Rice weevils obtained from Ibadan proved to have the highest level of insecticide resistance to
pirimiphos methyl. Ibadan has a tropical climate and falls within savannah forest, which is characterized
by high temperature throughout the year. Temperature affects the activity of insects, therefore, the
population growth rate of the pest is always high throughout the year (Lale and Ofuya, 2001). Ibadan
also has a large human population and there are many farms and markets within and around the
metropolis. More insecticides might be used in the region to treat produce. Although pirimiphos methyl
is not normally used directly on rice, the walls and storage pallets in rice stores are treated. It is also used
to treat produce like wheat, cowpea and maize which are marketed together with rice and S. oryzae can
cross infest from treated produce and develop resistance to pirimiphos methyl. Fragoso et al. (2002) and
Pereira et al. (2006) attributed the variation in the resistance of insect from different locations to greater
use of insecticides and usage pattern in those locations.