Leo W. Beukeboom

Invasive alien plants (weeds) are a growing, worldwide problem. Importation (‘classical’) biological control, using exotic insects, may be a key component in controlling invasive plants. Two‐thirds of plant species targeted worldwide by biocontrol programs experience some level of control, but the success rates are geographically variable and still hard to predict. Better understanding of the interactions of the species involved, and the effects of changing climate, may help to further improve both the efficacy and the predictability of this method. This requires pre‐release studies, as well as post‐release studies that quantify the impact at the population level to validate methodologies and assumptions of pre‐release studies. The 12 original papers of this special issue include pre‐ and post‐release studies on 13 insect species from five orders. The studies cover a wide range of methodologies and altogether they highlight that both target and non‐target impact are highly context dependent. This dedicated issue includes directions for methodological improvements to better assess ecological host ranges of agents and to avoid rejection of safe agents.