Economic evaluation of the successful biological control of Azolla filiculoides in South Africa

Abstract

Azolla filiculoides (red water fern) is a floating fern native to South America which has invaded aquatic ecosystems in South Africa. Thick mats of A. filiculoides on dams and slow-moving water bodies cause economic losses to water-users. Affected water users were surveyed using a questionnaire to assess the importance of the weed. Among those most seriously affected were farming (71%), recreational (24%), and municipal (5%) users. The average water area covered by A. filiculoides (per water-user) was 2.17 ha, with an expansion rate of 1.33 ha per year. The frond-feeding weevil Stenopelmus rufinasus was released as a biological control agent at the end of 1997. Within 3 years, the weevil had reduced the weed population to the point that it was no longer considered a problem in South Africa. The results reflect the dynamics of biological control on site-specific survey information, and place higher benefit–cost ratios achieved in other national level studies in a better context. It also raises the important policy question of who is responsible to finance such control programs in future, because on-site benefits of control are enough to justify the program in its own right. The paper concludes with recommendations on a financial mechanism to address biological control of invasive species in a sustainable manner.