Author
Hester Williams
williamshes@karen
Bioeconomy Science Institute – Manaaki Whenua – Landcare Research Group
Lincoln, New Zealand
Coauthors
Ronny Groenteman, Bioeconomy Science Institute – Manaaki Whenua – Landcare Research Group, Lincoln, New Zealand
Abstract
Ragwort (Jacobaea vulgaris) was first recorded in New Zealand in 1874. By 1930, it had become a widespread pasture weed in areas with over 870 mm annual rainfall, displacing desirable species and poisoning livestock.
Biocontrol began in the 1920s, with four of five released agents establishing. The flea beetle (Longitarsus jacobaeae) was most successful, reducing ragwort by 90–100% at many sites within 2–10 years. It was less effective in wetter regions, prompting the release of the plume moth (Platyptilia isodactyla) in 2006, which is now helping control ragwort in high-rainfall areas. As a result, both ragwort and its biocontrol agents became relatively rare.
Recently, stakeholders reported ragwort resurgence in areas where biocontrol had previously worked. MWLR began investigating in 2023, confirming flea beetle presence at many sites via email and photo follow-ups, though plume moth presence was harder to verify.
In Apr 2025, a pilot study at five Dunedin sites found ragwort densities of 0.5–2.7 plants/m². Flea beetle densities exceeded 4 adults per rosette at three sites (considered effective), but were lower at two. A second field visit is planned for Oct 2025 in South Canterbury, where flea beetle releases, requested by landowners, are scheduled for Mar 2026. Environmental data will be used to assess factors influencing biocontrol success. Results to follow.
Keywords
ragwort
flea beetle
plume moth
status
Highlights
Ragwort is regarded as a long-standing successful biocontrol system
Anecdotal reports received of ragwort resurgence
Results suggest ragwort biocontrol is still stable