Giant Buttercup in the Spotlight
Landcare Research recently joined forces with AgResearch to start looking at classical biocontrol options for giant buttercup (Ranunculus acris), a weed estimated to cost the dairy industry around $156 million annually.
Giant buttercup is primarily a pest of improved pasture. It is prevalent on dairy farms in South Auckland, Hawke’s Bay, Southland, Taranaki, Wairarapa, West Coast and the Tasman District. Sheep don’t seem to mind the acrid-tasting toxin that the flowering plant produces (a glycoside), but it is not palatable to cattle. While currently only these seven of the 17 dairy regions are known to have significant populations of giant buttercup, modelling by AgResearch indicates that all regions are climatically suitable for the weed and therefore vulnerable.
Giant buttercup first established in New Zealand around 1910 and originates from Europe and Asia. It reproduces through seed and vegetatively by rhizomes and through nodal rooting (layering) of collapsed flower stems. The seed is easily spread via stock, agricultural equipment, flood waters and hay. Rhizome fragments are also spread by stock, machinery and flood waters. The longevity of the seed in the soil appears to vary depending on soil moisture and other climatic conditions. Rhizome fragments readily survive drought.
Graeme Bourdôt from AgResearch has been working on giant buttercup since the 1990s after it started to show resistance to phenoxy herbicides. Graeme said, “The historical reliance by dairy farmers on MCPA and MCPB during the 1950s caused the resistance to develop.” Newer herbicides also have limitations. “Thifensulfuron methyl damages clovers, which further promotes the growth of giant buttercup, and the plant can evolve resistance to flumetsulam, leaving many dairy farmers with no options for selective removal of the weed from their pastures,” added Graeme. Biocontrol may therefore offer the only long-term option for managing giant buttercup.
Past research has focused on developing a mycoherbicide formulated from a naturally-occurring fungus, Sclerotinia sclerotiorum. Not only is the fungus able to kill giant buttercup plants with negligible risk to neighbouring crops, it is also harmless to pasture grasses and clovers. “Unfortunately there has been little interest in developing a commercial product by the bigger agrochemical companies despite recent modelling showing it would be economically viable to do so,” said Graeme. However, renewed interest by smaller companies in developing specialist environmentally sensitive products for weed management may still hold the key to its commercial development. Field trials conducted by AgResearch indicated that the mycoherbicide gave good (and environmentally safe) control of giant buttercup, and scientists have speculated that its use could be complemented with classical biocontrol agents such as insects.
Late last year, a team from AgResearch and Landcare Research visited Golden Bay, Taranaki and Southland to talk to farmers about the giant buttercup problem and collect samples of the plant and its associated endophytes, pathogens and insect fauna. “Endophytes occur naturally in all plants and are of interest because they can interfere with the effectiveness of biocontrol agents, especially pathogens,” explained Simon Fowler. As well as looking for insects damaging the plant, the team also looked for any that might disrupt biocontrol agents if they were introduced. Plants were also collected for DNA analysis. “We will be assessing the genetic variability of giant buttercup around New Zealand as well as trying to pinpoint which region of Europe the plants originated from. That way we can source potential agents from the best place,” said Dagmar Goeke. “Widespread weed species like giant buttercup are often very variable genetically over their native range, so it is critical that we identify the origin of the plants that are invasive in New Zealand.
Giant buttercup doesn’t occur exclusively on dairy farms, it also grows on roadsides, river flats, wetlands and anywhere else damp and warm. The weed is likely to have spread considerably over the past decade as the dairy industry has intensified and there are now localised populations in parts of the West Coast, Canterbury and Southland, but these are mainly on non-agricultural land. Now is definitely the time to thoroughly explore all biocontrol options for giant buttercup.
This project is funded through AgResearch’s core-funded pasture weeds programme.
Simon FowlerGraeme Bourdôt