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

Abstract Azolla filiculoides (red waterfern) 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. Based on year 2000 data, the cost savings (per user per hectare) resulting from the biological control program included a reduction of on-site damages caused by the weed to the value of US$589 per hectare per year. The average cost per hectare per year for the biological control program for the period 1995–2000 amounted to US$278, excluding investment costs of USD$7700 in 1995. These historic costs and benefits were adjusted to constant year 2000 values. The predicted spread of the weed was calculated on the basis of a sigmoid-curve rate of spread model. The net present value (NPV) of the program was calculated from 1995 onwards and discounted at 8%. This resulted in a NPV of US$1093 per hectare and US$206 million for South Africa as a whole. For the year 2000, the benefit–cost ratio was calculated at 2.5:1, increasing rapidly to 13:1 in 2005, and 15:1 in 2010 as the costs of the biological control program are expected to decrease. These indicators reinforce the overall economic viability of biological control, but do not necessarily confirm the viability of biological control on each management unit itself. 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.