Constraints in weed biological control: contrasting responses by implementing nations

The compilation of papers in this Special Issue (SI) derives from a Symposium at the 25th International Congress of Entomology, in 2016, entitled “Rise or demise? A global outlook on the future of classical biological weed control”. In the SI-opening-paper, a summary of the 5th edition of the world catalogue of weed biocontrol agents and their target weeds provides a comprehensive international perspective. Weed biocontrol implementation is beleaguered by perceptions of risk and restrictive regulatory procedures, notably in the USA, and less so in Canada. Thus, most of the papers in this SI comprise accounts of innovative responses to these challenges from scientists in the USA. Political and funding issues have inhibited weed biocontrol in Australia over the past decade, but there appears to be a gradual reversal of this trend in recent years. In contrast, in New Zealand and in South Africa, the practice is flourishing, and there are significant recent initiatives in Europe. Overall, the contributions in the SI suggest an optimistic prognosis for weed biological control.