The braconid larval parasitoid Cotesia flavipes Cameron was introduced from Asia into East and Southern African region starting in 1993 to control the invasive exotic maize stemborer Chilo partellus (Swinhoe) (Lepidoptera: Crambidae). A quasi-experiment was constructed using farmers that applied sublethal dosages of pesticide in assessing the counterfactual for the impact of the establishment of the introduced parasitoid in five countries in the region. Logistic regression method was used to estimate the probability that farmers’ would inadvertently exclude parasitoids in their maize field. Thereafter the propensity score matching method was used to find corresponding farmers excluding parasitoids among those who did not for yield comparison. The geographical information system (GIS) application software with ecological, agricultural and land cover data bases was used to classify and select the ecological zones surveyed, to estimate the spread of the parasitoid. Results indicate a rampant use of pesticides at sublethal dosage across the countries. Intensifying maize production by is likely to improve the effectiveness of parasitoids. The annual value of the economic impact of the introduced parasitoid is estimated to range from US $ 43 m – 76 m. It is apparent that the appropriate matching algorithm depends on the distribution of yield data available. The study demonstrates the potential of using biological agents to improve yields among the poor households who can seldom afford purchased inputs.