Insecticide resistance management in Rice weevil, Sitophilus oryzae (L.) (Coleoptera: Curculionidae)

Abstract

The research work entitled "Insecticide resistance management in rice weevil, Sitophilus oryzae (L.) (Coleoptera: Curculionidae)" was done at College of Agriculture, Vellayani during 2019 to 2021.The objectives of the study were to assess the resistance levels in rice weevil, biochemical basis of resistance and screening of new molecules for the management of S. oryzae. Different sample populations of S. oryzae were collected from the three godowns of Food Corporation of India (FCI) viz., Kollam, Valiyathura, and Thikkodi, as well as a susceptible lab culture of S. oryzae maintained without pesticide exposure, was obtained from the Division of Entomology, Indian Agricultural Research Institute (IARI). Resistance levels of the commonly used insecticides in FCI viz., malathion and deltamethrin were evaluated in the three populations collected from Kollam, Valiyathura and Thikkodi by film method of bioassay and compared it with the susceptible lab population from IARI. Kollam population showed highest resistance to both malathion and deltamethrin with resistance ratios of 14.94 and 9.03 followed by Valiyathura with resistance ratios of 11.39 and 7.79, respectively. Thikkodi population showed the least resistance to malathion and deltamethrin, with resistance ratios of 8.74 and 5.48, respectively. Malathion resistance was higher in all the three populations than the deltamethrin resistance. The most resistant population selected from the previous experiment i.e., the Kollam population was further assayed for its biochemical basis along with the susceptible IARI lab culture. The Kollam population was found significantly higher in total protein (6 mg ml-1 ) and activity of detoxifying enzymes viz., carboxylesterases (0.65 µmol min-1 mg protein-1 ), glutathioneS-transferases (0.41 µmol min-1 mg protein-1 ) and cytochrome P450 (0.74 p mol min-1mg protein-1 ) than the lab sample. The sample population collected from Kollam was again screened for their susceptibility to new generation insecticides like fipronil, indoxacarb and chlorantraniliprole by film method of bioassay. Fipronil was found to be more toxic with LC50 value of 5.86 ppm followed by indoxacarb (90.57 ppm) and chlorantraniliprole (4041.43 ppm). Fipronil and indoxacarb were 619.8- and 40.13-fold toxic to resistant population of S. oryzae with respect to malathion. The newer insecticides in the previous experiment along with malathion and deltamethrin were further screened simulating the conditions of FCI godown. The insecticides were sprayed on jute bags containing 1 kg of rice grains and 50 adult beetles of Kollam population of S. oryzae. The dosages of newer insecticides were taken 10 times more the LC50 value obtained in the laboratory bioassay and FCI recommended dosage was taken for malathion and deltamethrin. Fipronil 5% SC @ 0.006% recorded significantly higher mortality of 86.5 and 89.5 per cent at 24 and 48 h of treatment followed by indoxacarb 14.5% SC @ 0.09% and chlorantraniliprole 18.5% SC @ 4.04%. The persistence of the most effective insecticide fipronil along with malathion and deltamethrin on rice grains were further studied by recording the residue levels at different time intervals after spraying. Dosages were same as the previous experiment. Insecticides were sprayed on jute bags containing rice grains and residues were analysed at 0 (2 h after spray), 1, 3, 5, 7,10, 15, 20 and 30days intervals using the methods validated prior to residue estimation. Fipronil residues were below the limit of quantification at 2 h after spraying. The residues of deltamethrin and malathion persisted up to 3 and 5 days after spraying with a half-life of 2.48 and 2.71 days, respectively. From the present study it is revealed that S. oryzae collected from Kollam, Valiyathura and Thikkodi were resistant to the commonly used insecticides viz., malathion and deltamethrin and it is confirmed here by the presence of higher levels of detoxifying enzymes in the resistant population. Further screening of newer insecticide molecules against the resistant population of S. oryzae suggested that fipronil 5% SC @ 0.006% is highly effective and less persistent on grains when compared to malathion and deltamethrin.