ABSTRACT
Natural variation is best defined as a numerical difference in response that is detected each time a bioassay is repeated with one genetic strain, either within a single generation or more than one generation. In contrast, the hypothesis of parallelism was rejected only when C. occidentalis was treated with carbaryl. These results may indicate the incremental contribution of each sibling group to the natural variation among cohorts within a generation. Biotic and abiotic factors affect arthropod development, with the result that a range of different developmental stages is present at any given time. A primary source of variation in response at the population level is differential responses of these life stages. Results of repeated bioassays suggest the extent to which natural variation within strains of arthropod species reared even under highly controlled conditions can complicate detection of other differences related to pesticide resistance, varietal differences of host plants, or product quality.
