Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) is a Neotropical moth that has diverged into corn, Zea mays L., and rice, Oryza sativa L., host strains because these plants are their most frequently used hosts. The corn strain also has been found in cotton, Gossypium hirsutum L., and sorghum, Sorghum bicolor (L.) Moench, and the rice strain in small grasses and pasture grasses. Studies of the reproductive isolation between these two strains have provided ambiguous results from populations in the United States. In Colombia, we tested pre- and postzygotic isolation in these strains. Both strains showed postzygotic isolation for several life-history traits, including number of egg masses, number of larvae, number of females, pupal developmental time, female and male longevity, and female and male pupal weight. We observed a reduction of the number of hybrid females and a reduction in fertility in hybrids in S. frugiperda. These results suggest the possibility of Haldane's rule. Heterosis in the F1(2) and F2(1) generations was observed for number of larvae and adult longevity. This line presented a high standard deviation, suggesting instability in this cross. A possible effect of the X chromosome may explain the reduction in viability and sterility in F1 hybrids of host strains of S. frugiperda. No temporal isolation was observed between the corn and rice strains. Differences in longevity between corn and rice strains might be another form of temporal isolation between these strains, because differences in adulthood time might reduce the encounters between them and thus hybridization.