Resistance to pyrethroid insecticides in the tobacco budworm (Lepidoptera:Noctuidae)

The objectives of the present study were to evaluate the resistance spectra of different pyrethroid-resistant tobacco-budworm strains, to determine alternate insecticides for controlling these insects, to evaluate synergism of insecticides by chlordimeform, and to identify the mechanisms of resistance and possible biological deficiencies associated with resistance to pyrethroid insecticides in the tobacco budworm.;Insecticide bioassays with neonate tobacco-budworm larvae using the glass-vial technique revealed that the resistance spectra were approximately the same for all pyrethroid-resistant strains. However, resistance levels varied for each insecticide against each resistant strain. Chlordimeform synergized all insecticides tested, but synergism was variable for different insecticides and different strains. Extensive bioassays with a pyrethroid-resistant strain (ICI) showed that the main mechanism of resistance expressed in neonate larvae is target-site (kdr) resistance, since the resistance extended to all the pyrethroids tested. No cross resistance was observed in pyrethroid-resistant neonate larvae to the organophosphates monocrotophos, methyl parathion, profenofos, sulprofos, and acephate, the oxime carbamates methomyl and thiodicarb (only one resistant strain, Uvalde, was probably resistant to this insecticide), and the cyclodiene endosulfan.;Bioassays with Stoneville (susceptible) and ICI (resistant) third instars showed resistance to methyl parathion, but no resistance to another phosphorothionate chlorpyrifos, the S-alkyl phosphorothiolates profenofos, sulprofos, and acephate, the carbamates methomyl and thiodicarb, and endosulfan. Chordimeform synergized most insecticides against both tobacco budworm strains. The high level of resistance to cypermethrin at this stage, the resistance to methyl parathion, and the high level of cypermethrin synergism by piperonyl butoxide are evidence for the presence of metabolic resistance in the ICI strain in addition to target site resistance. Based on the synergism data, increased mixed-function oxidase activity seems to be the main factor responsible for resistance in third instars.;The resistance spectra were similar for adults and neonate larvae. Cypermethrin resistance was present in both adult sexes and there were no significant differences between sexes. Results were similar to those obtained with neonate larvae, indicating either life stage can be tested to determine the presence of pyrethroid resistance in the tobacco budworm. Resistance to methyl parathion and thiodicarb (only in the Hearne strain) was also present in adult males but no resistance to acephate was found.;Biological differences were found between the Stoneville and ICI strains. Differences include longer developmental period, reduced egg production, and lower number of females producing offspring in the resistant (ICI) strain. Based on these differences and on the results of bioassays, a general approach for managing pyrethroid resistance in the tobacco budworm using insecticides is proposed.