STUDY ON INSECTICIDE RESISTANCE IN SITOPHILUS ZEAMAIS INFESTING STORAGE GRAIN IN SINGAPORE AND MALAYSIA

Abstract

A survey of insecticide resistance in sitophilus zeamalo using the FAO impregnated filter paper method showed that all the field strains viz MSZl, MSZ2, MSZ3, SSZ4 and SSZ5 collected from Singapore and Malaysia were moderately resistant to lindane with their RF50 (Resistance Factor at the LD50 level) ranging from x6 to x10. On the other hand, only the MSZ2 and SSZ5 strains showed low resistance to malathion with HF50 values of x2.6 and x2.4 respectively. When triphenyl phosphate was added to malathion, it did not reduce the resistance levels, indicating that this resistance was not malathion-specific. Other studies have shown that nonspecific malathion resistance is associated with cross-resistance to other organophosphorus and carbamate insecticides. However, there was no cross-resistance observed to diazinon, fernitrothion, pirimiphos-methyl and carbaryl. This could be due to the low levels of malathion resistance. Studies of S. zeamais esterase activity and its inhibition by the above insecticides showed that resistance to malathion was due to reduced sensitivity of Cholinesterase. However, more specific methods should be used to confirm the results. Inhibition of the esterases by diazinon, fenitrothion, pirimiphosmethyl and carbaryl was similar in all the resistant and susceptible strains, Thus, this confirmed that there was no cross-resistance to these insecticides. Electrophoretic studies indicated that there were 5 bands, namely E1 to E5 from the fastest to the slowest in electrophoretic mobility. There was no difference in the numbers and staining intensity of the bands in the resistant and susceptible strains. E1 and E2 were identified as aliesterases, E3 and E4 were cholinesterases and E5 could not be ascertained. E4 ofMSZ2 was not inhibited by malathion and eserine indicating that an altered AchE resistance mechanism possibly existed. The resistance mechanism of SSZ5 could not be identified in the present study.