St. John’s wort beetles would not have been introduced to New Zealand now: A retrospective host range test of New Zealand’s most successful weed biocontrol agents

St. John’s wort, Hypericum perforatum, was a serious weed in New Zealand (NZ) pastures in the 1930s. Following successful biological control of the weed in Australia, two agents, the chrysomelid beetles Chrysolina hyperici and C. quadrigemina, were introduced to NZ in 1943 and 1965, respectively. At the time, the Australian host specificity testing, which had not included other Hypericum spp., was deemed sufficient for approval for release in NZ. We reviewed worldwide reports which suggest that St. John’s wort beetles will attack a range of Hypericum species in the field. A series of laboratory tests were conducted to simulate modern host-range-testing protocols. Larval feeding choice, larval development and female oviposition choice all suggested that at least two of NZ’s indigenous Hypericum species were suitable hosts for the two Chrysolina species, and a third indigenous Hypericum, while an unsuitable host, was not altogether avoided. We conclude that the two Chrysolina species would not have been approved for introduction to NZ under current risk assessment protocols. Had this happened, NZ would have missed out on one of its greatest biocontrol success stories, despite there being no evidence for subsequent impacts on the populations of indigenous congeners. This case stresses the need for better procedures to predict the realized host-range of an agent from the potential range in contained host-range testing, and raises issues surrounding risks and benefits to indigenous biodiversity.

Chrysolina beetle larval development on target host (Hypericum perforatum) and on four non-target hosts 10 days into a feeding experiment.Figure optionsDownload as PowerPoint slideResearch highlights
► St. John’s wort beetles would have failed current NZ host-range testing standards.
► NZ would have missed out on one of its greatest biocontrol successes.
► Failure of biocontrol may have been more damaging to NZ’s indigenous biodiversity.
► Geographic overlap and apparent competition become significant for risk assessments.
► Slightly higher tolerance to risk may improve net biodiversity gains from biocontrol.