The aim of the study was to evaluate the economic impact of biological control of cassava green mite (CGM) in Ghana by using the natural enemy Typhlodromalus manihoti. Both primary and secondary data were used for analysis. In total, 714 cassava producing households were drawn from 17 districts in seven regions of Ghana by employing a combination of proportional, purposive, simple random, and systematic random sampling techniques. Formal interviews with the use of standardized structured questionnaire were combined with field/farm visits to elicit information for the study. The ‘with and without’ comparison and trend analysis of secondary data were performed to evaluate project impact. The economic surplus model was used to estimate net benefits of biological control of CGM in Ghana in terms of Benefit-Cost Ratio (BCR), Net Present Value (NPV) and Internal Rate of Return (IRR). Following the release of about 2,344,817 CGM predators (T. manihoti) in seven regions of Ghana between the year 2007 and 2012, the study found evidence of T. manihoti presence in beneficiary communities. However, the level of establishment and extent of spread were markedly different across regions and districts in Ghana. The CGM biological control project in Ghana was found to have been successful in reducing CGM populations on cassava fields, reducing cassava root losses, improving productivity and profitability under varying climatic conditions and soil types. Biological control of CGM leads to higher economic gain to farmers and the country as a whole. Biological control of CGM was found to generate economic returns (NPV) of US$228.5 million, a BCR of 5,393.74 and an IRR of 3,424% at a discount rate of 20% for the period 2006 to 2046. Sensitivity analysis showed that the returns are robust even at higher discount rate (50%) and under pessimistic assumptions about yield gains. The overall project impact is expected to be higher when benefits to ecological and human health, which are usually difficult to quantify, are considered. Periodic inoculation in released communities and fresh releases in new communities are recommended if the country is to realize the full benefits of biological control of CGM.
Key word: Biological control, cassava green mite, economic gain, impact, Typhlodromalus manihoti.
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