Relative susceptibility of different larval instars of Spodoptera litura (Fabricius) to some synthetic pyrethroids

The results of bioassay with cypermethrin and fenvalerate against the larvae of Spodoptera lltura (Fabricius) indicated that LC₅₀ values increased gradually from the first to fourth instar. Based on LC₅₀ values, cypermethrin was respectively 6, 3, 4 and 10 times more toxic than fenvalerate to one, four, seven and nine-day old larvae of S. litura. There was no significant difference in the LC₅₀ values for cypermethrin or fenvalerate against different instars of S. litura calculated either by leafdip or direct spray methods. Assessement of relative susceptibility of different instars of S. litura by taking LC₅₀ of fourth instar larvae as unity, revealed that the first Instar larvae were 18 and 32 times more susceptible than the fourth Instar to cypermethrin and fenvalerate; second instar was 5 and 18 times more susceptible than the fourth instar, whereas the third instar larvae were only 3 and 7 times more susceptible than the fourth instar to cypermethrin and fenvalerate when tested by leaf dip and direct spray methods, respectively. The range of LC₅₀ values in the case of cypermethrin was 0.000125 – 0.002328 (leaf dip method) and 0.000114 – 0.002120 (direct spray method); and for fenvalerate 0.000754 – 0.024732 (leaf dip method) and 0.000720 – 0.022940 (direct spray method) between the first and fourth instar larvae of S. litura, respectively. The low LC₅₀ values, irrespective of the method used, suggest the occurrence of susceptible strain of S. litura and would, thus, provide the base line data for detecting resistance.