In the present study, two experiments were designed to investigate the lethal and sublethal effects of methoxyfenozide on Spodoptera litura (F.). The first experiment was conducted to explore the fitness cost of methoxyfenozide resistance in a methoxyfenozide-resistant strain (selected with methoxyfenozide for 13 consecutive generations), leading to resistance ratio to methoxyfenozide 83.0- and 2359-fold higher when compared to the field and susceptible populations with a fitness cost of 0.17. In the second experiment, second instars of the susceptible strain were treated with sublethal doses of methoxyfenozide (LC30, LC20, and LC10) by diet incorporation for larval feeding for 3 days. It was observed that higher concentrations of methoxyfenozide significantly prolonged the larval and pupal development time of S. litura as compared to the control treatment. The number of eggs per female, egg hatching, sex ratio, and longevity of adults of methoxyfenozide-treated groups was greatly reduced as compared to untreated S. litura. Our data clearly indicated that fitness cost of methoxyfenozide and its sublethal effects on S. litura has an important impact on its population dynamics.
Spodoptera litura is one of the most destructive polyphagous insect pests, with more than 120 host‐plant species. In our present study, a field‐collected population of S. litura when selected with spinosad for 11 consecutive generations resulted in the development of 3921‐fold resistance to spinosad as compared to the susceptible strain. The spinosad‐resistant strain of S. litura had a relatively high fitness cost (0.17) as compared to the susceptible strain. Furthermore, the lethal and sub‐lethal effects of different concentrations of spinosad were checked on the susceptible strain at different levels; i.e., LC40, LC30, LC20 and LC10, which revealed that the impact of spinosad on the life‐history traits of S. litura increased with the increase in concentration of spinosad. A significant impact of spinosad was recorded on the larval duration, pre‐pupal weight, pupal duration, pupal weight, reproductive potential and adult emergence. The outcomes of the current research clearly indicate that fitness cost of spinosad and its sub‐lethal effects have a significant impact on population dynamics of S. litura, for which it can be incorporated in integrated pest management.
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