The understanding of fall armyworm, Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), physiology has frequently suffered from discrepancies between different studies. One potential source of error is the assumption that laboratory colonies are sufficiently representative of wild populations that their biological parameters can be generalized. This is unlikely to be valid if the phenotype in question exhibits extensive genetic variation in the natural population, in which case laboratory lines can only be expected to contain a subset of the relevant genotypes. Here, I investigated whether this is a concern for experiments measuring larval developmental rate, a parameter frequently used to assess the relative resistance of different plant lines to fall armyworm herbivory. To estimate the genetic variation in this phenotype a simple selection experiment was performed to determine whether significantly different developmental rates could be isolated from inbred laboratory colonies representing the fall armyworm subgroup (rice-strain) that is the primary pest of pasture grasses. The results indicate that a strong genetic component influences larval development. Therefore, the average larval duration of the test population for a given treatment will depend on its genotypic composition that could vary significantly with different colonies. The implications of these findings on designing and evaluating feeding studies for fall armyworm are discussed.