OP Resistance in Spodoptera littoralis: Inheritance, Larval and Imaginal Expression, and Consequences for Control1

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Abstract
Larval resistance (R) against monocrotophos in an R-strain of Spodoptera littoralis (Boisduval) is characterized by a 250X difference of LD50 compared to a sensitive strain (S). Toxicological experiments with larvae of reciprocal crosses between S and R were inconclusive as to the R character. Analysis of P, F1, and F2 generations of moths by monocrotophos injections demonstrated the operation of a sex-linked gene for resistance. Biochemical tests of AChE activity and resistance to inhibition by 5 ppm monocrotophos showed that the sexlinked gene in R did not control an insensitive AChE. The biochemical mechanism which it controls could not be identified as yet. The present and earlier results point to R in S. littoralis as being due mainly to the very efficient MFO system in larvae. An insensitive AChE, offering limited protection for the imago, may have evolved as a specific response of the adult stage to selection pressure occurring as a side effect of larval control in the field. A sexlinked R-gene as demonstrated here also may be specific for the imaginal phase. Moths and eggs seem to be less capable to muster such efficient protective mechanism as the MFO-system in larvae. Therefore, they are suggested to be the preferred targets for future control of important lepidopteran pests.