Compatibility and interaction between the entomopathogenic bacteria Bacillus thuringiensis (Berliner) and some synthetic insecticides used to control cotton leafworm, Spodoptera littoralis (Boisd.)

As a selective biological agent, Bacillus thuringiensis var. kurstaki (Berliner) (Bt) has been widely used to control Spodoptera littoralis (Boisd.). The objective of this study was to evaluate the compatibility between a local isolate B. thuringiensis and some recommended insecticides at the recommended field rate/concentration (RC), half and 1/4 RC in culture medium. Results showed a significant difference in the number of Bt colonies formed following direct exposure to the tested insecticides (chloropyrofosmethyl, lambda-cyhalothrin, methomyl and flufenoxuron). RC, half and 1/4 RC of Chloropyrofos-methyl as well as RC of lambda cyhalothrin inhibited the growth of Bt by 100%. In contrast, emamectin benzoate at all concentrations used and methomyl at (half & 1/4 RC) had no effect on the number of Bt colonies when compared to control. The comparative efficacy of tested compounds against 2nd larval instar of S. littoralis revealed that emamectin benzoate was the most effective compound (with LC50 = 0.0503 ppm) compared to the other compounds, while methomyl was the least toxic compound (with LC50 = 7.42 ppm) after 48h from treatment. Interaction bioassay showed potentiation effect of emamectin benzoate at zero and 24 h, while lambda-cyhalothrin and flufenoxuron exhibited additive effect. In contrast, antagonistic effect was observed with chloropyrofos-methyl and methomyl treatments at zero time, while 24h after treating the larvae with Bt, both exhibited potentiation and additive effect, respectively. The findings of the present study suggested that, application of the tested insecticides after 24h from larval exposure to Bt were more effective than when combined with Bt at zero time or individual application.