Variations in the efficacy of mixed formulations comprising vegetable oils and synthetic pyrethroids against susceptible (Delhi) and resistant (Guntur) larval populations of Spodoptera litura (Fabricius).

The non-toxic vegetable oils, viz., karanj oil. neem oil, citronella oil and piperonyl butoxide were separately used in the mixed formulations with eypermethrin and fenvalerate in four ratios (1:1, 1:2, 1:5, 1:10) and evaluated against 7±1-day old larvae of Spodoptera litura (Delhi and Guntur populations) by leaf-dip method. The third instar larvae of S. litura obtained from Guntur district (Andhra Pradesh) were 4- and 5-fold resistant to cypermethrin and fenvalerate, respectively, when compared with the Delhi population. The categorization of the type of action was based on joint action ratio (JAR). Karanj oil when combined with either cypermethrin or fenvalerate proved synergistic against Delhi population of S. litura, which the combinations were antagonistic to Guntur population. Piperonyl butoxide synergised cypermethrin against both the Delhi and Guntur populations, whereas fenvalerate synergism with piperonyl butoxide was evident only against Delhi population. The well known neem oil and citronella oil produced additive effect with cypermethrin against both the populations. whereas fenvalerate exhibited additive action with neem oil against Delhi population and synergism against Guntur population. This indicated the potential value of certain insecticide mixtures for the control of different populations of S. litura. The synergism exhibited by mixing cypermethrin with piperonyl butoxide might be a feasible substitute for cypermethrin alone. especially in areas where resistance was high to this pyrethroid. Also, the mixture of fenvalerate and neem oil could produce significantly greater control than fenvalerate alone in areas where resistance to feuvalerate was observed.