Establishment of exotic insect species within New Zealand: The role of propagule pressure, source variation and dispersal capabilities

Abstract

Colonisation is a critical process in ecology, occurring when a species successfully spreads into a novel environment. Developing a comprehensive understanding of the variables contributing to colonisation is essential to mitigate the economic and environmental harm of an exotic species. Successful colonisation is influenced by a multitude of factors including; propagule pressure, genetic diversity and plasticity of traits within the population. To investigate these factors on establishment, two studies were conducted. Firstly, a study utilising historical interception data to determine the impacts of propagule pressure and source variation on establishment success of exotic insect species within New Zealand, and secondly, an experiment investigating the role of dispersal abilities in colonisation and establishment, using the Hadda beetle, Henosepilachna vigintioctopunctata as a case study. Historical border interception records from 1966-1982 were utilised to examine the establishment success of exotic insects in New Zealand using four predictor variables: propagule pressure, propagule number, propagule size and the number of sources (countries). Using general linear models, each predictor variable had a significant positive impact on establishment success of exotic insect species. The number of sources was often the best variable when evaluated using; Akaike information criterion (AIC) and Bayesian information criterion (BIC) values, cross-validation criteria, and Receiver Operating Characteristic (ROC). Despite these predictors having a significant positive impact on establishment success and having good predictive abilities, the surveillance sensitivity of the data was very low (i.e. probability of a species being intercepted, given it is established). Essentially, many species are establishing but are bypassing interception. The second study investigated the dispersal capabilities of the Hadda beetle, H. vigintioctopunctata, a recently established exotic species in New Zealand. Dispersal propensity and flight parameters were examined in terms of environmental and biological characteristics of the individuals (age, sex, mating status, size). Flight initiation was influenced solely by temperature; the minimal temperature for flight initiation was 19°C, and optimal temperatures were between 25 and 30°C. Flight mills were used to assess four flight parameters (total flight distance, frequency, duration and speed) of adult Hadda beetles. Sex, body size, mating status and age of the Hadda beetles had a significant influence on flight parameters. Understanding the flight capacities of the Hadda beetle has provided valuable information for predicting the ability of this species to continue to colonise new areas via dispersal; information which can be used for pest management of this established exotic species.