Conclusion

As in the case of the mosquitos on the Mediterranean coast, many instances of insecticide resistance in insects are due to a single allele that is partially or fully dominant over the allele that determines susceptibility. In this free course, Mosquito resistance to insecticides, you have learned that these resistance alleles increase in frequency very rapidly when insecticide is applied because susceptible genotypes suffer very high mortality. However, in the absence of insecticides, resistant genotypes are 5% to 10% less fit than susceptible ones and so decline in frequency. Resistance to insecticides therefore illustrates a trade-off – traits that are advantageous in one environment can have effects that are disadvantageous in other environments.