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Understanding the molecular basis of repeated evolution improves our ability to predict evolution across the tree of life. Only since the last decade has high‐throughput sequencing enabled comparative genome scans to thoroughly examine the repeatability of genetic changes driving repeated phenotypic evolution. The Asian corn borer (ACB), Ostrinia furnacalis (Guenée), and the European corn borer (ECB), Ostrinia nubilalis (Hübner), are two closely related moths displaying repeatable phenological adaptation to a wide range of climates on two separate continents, largely manifesting as changes in the timing of diapause induction and termination across latitude. Candidate genes underlying diapause variation in North American ECB have been previously identified. Here, we sampled seven ACB populations across 23 degrees of latitude in China to elucidate the genetic basis of diapause variation and evolutionary mechanisms driving parallel clinal responses in the two species. Using pooled whole‐genome sequencing (Pool‐seq) data, population genomic analyses revealed hundreds of single nucleotide polymorphisms (SNP) whose allele frequencies covaried with mean diapause phenotypes along the cline. Genes involved in circadian rhythm were over‐represented among candidate genes with strong signatures of spatially varying selection. Only one of two circadian clock genes associated with diapause evolution in ECB showed evidence of reuse in ACB (period [per]), but per alleles were not shared between species nor with their outgroup, implicating independent mutational paths. Nonetheless, evidence of adaptive introgression was discovered at putative diapause loci located elsewhere in the genome, suggesting that de novo mutations and introgression might both underlie the repeated phenological evolution. [ABSTRACT FROM AUTHOR]