Comparative studies between long-term bioassay and discriminating dosage as means for monitoring resistance in S. littoralis larvae

The results of monitoring resistance through long-term bioassay (topical application) showed El-Beheira field strain of the cotton leafworm Spodoptera littoralis had resisted a wide range of the tested insecticides (including organophosphates, carbamates and pyrethroids) except for malathion. The LD₅₀ values of El-Beheira field strain were greater than that of the laboratory-susceptible one for the most of the tested insecticides indicating different ranges of R/R values. For organophosphate insecticides the highest level of resistance was obtained for pirimiphos-methyl (WR 443.53), while moderate resistance was obtained for chlorpyifos and chlorpyrifos-methyl. In the meantime a low level of tolerance was observed to profenofos. Moreover, El-Beheira field strain was slightly changed from susceptibility to malathion (RIR = I .04). For carbamate insecticides El-Beheira field strain was moderately resistant to methomyl while thiodicarb was extremely high in inducing resistance. For synthetic pyrethroid insecticides, El-Beheira field strain was extremely resistant to deltamethrin (RJR = 571.4), while a slight tolerance was achieved for cypermethrin. Moreover, this strain was moderate resistant to fenvalerate. The discriminating dosage technique was used by estimating one dosage (LD₉₉ of the laboratory susceptible strain) for the tested insecticides to monitor for resistance in S. littoralis field strain. The data showed, that the field strain was resistant to eight out of the ten tested insecticides or had survivor percent value of more than 50% (these insecticides were deltamethrin pirimophos-methyl, fenvalemte, thiodicarb, profenophos, chlorpyriphos-methyl, and chlorpyrifos). In contrast, for malathion the percent of survivors was 0.33. So, field strain revealed susceptibility for this insecticide. The dosage technique is a simple, inexpensive and discriminate well between resistant and susceptible populations. The data of a—linating dosage for the tested insecticides against El-Beheira field strain ofS. liftoralis confirmed the long-term bioassay data in which high level of resistance was obtained for deltamethrin and pirimiphosmethyl, since a hundred percent of the tested insects were survived. kn the meantime El-Beheira field strain ofS. littoralis exerts susceptibility malathion. The results suggest the discriminating dosage technique for monitoring insecticide resistance in field populations. Moreover, this technique gave quick information about resistance level provide a simple and quick test for distinguishing between the resistant and susceptible populations. Therefore, this technique is useful to monitor resistant species and thus contribute to their control. Generally, the long-term bioassay technique needs long time and large numbers of insects but it provides an accurate information about the level of resistance.