site conditions.