The ovicidal action of insecticides and insect growth regulator/insecticide mixtures on the eggs of various ages of susceptible and diflubenzuron‐resistant strains of Spodoptera littoralis boisd

Abstract The ovicidal action of four insecticides and three insect growth regulators, on eggs of various ages of the susceptible (S) strain of Spodoptera littoralis, was investigated. The results revealed that diflubenzuron was the most toxic compound tested on 0‐1‐day‐old eggs, followed by triprene and then methoprene; chlorpyrifos came next in its toxicity, followed by cypermethrin then fenvalerate; methomyl was the least effective. With the progress of embryonic development, 1‐2‐day‐old eggs showed slight tolerance to the action of the pesticides and juvenoids, but apparently resisted the action of diflubenzuron. Eggs 2‐3 days old showed increased tolerance to the insecticides and diflubenzuron and, to a lesser extent, to the action of the two juvenoids. A total of 21 insecticide or insect growth regulator‐insecticide mixtures, based on their toxicity equivalents (LD25 + LD25), were applied jointly on 0‐1‐day‐old eggs of the S strain. The mixtures which produced high synergism on this stage could be arranged, according to their decreasing order of potentiation, as follows: fenvalerate/diflubenzuron, cypermethrin/triprene, methomyl/methoprene, cypermethrin/methoprene, fenvalerate/ methoprene, cypermethrin/methomyl, cypermethrin/diflubenzuron, fenvalerate/triprene, diflubenzuron/methoprene and methomyl/triprene. The mixtures that produced synergistic effects on 0‐1‐day‐old eggs were also tested against the 1‐2 and 2‐3‐day‐oldeggs of the S strain. Theresults indicated that themixtures fenvalerate/diflubenzuron, and cypermethrin with either triprene or diflubenzuron, when tested on 1‐2‐day‐old eggs, produced levels of synergism nearly similar to those achieved on newly deposited eggs. On the other hand, the mixtures cypermethrin/methomyl, and fenvalerate with methoprene or triprene, showed comparatively lower levels of synergism. However, the synergistic action of the mixture cypermethrin/methoprene, previously detected on newly deposited eggs, became an additive effect on 1‐2‐day‐old eggs. Only the mixtures fenvalerate/diflubenzuron, cypermethrin/triprene and fenvalerate/methoprene showed synergism on 2‐3‐day‐old eggs. The mixtures which showed synergistic effects on 0‐1‐day‐old eggs of the S strain were further tested on eggs of various ages of the diflubenzuron‐resistant (Rd) strain. As regards 0‐1‐day‐old eggs, only the combinations cypermethrin/triprene, methomyl/methoprene and fenvalerate/methoprene produced additive effects. The mixture cypermethrin/diflubenzuron was still synergistic against newly deposited eggs of the Rd strain, in a manner similar to that indicated with the S strain. Apparent levels of antagonism were shown to the other insecticide‐juvenoid mixtures as well as to the diflubenzuron/methoprene combination. The combinations cypermethrin/methomyl and fenvalerate/diflubenzuron had some effect on the freshly deposited eggs of the Rd strain but produced lower levels of potentiation. With the increase in age of the eggs, the mixture cypermethrin/diflubenzuron showed a lower level of synergism on the 1‐2 day‐old eggs than was observed in younger eggs. The additive effects, previously detected with some of the mixtures on newly deposited eggs became an antagonistic effect for the 1‐2‐day‐old eggs. The eggs at this stage were more able to resist thejoint action of the other mixtures tested. Moreover, with further increase in the age of the eggs, the combination cypermethrin/diflubenzuron produced a lower level of potentiation on 2‐3 day‐old eggs, followed by fenvalerate/diflubenzuron at an even lower level, while the other mixtures showed various levels of antagonism.