Abstract:
The case for eradication of an invasive phytophagous insect is predominantly based on the estimated damage it could do to commercial crops versus the cost of eradication. As a result, incursion responses are stood down for many insects and the potential for native plants to be hosts, and to be impacted upon, is not always fully accounted for during risk assessments. Hence the lack of a thorough assessment could result in a decline in certain native plant populations. The Henosepilachna vigintioctopunctata (hadda beetle) incursion is one such instance where the eradication response was stood down without thorough assessment of the risk to native plant species. This herbivore feeds on three plant families for which New Zealand has ecologically important native representatives. However, the specificity of the beetle and its ability to fully develop through its life-cycle on native New Zealand plants is unknown. In this study, retrospective host specificity testing was done to determine the risk to New Zealand native plants by: 1) establishing if the beetle will fully develop on native plant species, and 2) assessing potential feeding damage. Additionally the invasive weed Solanum mauritianum was assessed for the potential for the beetle to be a biological control agent. Host specificity was determined through no-choice and multi-choice feeding tests, while feeding damage was assessed through a herbivory damage scoring system, to measure the impact and host preference of the beetle. The results of this study indicate that the native species Solanum aviculare, Solanum albiflorum and Solanum laciniatum are all viable hosts for H. vigintioctopunctata. Conversely, the weed Solanum mauritianum is unlikely to be a viable host for H. vigintioctopunctata, as there was little oviposition or feeding damage on the species. If Solanum nigrum, the preferred host, were to become scarce in the wild, H. vigintioctopunctata could complete a full life-cycle on the native test plants, potentially tipping the native plant species into further decline via herbivory damage. This study highlights the importance of comprehensive risk assessment in invasive species management decisions.