Development of malathion resistance in Sitophilus oryzae (L.) in Punjab (India)

Susceptibility status of Sitophilus oryrzae (L.) collected from 33 locations in Punjab (India) to malathion was established in the laboratory using filter-paper impregnation bioassay technique. The LC50 values of malathion against S. oryrzae ranged from 0.238 to 0.447%, 0.209 to 0.404% and 0.173 to 0.377% for populations from godowns, flour mills and rural houses, respectively as compared to 0.025% of the susceptible strain reported in 1975. This showed resistance ratio (RR) ranging from a minimum of 7.0 to a maximmn of 17.9 times. About 60% populations showed a resistance ratio of more than 10.0. The populations from godowns showed maximum tolerance to malathion (RR, 9.5 to 17.9) followed by flour mills (RR, 8.4 to 16.2) and rural houses (RR, 7.0 to 15.1). The results showed low levels of malathion resistance in all the populations collected from different parts of the State. This is the first report of development of resistance in the pest to malathion in this part of India.