Munir Ahmad

Resistance to Selected Organochlorin, Organophosphate, Carbamate and Pyrethroid, in Spodoptera litura (Lepidoptera: Noctuidae) from Pakistan

Abstract The toxicity of the most commonly used insecticides of organochlorine, organophosphate, pyrethroid, and carbamate groups were investigated against Spodoptera litura (F.) (Lepidoptera: Noctuidae) populations collected for three consecutive years (2004–2006). For a chlorocyclodiene and pyrethroids tested, the resistance ratios compared with Lab-PK were in the range of 10- to 92-fold for endosulfan, 5- to 111-fold for cypermethrin, 2- to 98-fold for deltamethrin, and 7- to 86-fold for β-cyfluthrin. For organophosphates and carbamates, resistance ratios were in the range of 3- to 169-fold for profenofos, 18- to 421-fold for chlorpyrifos, 3- to 160-fold for quinalphos, 6- to 126-fold for phoxim, 7- to 463-fold for triazophos, and 10- to 389-fold for methomyl and 16- to 200-fold for thiodicarb. Resistance ratios were generally low to medium for deltamethrin and β-cyfluthrin and high to very high for endosulfan, cypermethrin, profenofos, chlorpyrifos, quinalphos, phoxim, triazophos, methomyl, or thiodicarb. Pairwise comparisons of the log LC50 values of insecticides tested for all the populations showed correlations among several insecticides, suggesting a cross-resistance mechanism. Integration of timely judgment of pest problem, delimiting growing of alternate crops such as arum, rotation of insecticides with new chemicals, and insect growth regulators in relation to integrated pest management could help in manageable control of this important pest.

Cross-Resistance and Genetics of Resistance to Indoxacarb in Spodoptera litura (Lepidoptera: Noctuidae)

Abstract Bioassays (at generation G1) with a newly collected field population of Spodoptera litura (F.) (Lepidoptera: Noctuidae) from Multan, Pakistan, showed resistance ratios of 15, 23, 37, and 16 for indoxacarb, spinosad, abamectin, and emamectin, respectively, compared with a laboratory susceptible population, Lab-PK. At G1, the field population was selected with indoxacarb by using single pair crosses. For Indoxa-SEL, bioassay at G4 found that selection increased resistance ratio to 95 for indoxacarb compared with Lab-PK. Selection with indoxacarb significantly increased resistance to spinosad and emamectin; however, resistance to abamectin was observed to drop. A significant reduction in the resistance to indoxacarb was observed in Indoxa-SEL at G9, indicating unstable resistance to indoxacarb; however, it was stable for fipronil. Synergism tests with microsomal oxidase and esterase-specific inhibitors suggested that the indoxacarb resistance was associated with microsomal oxidase. Reciprocal genetic crosses between Indoxa-SEL and Lab-PK populations indicated that resistance was autosomal and incompletely dominant. Tests of monogenic inheritance suggested that resistance to indoxacarb was controlled by more than one locus.

Fitness costs limit the development of resistance to indoxacarb and deltamethrin in Heliothis virescens (Lepidoptera: Noctuidae).

Abstract Insecticide resistance in Heliothis virescens (F.) (Lepidoptera: Noctuidae) has been documented from all over the world and is often associated with reduced fitness. Fitness costs could delay the development of resistance depending upon the prevailing conditions. We were interested in establishing whether a field-collected population from Washington County, MS, was resistant to spinosad, indoxacarb, and deltamethrin and whether any such resistance was associated with fitness costs. Bioassays results showed that the insecticides were equally toxic to the field population. Upon laboratory selection (generations [G]3 to G8), the resistance ratio increased only 2-, 3-, and 1-fold for spinosad, deltamethrin, and indoxacarb, respectively, compared with the field population. In contrast, the resistance ratios increased 213-, 65-, and 55-fold compared with an unselected population at G9. The estimated realized heritability (h2) after six generations of selection was 0.17, 0.03, and 0.12, respectively, and the number of generations required for 10-fold increase in LC50 of Spino-SEL, Indoxa-SEL, and Delta-SEL was estimated to be 14.3, 50, and 14.3. Comparison of life traits between the selected and unselected populations revealed that the selected populations laid a significantly lower number of eggs and that a lower percentage of eggs hatched. This also was reflected in both the net replacement rate and the intrinsic rate of population increase, which were both lower for the selected populations. It also was observed that the mean relative growth rate of the larvae was lower for the selected populations; not only did the larvae take longer to pupate but the mean weight of the prepupae from the selected populations was lower. Our data suggest that due to fitness costs the development of resistance to the insecticides was limited such that after six generations of selection the larvae were no less susceptible to the insecticides than the field population although were considerable more resistant than the unselected population.