Synergistic Effects of Sesame Oil with Cypermethrin on the Survival and Detoxification Enzyme Activity of Plutella xylostella L. Larvae

Abstract

                Two types of insect-toxicity tests, (1) contact and (2) no-choice leaf dipping test, were conducted using the insecticide cypermethrin without piperonyl butoxide (PB), cypermethrin with PB and cypermethrin with sesame oil against the 2^nd –3^rd instar larvae of Plutella xylostella L. Sesame oil showed good synergism with cypermethrin yielding synergistic ratios (SR) that ranged from 1.54 – 6.33 in the contact method and 2.04-5.88 in the no-choice leaf dipping method and were comparable to using PB, which showed SR’s of 6.33 and 6.71, respectively. Both PB and sesame oil mixed together with cypermethrin inhibited monooxygenase activity by approximately two-third but induced glutathione–S-transferase ca. 2-3 folds in both methods. The synergists had no effect on esterase activity (CF ca. 1.2).

                 Residues of cypermethrin in the larvae increased by 1.29 – 2.57 folds in the treatments with added sesame oil compared to a 2.86 fold increase when PB was added, using the contact method. The no choice leaf dipping method revealed that cypermethrin residue levels increased by 2.82 – 6.91 fold with added sesame oil and 8.27 fold with added PB. This indicated that both PB and sesame oil played the same role in the inhibition of an enzyme, possibly monooxygenase. Field trials with Chinese kale showed the same trends that were evident in the laboratory work. The addition of sesame oil to the insecticide reduced the larval population by 70-80 percent while the addition of PB reduced the larval population by up to 88 percent in the kale crop. Monooxygenase activities of insect larvae collected in the field from kale sprayed with cypermethrin plus synergist (sesame oil or PB) were lower than those from kale treated with insecticide alone. The results in terms of synergism and changes in enzyme metabolism were discussed.