Divergent natural selection on different host plants may be a crucial factor in promoting the remarkable diversity of phytophagous insects, and might occur in any geographical context. Because the intensity and consequences of divergent selection on different hosts can vary depending on the degree of gene flow between conspecific insect populations, elucidating the geographical context and degree of host specificity in the incipient phase of differential host use is indispensable to understanding the diversification process in phytophagous insects. Henosepilachna diekei Jadwiszczak & Węgrzynowicz (Coleoptera: Coccinellidae) is a tropical ladybird beetle occurring mainly on two host species from different plant families, Asteraceae and Lamiaceae. We investigated the geographical distribution of H. diekei across Java, Indonesia, in relation to the availability of the two hosts, and examined the host specificity of beetles in the laboratory. We also investigated genetic relationships among local populations of beetles using mitochondrial NADH dehydrogenase subunit 2 gene sequences. Geographic variation in host use by H. diekei was largely determined by skewed geographical distributions of the hosts, although there was a synergistic effect with extremely divergent host specificity by the beetles. The molecular analyses suggested that genetic differentiation among the beetle populations has occurred and has been maintained by the effects of both geographical distance and divergent host specificity. The geographical distribution of H. diekei populations differing in host specificity suggests that geographical distance, local host-plant availability, and divergent host specificity contribute synergistically to promote the genetic differentiation and subsequent diversification of phytophagous insects on different hosts.