Impact of a Local Nigerian Rice Variety and Wheat on the Tolerance of Geographical Strains of Sitophilus oryzae to Dichlorvos

Rice weevil, Sitophilus oryzae, a cosmopolitan pest of stored cereals (including wheat and rice) is moved around inadvertently through global cereal trade. In this study, we consider the effect of strain origin and host food on the tolerance of S. oryzae to dichlorvos, a common insecticide used to protect stored grains in Nigeria. Three geographical strains of S. oryzae (Nigeria, Trinidad and Morocco) were bred separately on Ofada rice (a Nigerian local rice variety) and wheat and then exposed to dichlorvos. The data were analyzed using analysis of variance and General Linear Model, probit analysis was used to estimate LC 50 and LC 95 values. The strain by food combinations varied significantly in their tolerance to dichlorvos. Overall, the Nigerian strain was the least tolerant. Nonetheless, for all strains novel host food influenced their tolerance positively; wheat in the case of the Nigerian strain and Ofada rice for the exotic strains. The results were discussed in terms of varying body size among the strains and physiological effect when insects find themselves in novel environments. The risk posed to local Nigerian rice varieties is considered through the import of exotic strains of S. oryzae, especially when entering the country on wheat