Eight decades of invasion by Chromolaena odorata (Asteraceae) and its biological control in West Africa: the story so far

Chromolaena odorata (L.) R.M. King and H. Robinson (Asteraceae) is a perennial weedy shrub of neotropical origin and a serious biotic threat in its invasive range. The Asian-West Africa (AWA) biotype of C. odorata present in West Africa is both morphologically and genetically different from the southern African (SA) biotype. The AWA biotype was first introduced into Nigeria in the late 1930s and rapidly spread across West Africa. Currently, 12 of the 16 countries in West Africa have been invaded, with significant negative effects on indigenous flora and fauna. However, locals in West Africa have found several uses for the weed. As chemical, physical and other conventional methods were unsustainable, costly and largely ineffective, three biological control agents, Apion brunneonigrum (Coleoptera: Brentidae), Pareuchaetes pseudoinsulata (Lepidoptera: Erebidae) and Cecidochares connexa (Diptera: Tephritidae), have been released in West Africa between the 1970s and the early 2000s. However, only C. connexa and P. pseudoinsulata established, contributing to the control of the weed, in six and four countries in West Africa respectively. Limited research funding, the absence of post-release evaluations of the established agents, and the 'conflict of interest' status of C. odorata (i.e. being beneficial for local use but damaging to ecosystem services and agriculture), are serious factors deterring the overall biological control effort. Here, using historical records and field surveys, we examine the invasion history, spread, impacts, and management of C. odorata in West Africa and make recommendations for the sustainable management of C. odorata in the region.