Correlates of spread rates for introduced insects

Aim: The rate at which introduced insects disperse into novel habitats is a key determinant of the impacts of biological invasions and the efficacy of importation biological control programs. Understanding which life-history traits or abiotic factors moderate spread rates is important for designing trapping and eradication programs for invasive insects and for designing release protocols of imported natural enemies. Our aim was to identify variables that can explain variation in spread rates of introduced insects. Location: Global. Spread rates from 30 different countries were compiled. Time period: 1976–2014. Major taxon studied: Insecta. Methods: We compiled 147 published spread rates of 86 non-native insects. Spread rates were averaged per species so that each insect was only represented once in statistical analyses. Simultaneously, we collected information on several variables associated with the introduced insects such as adult longevity, diet breadth, diet preference, eusociality, fecundity, taxonomic order, role of passive dispersal (important versus not important), size, type (invasive species versus biological control agent), voltinism (i.e., the number of generations per year) and duration of spread. In addition to analysis of the complete data set, analyses of several subsets of the data were conducted to determine robustness and sensitivity of findings. Results: In the global analysis, voltinism was clearly the most significant predictor of spread rates. Insects with more than one generation per year spread faster than those with one or fewer generations. Analyses limited to either invasive species or importation biological agents further confirmed that multivoltine insects spread faster on average than univoltine insects. Main conclusions: Whereas previous work has shown that accidentally introduced insects spread faster than those introduced intentionally, the identification of voltinism as a key predictor of spread may enhance risk analyses and accuracy of forecasting, especially where a changing climate may alter patterns of voltinism.