(SEE COMMENTS)

Authors: HUNTER, GAVIN - Csiro, Black Mountain Laboratories; EGLI, DANIELLA - University Of Kwazulu-Natal; ENCINAS-VISO, FRANCISCO - Csiro, Black Mountain Laboratories; Mc Cue, Kent; Portman, Scott; ZEIL-ROLFE, ISABEL - Csiro, Black Mountain Laboratories; MORAN, PATRICK - Former ARS Employee; GOODEN, BEN - Csiro, Black Mountain Laboratories

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/12/2025
Publication Date: 9/22/2025
Citation: Hunter, G., Egli, D., Encinas-Viso, F., Mc Cue, K.F., Portman, S.L., Zeil-Rolfe, I., Moran, P., Gooden, B. 2025. (SEE COMMENTS). Biological Control. https://doi.org/10.1016/j.biocontrol.2025.105893.
DOI: https://doi.org/10.1016/j.biocontrol.2025.105893

Interpretive Summary: The invasive vine known as Cape-ivy (Delairea odorata), native to South Africa, smothers native plants and clogs coastal streams along the California and southern Oregon coast of the USA. It is also invasive in Hawaii, and globally in southern Europe, Australia and New Zealand. A team of researchers led by the Commonwealth Science and Industry Organisation in Australia used cutting-edge molecular DNA and statistical techniques to determine the genetic diversity of Cape-ivy populations in all of these areas in relation to its native range. Over 1,000 leaf samples of Cape ivy were collected from 122 sites, including from seven sites in California, one in Oregon and three in Hawaii. The DNA results indicated that in all of the world where Cape-ivy has invaded except California and Oregon, there is only one genotype (collection) of Cape-ivy, and it is clonal-it reproduces exclusively by vegetative means (the flowers are sterile-they do not produce seed). These accession likely came from coast of the Western Cape Province of South Africa, perhaps from the area near Cape Town. In California, by contrast, there is more genetic complexity and evidence of multiple introductions, including accession that can reproduce sexually (through flower pollination and seed production). The global clonal accession is present in California, but other sites there have this sexually-reproducing accession. It is genetically very close to an accession collected in southern France. Cape-ivy was cultivated as an ornamental plant in the eastern and midwestern U.S from the 1860s onwards, where it was marketed as "German ivy" and used as a window garden box plant, suggesting that the horticultural industry for this plant was based on plants imported from Europe, not directly from South Africa. Some whole plants or cuttings from that industry made their way westward to southern Oregon and California, perhaps via the then-new Transcontinental Railroad. The clonal accession was introduced to California later on, at an unknown point in time. At some California sites, both the clonal and sexually-reproducing accessions are mixed, and the accession that can make seed has crossed with other Cape-ivy plants at the site, creating more genetic diversity. Overall, the native South African Cape-ivy populations that are genetically closest to the invasive populations in California are from coastal Western Cape Province of South Africa. The results have implications for biological control of Cape-ivy using insects as biological control agents. One agent has been released, a shoot tip-galling fly (Prafreutreta regalis), and another agent is being considered for release, a leaf- and stem-mining moth (Digitivalva delaireae). Both insects were sourced from coastal Western Cape Province. The genetic results suggest that these insects should do well on the invasive Cape-ivy populations in California, as those populations are well-matched to the area of the native range where the Cape-ivy came from originally. Indeed, the shoot-galling fly has established populations at 14 sites along the California coast.

Technical Abstract: We investigated global patterns of population genetic diversity and structure for the clonal invasive scrambler, Cape ivy (Delairea odorata Lem., family Asteraceae), to help optimize exploratory surveys of candidate biocontrol agents in its native range, South Africa. We collected more than 1,000 Cape ivy samples from 122 populations throughout South Africa and its introduced ranges (Australia, New Zealand, mainland USA, Hawai’i and southern France), and performed population genetic diversity and structure analyses based on single nucleotypic polymorphisms (SNP’s) (derived from DArTseq genome complexity reduction-based sequencing). On average, population genetic diversity values were six times higher in South Africa than all introduced populations, although these values were three times higher for the mainland USA populations than those in Australia, Hawai’i and New Zealand. Populations were strongly differentiated with limited gene flow between the native and introduced ranges (intercontinental scale) and between sites within each range (regional scale). All Australian, New Zealand, Hawai’ian and some Californian populations had Fst values approaching zero, likely arising from a single clonal accession derived from the same ancestral population, likely from the Western Cape province of South Africa. The remaining mainland USA populations showed signs of multiple introductions and admixture patterns. Our results show that human-assisted dispersal of Cape ivy through the ornamental plant trade can overcome genetic (bottleneck) barriers to global invasion, especially for clonal accessions of the plant. We recommend that host-specificity testing of candidate biocontrol agents for Cape ivy in the Australian and New Zealand contexts should source agents from the Western Cape province of South Africa.