| The insect pest usually occurs simultaneously with disease on plant under field conditions.The majority of plant viruses are transmitted to their host plants by insect vectors,thus viruses may evolve mechanisms to alter the performance of their vectors in favor of viruses spread.In addition,viruses can also indirectly affect the interaction between insects and their natural enemies.Consequently,natural enemy-vector-virus interactions should be investigated to evaluate the continuing spread of insect-transmitted viruses.The green peach aphid or the peach-potato aphid,Myzus persicae(Sulzer),is the main insect pest on tobacco and widely distributed in China.M.persicae has been mainly regulated with chemical insecticides.However,the application of chemical pesticides in a large quantity for a long time will not only enhance the insecticide resistance of M.persicae,but also lead to environmental pollution and threaten human health.Potato virus Y disease on tobacco is a devastating disease caused by potato virus Y(PVY).The most efficient vector of PVY is M.persicae,which transmits the virus in a non-persistent manner during brief probes of the plant epidermis.Currently,the control of PVY disease mainly relied on reducing the population quantity and limiting the spread area of M.persicae.Although the prevention and control of M.persicae with chemical pesticides can reduce the transmission of PVY,but the results are not satisfactory.Besides,people pay more and more attentions to pesticide residues.Therefore,other effective and environmentally friendly methods to control M.persicae and PVY disease should be developed.Aphidius gifuensis(Ashmead)is a primary endoparasitoid against M.persicae in agricultural fields.The damage caused by the virus transmitted by aphids in the feeding process is much more harmful than its own sap-sucking from host.Thus,the common sense of a biological control agent in a pest/vector model system will generally be assumed as a way to control the target vector in order to reduce vector-borne disease in the field.So far,although there are many researches on the application of A.gifuensis to control M.persicae,but few documents had ever focused on whether A.gifuensis could reduce the diffusion of PVY through the control of aphid.Besides researches on the effect of control,in order to comprehensively evaluate the persistence of A.gifuensis in prevention of M.persicae and PVY,the impact of PVY-infected tobacco plants on M.persicae and A.gifuensis should be taken into account to define whether it is feasible to affect the control effect of A.gifuensis by regulating the behaviors of M.persicae.Accordingly,this dissertation,using the electrical penetration graph(EPG)and RT-qPCR technology,researches on the feeding behavior of M.persicae parasitized by A.gifuensis,the quantity of carrying PVY and the rate of PVY transmission and further verify the control effect A.gifuensis to M.persicae and PVY.Besides,the influence of PVY-infected tobacco plants on the characteristic of feeding behavior of M.persicae,biocompatibility and A.gifuensis adaptability is also investigated,having made certain that PVY-infected tobacco plants will affect A.gifuensis by adjusting M.persicae's probing and growing development.The main results of this dissertation are as follows: 1 M.persicae's behavioral characteristic and biocompatibility of probing on PVY-infected tobacco plantsThe influence of PVY-infected tobacco on M.persicae's feeding behavior and life table parameters was studied using EPG technology and age-specific life table method.The results showed that there were nine EPG waveforms of aphids feeding on tobacco: non-penetration waveform(waveform np),pathway waveforms(waveform A,B,C,pd,and F),xylem sap ingestion(waveform G),saliva secretion on phloem(waveform E1),and phloem sap ingestion(waveform E2).The total duration of np waveform and count probes of M.persicae on PVY-infected tobacco plants were less than that of healthy plants.The duration of first probe of aphids on PVY-infected tobacco was 62.02 min,significantly longer than that of the control(27.37 min).During the phloem phase,the count and total duration of M.persicae salivating(E1 waveform)on infected tobacco plants were less than that on healthy plants;while the duration of passive ingestion(E2 waveform)was significantly longer than that of control.These results indicated that PVY-infected tobaccos could increase the fitness of M.persicae.The results of life table showed that nymph development time and mean generation time of M.persicae on PVY-infected tobacco plants were significantly shorter than that on healthy tobaccos,and the survival rate in nymphal stage was slightly higher than that on control tobaccos,indicating that PVY-infected tobacco is benefit for the development of M.persicae in nymphal stage.But after M.persicae developed to adult,the total preoviposition period(TPOP)of female M.persicae on infected tobaccos extended,and its life longevity shortened.Additionally,the fecundity,intrinsic rate of increase and net proliferation rate all were significantly lower than that of healthy plants.Further identification of nutritional components of PVY-infected tobaccos(inoculation 12 d)showed that,besides the contents of methionine,histidine and proline,the contents of other 14 kinds of amino acid in infected tobacco plants have significantly risen.The total amino acid contents and total sugar contents of PVY-infected tobaccos were also apparently higher than that of healthy tobaccos.In a word,it is clear that in the earlier stage of PVY infection,the increase of contents of amino acid and sugar in tobacco plants could increase the compatibility of the host with M.persicae.It is reckoned that the low proliferation rate and survival rate in adult stage may be caused by the deterioration of nutritional status of tobacco after infection.2 Effects of A.gifuensis on the feeding behavior and virus-carrying ability of M.persicae 2.1 Effects of A.gifuensis on the feeding behavior of M.persicaeThe feeding behavior of parasitized M.persicae was studied to investigate the direct effects of A.gifuensis on the feeding activities of aphids.A four-channel DC-EPG device was used to monitor the feeding behavior of M.Persicae on healthy tobacco plants.Before recording,apterous aphids were removed from tobaccos after parasitization by A.gifuensis on days 3,4,and 5.Unparasitized aphids were used as the control group.The results showed that parasitized aphids had different behavior than control aphids on several EPG variables related to pathway phase,xylem phase and phloem phase.Three and four days parasitized aphids increased the number of probes and short probes(C < 3 min)than that of control and aphids of 5 days parasitized,although these differences were statistically significant only on day 4.Independently of the day after the aphids have been parasitized,they had shorter duration of first probe than control aphids.On the xylem phase,the total duration and percentage of waveform G produced by parasitized aphids on day 5 were longer and higher,respectively,than those produced by the control group.This result demonstrates that parasitized aphids on day 5 may need to ingest a larger amount of xylem sap to maintain their water balance.A.gifuensis significantly affected the feeding activities of M.persicae in the feeding on phloem phase compared with the control.The contribution of time(percentage)of E1 to the phloem phase was also significantly lower in control group than in the parasitized aphids.The total duration of E2 waveforms in aphids parasitized on days 3,4,and 5 was 61.22 ± 15.13,79.53 ± 27.79,and 21.00 ± 11.97 min,respectively;these values were shorter than those of the control(114.64 ± 23.80 min).However,the duration time of M.persicae feeding from phloem was significantly reduced in parasitized aphids only on day 5.2.2 PVY carrying quantity of M.persicae having been parasitized by A.gifuensis for different periods.RT-qPCR technology was employed to detect the PVY carrying quantity of M.persicae parasitized by A.gifuensis in different periods.The results showed that the A.gifuensis could not significantly affect the PVY acquisition of M.persicae.Compared with the parasitized aphids,the relative quantity of PVY carrying of the control aphids after inoculation for 10 min was relatively high.The PVY carrying quantity of M.persicae parasitized by A.gifuensis on days 3 and 4 slightly reduced,and that of M.persicae parasitized for 5 days reached the minimum.Although,compared with the control group,the PVY carrying quantity of parasitized aphids after inoculation for 10 min relatively declined,but no significant difference was detected.3 Influence of parasitization of A.gifuensis on M.persicae's feeding behavior and virus transmission on PVY-infected tobacco plantsM.persicae parasitized by A.gifuensis on day 5 were taken as the research objects to detect 24 EPG indexes of parasitized M.persicae probing on PVY-infected and healthy tobacco plants.It was found that the total duration of np wave of aphids on PVY-infected tobaccos was shorter than that of the control group.In probing process,the number of F and duration of F waves on PVY-infected plants also obviously reduced.There was no difference in the pd waveform.The phloem parameters showed that the time of salivating(waveform E1)of parasitized M.persicae on infected tobacco plants decreased.However,the time of passive sucking juice(waveform E2)significantly extended.The percent transmission of PVY by the aphids parasitized on day 5 was 22.67%,which was lower than that in the control group(28.33%),but no significant difference was detected.To sum up,A.gifuensis can impede the feeding behavior of M.persicae,but unfortunately does not provide an immediate solution for disrupting the transmission of PVY,which further verified that the transmission of PVY was mainly related to pd wave.4 Influence of M.persicae probing on PVY-infected tobacco plants on A.gifuensis adaptabilityIn order to make clear whether PVY-infected tobacco plants can indirectly affect A.gifuensis by adjusting M.persicae's feeding behavior or growing development,the adaptability of A.gifuensis to M.persicaes on infected tobacco and healthy tobacco plants(control group)were evaluated respectively.Results showed that the parasitism rate of A.gifuensis to M.persicaes on PVY-infected tobacco plants was 33.67%,significantly lower than that of the control group(64.67%),but there was no difference in mummified M.persicae eclosion rate and the female-male ratio of progeny.The average survival time of adult A.gifuensis on infected tobacco plants was 1.48 d,shorter than that of the control(2.25 d).In addition,the body size of adult A.gifuensis on PVY-infected plants was also smaller than that on healthy plants.The results indicated that PVY infection had an adverse effect on A.gifuensis.5 Influence of parasitization of A.gifuensis on protective enzyme and detoxifying enzyme activity in M.persicaeThe activity of acetylcholine esterase(AChE),acid phosphatase(ACP),alkaline phosphatase(AKP),catalase(CAT),peroxidase(POD)and total superoxide dismutase(T-SOD)in the body of M.persicae parasitized by A.gifuensis were detected.The results showed that the activity of AChE in parasitized aphids was significantly lower than that of the control group at the time of 12 h and 24 h;the activity of ACP increased at 12 h and 24 h;The activity of AKP was slightly lower than that of the controls at 4 h,24 h and 48 h but was higher than the control group at 8 h and 12 h;the activity of CAT was significantly higher than that of the control group at the stages of 12 h and 24 h;POD activity slightly decreased at the time of 4 h,8 h and 48 h but significantly increased at 24 h;T-SOD activity did not significantly differ from that of the control group.The results demonstrated that in M.persicae,after parasitized by A.gifuensis,the activity of AchE was inhibited,while the activity of ACP,CAT and POD were activated in later stage of parasitism,and the activity of AKP and T-SOD had no significant differences between the infections and the controls.6 Study on field control effect of A.gifuensis on M.persicae and PVYThe control effect of A.gifuensis on wingless aphid,winged aphid and PVY in three districts in Chongqing were studied.Results showed that releasing A.gifuensis was effective for inhibiting the growth of M.persicae population,but the prevention and control effect before deposit apiary was relatively unsatisfactory.Ten days after releasing A.gifuensis,A.gifuensis in Fengjie and Qianjiang can control about 40% wingless aphid,while the control rate in Wushan was relatively higher,indicating that the control effect of A.gifuensis on M.persicae varied from place to place.The control effect of A.gifuensis on winged aphid was approximate to that on wingless aphid.The corrected control effect in the early stage of deposit apiary was unsatisfactory,but it became better on the whole with the passage of time,which reached up to 63.39%.In the three regions of Fengjie,Qianjiang and Wushan,the incidence of PVY in the deposit A.gifuensis area was lower than that in the control area,indicating that A.gifuensis can indirectly control the incidence of PVY.To sum up,A.gifuensis can impede the feeding behavior of M.persicae,which sheds light on the biological basis of A.gifuensis as a natural enemy.Additionally,releasing A.gifuensis can control the growth of M.persicae population and reduce the incidence of PVY in field.However,since PVY can reduce the adaptability and parasitism rate of A.gifuensis to and on M.persicae by controlling the development and proliferation of M.persicae after infecting tobacco plants,it is reckoned that PVY will reduce the effect of prevention and control of A.gifuensis after infecting tobacco plants.Therefore,to enhance the effect of control of A.gifuensis over M.persicae and viral disease,it is suggested to take extra controlling measures before releasing A.gifuensis to narrow the incidence range of virus as far as possible.Besides,this dissertation also shows that although A.gifuensis cannot prevent M.persicae from transmitting PVY by inhibiting its probing,it can reduce the viral transmission risk by preventing M.persicae from spreading.Consequently,our study lays the foundation for the prevention and control of aphids and virus diseases using the A.gifuensis. |
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