Stability, cross-resistance and effect of synergists, PBO and DEF, on deltamethrin resistant strain of Spodoptera exigua (Lepidoptera: Noctuidae) from Pakistan.

Spodoptera exigua (Hubner), is a polyphagous pest of vegetables and field crops. A field population of S. exigua from Lodhran was found resistant to conventional insecticides. It gave 65, 66, 92, 73, 34 and 29-fold resistance to deltamethrin, cypermethrin, chlorpyrifos, profenofos, abamectin and spinosad respectively compared with Lab-PK susceptible population. Field population was divided into two sub-populations. One was left unselected and the second (Del-SEL) selected for five generations at the dose equal to LC50. Bioassays at G5 for Del-SEL strain gave resistance ratios of 976, 421, 118, 30, 15 and 17-fold for deltamethrin, cypermethrin, chlorpyrifos, profenofos, abamectin and spinosad, respectively compared with Lab-PK. Resistance was found stable for all the insecticides in the field population but it was more stable in Del-SEL strain for deltamethrin, cypermethrin and chlorpyrifos than profenofos, abamectin and spinosad when reared without exposing to deltamethrin (G5 - G10). It indicated that cross resistance occurred between deltamethrin, cypermethrin and chlorpyrifos. There was no effect of delta selection on the toxicity of three insecticides i.e., profenofos, abamectin and spinosad, which indicated lack of cross resistance with deltamethrin. Deltamethrin resistance in Del-SEL strain of S. exigua was suppressed with the synergists such as piperonyl butoxide (PBO) and S,S,S-tributylphosphorotrithioate (DEF), suggesting the involvement of monooxygenase and esterase in the development of resistance in S. exigua. Due to the stability and cross resistance with other insecticides deltamethrin should be replaced with insecticides having different mode of action possibly with new chemistries in integration with other techniques like catch and kill method with light traps, altering cropping pattern i.e., rotation of host plants with non host plants etc.