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We report on laboratory hybridization between the indigenous Chrysoperla nipponensis and its close relative, C. carnea, which has been imported since 2001 from Germany to Japan as a biological control agent. Interspecific hybrids were obtained, and fully viable and fertile F1 and F2 generations were produced. Crosses between C. carnea females and C. nipponensis males showed 41.3% fertility, but the reciprocal cross showed only 9.5% fertility. Despite the low fertility of interspecific crosses, most F1 hybrids were fertile and were successfully backcrossed to both parental species. However, F1 males from C. carnea females ⊗ C. nipponensis males showed low fertility (zero except for 20% in one ease) when crossed with any females (F1 or backcross), In the one combination resulting in 20% fertility, an extremely long preoviposition period was observed, caused presumably by a mating delay. The absence of change in hatchability, adult emergence rate, total developmental period, or deviation from the 50:50 sex ratio from parents to the F2 generation of hybrids suggests that there is no genetic incompatibility between C. carnea and C. nipponensis resulting from hybrid breakdown. Because the courtship songs of the introduced C. carnea and the indigenous C. nipponensis are quite different, interspecific mating between the two close relatives may not occur under natural conditions. It will be necessary to monitor the establishment of the introduced C. carnea and its possible hybrids with the indigenous C. nipponensis in the wild to minimize any irreversible ecological risks, such as loss of genetic identity. [ABSTRACT FROM AUTHOR]