AbstractThe joint action of insecticides, or of mixtures of insect growth regulators and insecticides, on the susceptible (S) strain and diflubenzuron‐resistant (Rd) strain of the cotton leafworm Spodoptera littoralis Boisd. was investigated. The joint action of the insecticides and/or insect growth regulator mixtures was determined by mixing them in proportion to their activity equivalents at the LD25 or ED25 levels. A total of 15 mixtures of two synthetic pyrethroids, two organophosphorus, one carbamate and one organochlorine insecticides, were applied to the fourth‐instar larvae of the S and Rd strains. The insecticide mixtures cypermethrin/methomyl and cypermethrin/endrin exhibited high and moderate levels of synergism on the S strain, respectively. However the mixtures chlorpyrifos/methomyl, phosfolan/methomyl, and phosfolan/endrin produced antagonism, while the other mixtures showed varying levels of additive effects. The response of the fourth‐instar larvae of the S strain, to the joint action of diflubenzuron/juvenoid, diflubenzuron/insecticide, or insecticide/juvenoid mixtures, revealed that diflubenzuron produced high levels of synergism when combined with methoprene and progressively less with fenvalerate, methomyl and cypermethrin. On the other hand, the mixture diflubenzuron/triprene was antagonistic. Fenvalerate with the two juvenoids produced synergism while methomyl showed an additive effect with methoprene. However, the mixtures cypermethrin/methoprene, cypermethrin/triprene and methomyl/triprene produced antagonism. The mixtures that produced potentiation on the fourth‐instar larvae of the S strain lost their high potency when tested against the Rd strain. The results also indicated that insecticide/juvenoid mixtures, when applied on 2‐day‐old pupae of the S strain, were synergistic, except in the case of cypermethrin/methoprene and methomyl/triprene mixtures, for which additive effects were observed. When the mixtures that had synergistic effects on the S strain were tested on the Rd strain, the results revealed that their synergistic effects were apparently reduced. This was attributed to the fact that the generalised levels of tolerance in the Rd strain towards various compounds may have influenced the several defence mechanisms to act against the synergistic action of the chemical mixtures.