Propylea japonica (Coleoptera: Coccinellidae) is a natural enemy insect with a wide range of predation in Chinese mainland and is commonly used in pest management.  However, its genetic pattern (i.e., genetic variation, genetic structure, and historical population dynamics) is still unclear, impeding the development of biological control of insect pests.  Population genetic research has the potential to optimize strategies at different stages of the biological control processes.  This study used 23 nuclear microsatellite sites and mitochondrial COI genes to investigate the population genetics of Propylea japonica based on 462 specimens collected from 30 sampling sites in China.  The microsatellite dataset showed a moderate level of genetic diversity, but the mitochondrial genes showed a high level of genetic diversity.  Populations from the Yellow River basin were more genetically diverse than those in the Yangtze River basin.  Propylea japonica has not yet formed a significant genealogical structure in China, but there was a population structure signal to some extent, which may be caused by frequent gene flow between populations.  The species has experienced population expansion after a bottleneck, potentially thanks to the tri-trophic plant–insect–natural enemy relationship.  Knowledge of population genetics is of importance in using predators to control pests.  Our study complements existing knowledge of an important natural predator in agroecosystems through estimating its genetic diversity and population differentiation and speculating about historical dynamics.

With the increased cultivation of Bt crops in China, Apolygus lucorum and other mirid bugs have emerged as important agricultural pests because they are insensitive to the Bt proteins.  In addition, the reduction of pesticide applications after planting Bt crops also increases the severity of mirid bug outbreaks.  Peristenus spretus is a parasitoid of mirid nymphs, but its sensitivity to Bt proteins is not known.  In the current study, we developed a dietary exposure assay to assess the effects of Bt proteins (Cry1Ab, Cry1Ac, Cry1F, Cry2Aa, and Cry2Ab) on P. spretus adults using a diet consisting of a 10% honey solution with or without Bt proteins at 400 µg g^–1 diet.  The results showed that the survival and reproduction of P. spretus adults were reduced by the cysteine protease inhibitor E-64 (a positive control) but were not affected by any of the five Bt proteins.  The activities of digestive, detoxifying, and antioxidant enzymes in P. spretus were also unaffected by diets containing the Cry proteins, but they were significantly affected by the diet containing E-64.  We then developed a tri-trophic bioassay to determine the effects of the five Bt proteins on P. spretus larvae and pupae.  In this assay, A. lucorum nymphs fed an artificial diet containing Cry proteins were used as the hosts for P. spretus.  The results of the tri-trophic assay indicated that neither the pupation rate nor the eclosion rate of the P. spretus parasitoids were significantly affected by the presence of high concentrations of Bt proteins in the parasitized A. lucorum nymphs.  The overall results indicate that these two assays can be used to evaluate the toxicity of insecticidal compounds to P. spretus and that the tested Cry proteins are not toxic to P. spretus.