Effects Of Cadmium Transfer Along Soil And Broad Bean On Aphid And Its Natural Predator,Harmonia Axyridis

Ecosystems are contaminated with heavy metals due to frequent human activities.Plants are the route through which herbivores are exposed to heavy metals,while herbivores,such as herbivorous insects,are the intermediate mediators that transfer heavy metals to organisms at higher levels in the food chain.Compared with insecticides,predatory ladybugs and other natural enemy insects play an important ecological role in pest control,which can effectively reduce the economic losses of crops with pests and diseases.Feeding on heavy metal-contaminated prey may negatively affect the growth,development and reproduction of natural enemy insects,thereby reducing the effectiveness of pest control.Therefore,the pollution of agricultural ecosystems by heavy metals has become a serious issue of global concern.However,less research and attention have been paid to natural enemy insects through the food chain.Taking Vicia faba L.-Megoura crassicauda-Harmonia axyridis as the research model,the growth of V.faba was treated with different concentrations of cadmium solutions（0,3.125,6.25,12.5,25 and 50 mg/kg）,the transfer of cadmium（Cd）in various trophic levels was detected,and then the effects of Cd on the biological characters and related gene expression of broad bean,aphid and ladybug were studied.The relevant results not only provide help for phytoremediation of heavy metal pollution,but also provide a theoretical basis for further exploring the molecular mechanism of cadmium homeostasis in plants and insects under cadmium stress at the cellular and subcellular levels.In addition,it will contribute to a better understanding of the impact that accumulated cadmium may have on herbivorous insect,natural enemy insect and species at higher levels of the food chain,which lays the groundwork for protecting predatory insects to control pests and maintain ecosystem stability.The main research results are as follows:1.Bioaccumulation of cadmium in soil and different organisms and its effect on broad beanFirst,the survey results showed that only the content of chromium（Cr）in the soil of the three sampling sites was higher than the background value of the soil in Zhejiang Province.The contents of lead（Pb）and zinc（Zn）were close to the background value,and the content of Cd was lower than the background value.However,some excess rates of Pb and Cr in broad bean roots and stems were up to100%,and the excess rate of Cd in broad bean roots was 67%.Secondly,it was detected that the Cd content in broad beans after Cd treatment was significantly higher than that in the control group（0 mg/kg）,and it was proportional to the treatment concentration.After Cd treatment,the Cd contents in soil,roots,stems and leaves of broad bean were significantly higher than those in the control group,and the content of Cd in roots was higher than stems/leaves.The Cd content in aphids and H.axyridis in each treatment group was also significantly higher than that in the control group,and the Cd content was related to the concentration of heavy metal treatment.Meanwhile,for the 12.5,25 and 50 mg/kg treatment groups,the Cd content in the soil,roots and stems of broad bean infected with aphids was significantly lower than that in the uninfected treatment groups.However,for the 6.25 mg/kg treatment group,the Cd content in aphid-infected soil and broad bean leaves was significantly higher than that in uninfected.It indicated that the feeding of herbivorous insects can counteract the accumulation of heavy metals in plants.Moreover,Cd stress had no significant effect on the germination rate and seedling height of broad bean2.Effects of cadmium transfer along the system on developmental regulation and trehalose metabolism of Megoura crassicaudaRT-qPCR results showed that after Cd treatment,the expression levels of ecdysone receptor（Ec R）gene in the first,fourth and fifth generations of aphids were significantly up-regulated.Second,after Cd treatment,the first and fifth generations of adult aphid insulin receptor（In R）gene expression levels were significantly increased,while the second and third generation In R expression levels were decreased in most groups.After Cd treatment,the expression levels of insulin receptor substrate（IRS）gene were up-or down-regulated in five generations of aphids.In the first-generation,compared with the control group,the glycogen content and two trehalase activities of aphids treated with Cd decreased,while the trehalose content increased.There were no significant changes in carbohydrate content and trehalase activity in the fourth and fifth generations of aphids.3.Effects of cadmium transfer along the system on the growth,development and predation ability of Harmonia axyridisAfter feeding on Cd-contaminated aphids,the development time of each instar of H.axyridis was longer than that of the control group,especially from the 2^nd to the 4^thinstar.The weight of pupae in 6.25 mg/kg group and the weight of pupae and adults in25 mg/kg group were significantly lower than those in the control group.In addition,the survival rate of 2^nd-4^th instar larvae decreased in the Cd-treated group compared with the control group.At the pupal and adult stages,the highest survival rate was showed in the 12.5 mg/kg treatment group,which were 80%and 77.50%respectively,followed by the 3.125 and 25 mg/kg groups and the control group.After Cd treatment,the effect on pupation rate and emergence rate of ladybug was not obvious,but the abnormality rate of adults increased,mainly including difficulty in molting and deformed wings.The predation of 4^th instar larvae was higher than that of adults.The predation amount of the 4^th instar larvae in the Cd treatment group was significantly higher than that in the control group.However,when the aphid density was 60 and100,the predation amount of the 50 mg/kg group was significantly lower than that of the control group.4.Effects of cadmium transfer along the system on the fecundity of aphids and Harmonia axyridisThe number of offspring produced by the first to fifth generations of female aphids under Cd pollution was significantly lower than that of the control group.Secondly,the preoviposition period of female H.axyridis in the Cd treatment group was longer than that in the control group,and the 25 mg/kg treatment group（10±1.41 d）showed a significant effect.Although there was no significant change in the number of eggs laid by females in each group,the hatching rate（<72%）was significantly reduced.In addition,the change trend of female vitellogenin genes Vg1and Vg2 was similar,and the expression levels of Vg1 and Vg2 were significantly decreased in the Cd-treated group on the 4^th day after eclosion.On the 7^th day after eclosion,the expression levels of Vg1 and Vg2 were significantly increased in the6.25 and 25 mg/kg groups,while the expression levels of Vg1 and Vg2 in the 12.5 and50 mg/kg groups were significantly decreased.On the 10^th day after eclosion,the expression level of the vitellogenin receptor gene Vg R in the 3.125 mg/kg group was significantly decreased.At high doses of Cd,the ovaries of females on 2^nd day after eclosion were retarded and decreased in size compared with controls.In conclusion,the results showed that Cd could be transferred and accumulated along the soil-broad bean-aphid-ladybug system,and the Cd content was related to the treatment concentration.Secondly,the phenotypic changes of aphids were not obvious under Cd pollution,but it had an impact on the expression level of some genes and trehalose metabolism in aphids.Under Cd pollution,the survival rate of H.axyridis decreased,the abnormal phenotype appeared,and the predation was affected.Finally,under Cd contamination,the number of offspring produced by aphids was reduced,and female fecundity of H.axyridis was also affected,especially in terms of gene expression and quality of eggs laid.