Studies on mechanism and fitness cost of resistance in Spodoptera litura against some new chemistry insecticides in Pakistan

Abstract

The tobacco budworm Spodoptera litura (Fabricius) is one of the major insect pests of many economically important crops and vegetables. It attacks more than 112 cultivated plants species including cotton, maize, tobacco, groundnut, summer legumes and many vegetables. It has developed high resistance against all type of insecticides including conventional and new chemistry insecticides. Keeping in view the importance of this pest, experiments were conducted to find out mechanism of resistance, cross resistance, stability and fitness cost of resistance of S. litura against methoxyfenozide and spinosad. To investigate the mechanism, cross resistance and stability of S. litura to methoxyfenozide, a field collected population of S. litura selected with methoxyfenozide for thirteen consecutive generations resulted in the development of 83.24 and 2358.6-fold resistance to methoxyfenozide as compared to parental field population and susceptible laboratory population, respectively. The outcomes of synergism studies revealed methoxyfenozide resistance in S. litura to be monooxygenases (MO) mediated with high synergistic ratio (4.83) with piperonyl butoxide (PBO), while S, S, S-tributyl phosphorotrithioate (DEF) showed no synergism with methoxyfenozide (SR=1). This methoxyfenozide resistant strain showed a high cross resistance to deltamethrin (28.82), abamectin (12.87) and little to emamectin benzoate (2.36), however no cross resistance of methoxyfenozide and other tested insecticides was recorded. The results depicted the methoxyfenozide resistance in S. litura to be unstable with high reversion rate which decreased from 2358.6 to 163.9-fold (as compared to the susceptible strain) when reared for five generations without any insecticidal exposure. The present research supports the significance of MO-mediated metabolism in resistance to methoxyfenozide, which demands some tactics to tackle this problem. The resistance against methoxyfenozide in S. litura can be overcome by switching off its use for few generations or insecticides rotation having different mode of action. Similarly in order to investigate the mechanism of resistance, cross resistance and stability of spinosad resistance to S. litura, a field collected population of S. litura was selected with spinosad for eleven generations under controlled laboratory conditions. The resistance to spinosad in S. litura increased 3921-fold (after eleven generations of selection with spinosad) as compared to a susceptible population of S. litura. No cross resistance between spinosad and emamectin benzoate, methoxyfenozide, fipronil, indoxacarb, profenofos, lufenuron or deltamethrin was found in the spinosad selected population of S. litura. To find the possible mechanism of spinosad resistance in S. litura two synergists, piperonyl butoxide (PBO), S, S, S- tributyl phosphorotrithioate (DEF) were tested on the susceptible and resistant strains and on the un-selected field population. The values of the synergist ratios of PBO and DEF were 2.33 and 1.06 for the spinosad selected strain, 1.36 and 1.06 for the un-selected field population and 1.14 and 1.00 for the susceptible strain, respectively. As high PBO ratio indicates the role of microsomal O-demethylase in causing spinosad resistance in S. litura. The spinosad resistant and field populations of S. litura were reared without any selection pressure from the 12th to the 16th generation (G12-G16). The spinosad resistance decreased from 3921 to 678 -fold in the spinosad resistant population and from 31.1 to 15.1-fold in the un-selected population of S. litura as compared to the susceptible strain. Spinosad resistance in S. litura has a high reversion rate (−0.15) which indicates that spinosad resistance in S. litura is unstable and can be easily managed by switching off the selection pressure for a few generations or alternating with insecticides having different modes of action. To find out the fitness cost and sub lethal effects of methoxyfenozide to S. litura, two experiments were designed using the susceptible, field and methoxyfenozide resistant populations of S. litura. The first experiment was conducted to find out the fitness cost of methoxyfenozide resistance in a methoxyfenozide-resistant strain of S. litura for which a field collected population of S. litura was selected with methoxyfenozide for thirteen consecutive generations which resulted in the development of 83.0 and 2359-fold resistance to methoxyfenozide as compared to the field and susceptible population of S. litura and showed a fitness cost of 0.17 as compared to the susceptible strain of S. litura. In the second experiment this susceptible strain was treated with methoxyfenozide by incorporating different concentration levels of methoxyfenozide i.e. LC30, LC20 and LC10 into its artificial diet and feeding the 2nd instar larvae on this treated diet for three days. The effects of different concentrations of methoxyfenozide on the biological parameters of S. litura were determined. It was observed that higher concentrations of methoxyfenozide significantly prolonged the development period of larvae and pupae of S. litura as compared to the untreated control population. The larval mortality was 28.00%, 19.00% and 10.00% at LC30, LC20 and LC10 levels of methoxyfenozide, respectively. Similarly the pupal mortality recorded at LC30, LC20 and LC10 levels of methoxyfenozide were 13.00%, 8.00% and 5.00%, respectively. Methoxyfenozide also showed a significant effect on the adult longevity and survival. The number of eggs laid per female, egg hatching, female ratio and the survival time of the adults of methoxy-treated groups were greatly reduced as compared to the control population of S. litura. However the effects of methoxyfenozide were greatly minimized in the next offspring generation of the methoxy-treated parent generation of S. litura. The results clearly indicated that fitness cost of methoxyfenozide and its sublethal effects on S. litura have an important impact on its population dynamics. Thus it should be incorporated in the IPM program of S. litura in order to keep the pest population below economic injury level. Similarly in order to find the fitness cost and sub lethal effects of spinosad on S. litura, experiments were conducted by using the susceptible, field and spinosad-resistant populations of S. litura. The fitness cost of resistance to spinosad was determined in S. litura. The results of the fitness study showed that the spinosad resistance in resistant strain of S. litura had a relatively high fitness cost of 0.15 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 including LC40, LC30, LC20 and LC10, which revealed that the impact of spinosad on the biological parameters of S. litura increased with the increase in concentrations of spinosad. The results showed a significant impact of spinosad on the larval duration, pre-pupal weight, pupal duration, pupal weight, No. of eggs per female and adult emergence etc. The outcomes of the current research clearly indicate that fitness cost of spinosad and its sub-lethal effects on S. litura have a significant impact on its population dynamics which can be incorporated in the integrated pest management of S. litura.