Assessing the risk of exotic herbivorous insects to New Zealand’s native plants

Abstract

Despite the presence of >1400 exotic insect species in New Zealand, relatively limited literature is available to describe their impact on native biodiversity. In this project, I use an integrative risk assessment to identify the risk posed to a range of native plant species from exotic herbivorous insects. To do this, I determine the interactions between ‘assets’ (i.e. native plants) and the ‘hazards’ (i.e. exotic herbivorous insects that have already established). Firstly, I use literature collated within the online Plant-SyNZ database of exotic insects present in New Zealand and their associated host plants to identify: i) the most common exotic herbivorous insects, ii) the plants with the highest numbers of exotic herbivores, and iii) quantify the associations between them. Secondly, I compare the potential threat, as recognised from the above findings, with the realized threat by undertaking field surveys of five common plant species (karamu, kawakawa, mahoe, karo and red matipo) across 14 Regional Parks within the Auckland region to obtain the frequency and abundance of insects. I investigated how insect traits, plant traits, and environmental variables interact to shape these systems. Polyphagous and o-oligophagous insect species within the families Ricaniidae, Flatidae, Thripidae (subfamily Panchaetothripinae), Pentatomidae, Aphididae, Diaspididae and Coccidae were identified as posing a high-risk to native plants. Additionally, if these species had many associations with cultivated plants, then they posed an even higher risk to native plants. Vulnerable native plants were phanerophytes (plants taller than 25-50cm whose shoots don’t die back periodically to that height limit) which are found in coastal environments and are within the families Haloragaceae, Scrophulariaceae, Sapindaceae, Bignoniaceae, Solanaceae and Rutaceae. The passionvine hopper was the most widespread and abundant species collected across all plant species and most sites within the field surveys. This was an important finding as there is no literature available to describe the impact of the passionvine hopper on native plants. However, the large populations throughout the Auckland Region are very concerning in relation to the long-term implications of their feeding pressure on native plants. Future researchers must aim to carry out full risk assessments relating to the feeding pressure of the passionvine hopper on native plants and insects which fall into the identified high-risk groups.