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By Alexander Schmidt-Lebuhn and Ben Gooden 27 November 2022 3 min read

Senecio is a group of several hundred species of flowering plants in the daisy family. Like many daisies, Senecios can be problematic weeds. This is due to their small seeds that disperse easily on the wind, allowing them to spread widely. This is especially true for fireweed, Senecio madagascariensis, native to southern Africa but highly invasive elsewhere in the world.

Fireweed has spread to many countries around the world, most notably in Australia where it's a Weed of National Significance. Fireweed is also invasive in parts of South America, Japan and Hawai’i. It is a serious threat to agricultural productivity, being toxic to livestock, thus reducing pasture palatability and stocking rates.

In Australia, fireweed is a Weed of National Significance.

Fireweed’s closest relatives are also introduced weeds. They are the narrow-leafed ragwort Senecio inaequidens, which has spread to Mexico and Europe, and the gravel groundsel S. skirrhodon, which is present in New Zealand. Unfortunately, these invasive species can be difficult to tell apart from each other and even from some non-weedy, native Senecios. This is because their flower-heads look very similar, while their leaf shapes vary strongly within the same species.

Accurate identification of fireweed is important for detecting the presence of this destructive invasive species. Early detection allows us to build up a picture of the invasion history, contain the spread and prevent future incursions. It also means researchers can investigate possible biocontrol solutions for managing the weed. Such biocontrol agents might be insects or fungi sourced from the species’ native range, which can be released to attack the weed in its introduced range. They have to be chosen carefully because they should be well-adapted to the target weed for maximum impact and so they do not also attack related native species.

Senecio species in New Zealand

Our colleague, entomologist Dr Jenny Dymock, who provides biocontrol services to the Northland Regional Council in New Zealand, has strongly suspected for some time that S. madagascariensis may be present in NZ (where it is also known as Madagascar ragwort). She had noticed its probable presence in pastures in Northland, the subtropical region at the tip of the North Island. However, she had not been able to confirm this. Instead, this weed was thought more widely to be the gravel groundsel (S. skirrhodon). Although weedy and present in New Zealand since 1920, gravel groundsel is not a serious problem in agriculture.

We used high-throughput sequencing to compare the genetics of presumed fireweed in New Zealand with expertly identified fireweed specimens that are held in herbaria.

Dr Dymock sampled several populations of suspected fireweed and two of the gravel groundsel across the North Island. Our South African collaborator Daniella Egli collected samples from part of the area of origin of these species, in KwaZulu-Natal, for genetic comparison. We supplemented these fresh collections with specimens held at the Australian National Herbarium. We then extracted DNA from dried leaf material of all samples.

Fireweed growing in the Stanthorpe area in southern Queensland.

Assembling a family tree for fireweed

Using high-throughput sequencing to capture data from hundreds of genes enabled us to construct a phylogeny, or family tree, of fireweed that was much more finely resolved than previous studies. It revealed that both S. skirrhodon and S. madagascariensis species are present in the North Island of New Zealand. S. madagascariensis is widespread in grazed areas in Northland. S. skirrhodon is present throughout the North Island. It grows in sites such as beach dunes and disturbed areas like roadsides, railway lines and building sites.

The phylogeny we constructed is detailed enough to suggest fireweed may have spread to New Zealand from Australia. We need to sample more plants growing in their native habitat in southern Africa to confirm this possibility.

Knowing what invasive species are present in a country allows appropriate management strategies to be applied, such as containment or control. We are working to expand our current dataset to include populations of fireweed and its relatives from around the world. This will clarify if the various invasive populations are the species they are currently assumed to be. It will also increase our understanding of invasion pathways.

This research was supported by the Australian Government Department of Agriculture, Fisheries and Forestry, through the Established Pest Animals and Weeds Management Pipeline Program, building on work undertaken through the Agricultural Competitiveness White paper – Established Pest Animals and Weeds Measure.

The paper Genetic data confirm the presence of Senecio madagascariensis in New Zealand was published in the New Zealand Journal of Botany.

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