Monocrotophos and Profenofos: Two Organophosphates with a Different Mechanism of Action in Resistant Races of Spodoptera littoralis1

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Abstract
An Egyptian cotton leafworm, Spodoptera littoralis (Boisduval), sampled in 5 provinces of the Nile Delta showed uniform resistance (R) towards monocrotophos. The same populations were uniformly sensitive (S) towards profenofos, a new organophosphate insecticide. Against monocrotophos, all developmental stages of S. littoralis were resistant and differences in R-S factors depended either on the mode of application or the stage of development. In both laboratory and field strains, monocrotophos was synergized by inhibitors of microsomal oxidation. However, no synergist enhanced the sensitivity of the R-strain to the extent of reaching the S-level. The remaining “residual” resistance of 7× is probably not based on oxidative metabolism. Profenofos is not synergized in the laboratory R-strain. Its low R-S factor of 6×, therefore, cannot be due to oxidative metabolism. Rather, the residual resistance may be explained by an acetylcholinesterase which demonstrated insensitivity to inhibition by OP compounds. S/R ratios of the bimolecular rate constants (k1) are of the same order of magnitude as R-S factors of toxicity data for profenofos or the synergistic ratio monocrotophos/monocrotophos + synergist in the R-strain.

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