This study assesses the ex ante economic impact of the establishment of two parasitoids, the scelionid egg parasitoid Telenomus isis Polaszek from West Africa and a virulent strain of the braconid larval parasitoid Cotesia sesamiae Cameron from western Kenya, to control stemborer. Using a Cobb-Douglas production function, it was estimated that yields increased by 28.5% when pesticides were used to control stemborers. The benefit-cost ratio after the establishment of the parasitoids ranged from 1:1 to 777:1, with a net present value of up to US$192 million. The study demonstrates the potential of redistributing parasitoids indigenous to Africa for the control of African stemborers. Because the benefits of biological control are positively scale dependent, while the costs are generally scale irresponsive, biological control programs would accrue more benefits if parasitoids were released to a wider area.