The present paper reviews current studies on various aspects of reproductive isolation in a series of closely related phytophagous ladybird beetles called the Epilachna vigintioctomaculata complex, which is composed of two groups and four species: group A comprising E. vigintioctomaculata, and group B comprising one northerly distributed species, E. pustulosa, and two southerly distributed species, E. niponica and E. yasutomii.

Being broadly sympatric from middle through northern Japan, the two groups are reproductively isolated by a combination of several factors, each of which functions as an incomplete barrier to gene flow: difference of host plants, weak sexual isolation, and low hatching rates of eggs produced by interspecific matings. Conspecific sperm precedence further lessens the probability of producing hybrids in mixed populations. By contrast, the two sympatric species of group B, E. niponica and E. yasutomii, are reproductively isolated from each other solely by fidelity to different host plants. However, there are no effective barriers to gene exchange between either of these two species and the northerly distributed E. pustulosa.

These situations have relevance to various controversial issues in evolutionary biology, covering the mode of speciation in host specific insects, role of postinsemination barriers to fertilization, reinforcement of reproductive isolation, and treatment of allopatric populations in speciation studies. The E. vigintioctomaculata complex thus offers a rare opportunity to extend our understanding of the nature of animal species and their origin.