Resistance Monitoring,Resistance Risk Assessment And Control Techniques Of Bemisia Tabaci MED

The Whitefly Bemisia tabaci?Gennadius??Hemiptera:Aleyrodidae?is a species complex composed of at least 40 cryptic species.Damage from B.tabaci has occurred in various countries and regions,including Africa,the Middle East and the United States.The two most invasive members of the cryptic species complex posing the greatest to growers are Middle East-Asia Minor1?MEAM1?and Mediterranean?MED?.Bemisia tabaci not only causes sooty mould,mainly by piercing plants to suck their juice and honeydew,but also spreads plant viruses that damage crops,vegetables and ornamental flowers,resulting in economical loss.Currently,pesticide use is the main method for preventing B.tabaci infections.However,misuse of pesticide can lead to various problems,such as pesticide residues,environmental pollution,food safety issues and pesticide resistance development in B.tabaci.In order to determine the distribution and genetic diversity of different cryptic species of B.tabaci in China,we collected and analyzed a series of field sampling from six locations in Shandong,China.Furthermore,the resistance and biochemical mechanisms of B.tabaci to the insecticides were studied in the laboratory.In this experiment,we acquired resistance strains of B.tabaci MED to pymetrozine,as the experimental materials to study the risk of resistance,and to clear the cross-resistance and the biochemical mechanism of resistance.To resolve the problems in the control of B.tabaci in production,we turned to study a comprehensive method centred on physical controls.The main results were as follows:1.To reveal the distribution and infestation status of B.tabaci cryptic species,a systematic and comprehensive survey on its species composition was conducted in Shandong,China.We sampled collections of adult whiteflies from six locations in Shandong during2016²017.The results showed that the percentage of MED was 100%in four locations and was>90%in two locations?Weifang and Linyi?,where MEAM1 was found with low percentage?<5.7%?.The results indicate that MED has completely displaced MEAM1 in most regions of Shandong,China.2.The resistance of two sensitive populations?SS-B and SS-Q?to eight insecticides was evaluated with indoor bioassay.The results showed that the LC₅₀ values of the eight insecticides in SS-Q were higher than those in SS-B.Compared with the SS-B,the SS-Q had the highest resistance to nitenpyram and imidacloprid,with the resistance ratio of3.95-fold and 3.57-fold,respectively,the resistance to pymetrozine had the smallest difference?1.06-fold?.Under sensitive conditions,the B.tabaci MED has higher resistance than B.tabaci MEAM1.3.Insecticide resistance levels were determined in six field populations of B.tabaci collected in Shandong,China.Eight insecticide?avermectin,imidacloprid,thiamethoxam,nitenpyram,acetamiprid,chlorpyrifos,pyriproxyfen and pymetrozine?were included in this monitoring program.The results showed that six field populations were still sensitive to avermectin,all lower than 0.80 mg/L in terms of LC50 values.There is no significant difference to pymetrozine between the resistance and sensitive populations with the LC₅₀between 148.68 mg/L and 439.59 mg/L,and only Tai'an population has minor resistance to pymetrozine?3.27-fold?.The virulence of pyriproxyfen on eggs of different populations of B.tabaci were different.The Linyi,Qingdao and Jinan populations were sensitive to pyriproxyfen,the LC50 ranged from 15.28 mg/L²9.91 mg/L;Dezhou and Tai'an populations showed minor resistance?3.89 and 4.48-fold?,while Weifang population has showed resistance in low level?5.55-fold?.Various populations of insecticides have different levels of resistance,most of them have reached low to moderate level.All the populations showed minor or moderate resistance to imidacloprid?4.29 to 16.56-fold?;four populations showed low or moderate resistance to thiamethoxam?8.39 to 15.81-fold?;four populations showed resistance minor or low to acetamiprid?4.10 to 8.35-fold?;Weifang and Jinan populations showed low resistance to nitenpyram?5.87-fold and 6.79-fold?.To the chlorpyrifos,Dezhou and Weifang populations showed minor resistance?3.44-fold and 4.80-fold?,the Jinan population reached a low resistance level?6.74-fold?.Thus,avermectin and pymetrozine use in rotation should control B.tabaci and slow down the generation of insecticide resistance in the long term.4.Based on the field populations,the determination of the detoxifying enzymes?CarE,GSTs and MFO?activities were performed.The research showed that the GSTs and MFO in different populations of B.tabaci have enhanced with the increasing of population resistance level.However,the CarE activities of Dezhou,Jinan,Tai'an and Linyi populations have lower than sensitive population.The results showed that the MFO and GSTs was closely related of the population which have the moderate resistance level.5.Based on the leaf-dip bioassay,pymetrozine resistance in the susceptible SS-Q strain was continuously selected for 18 generations.In the first 10 generations,resistance developed very slowly and resulted in about a 2.16-fold increase in the resistance ratio.From 11th to18th generation,resistance reached 10.70-fold.The realized heritability?h²?of resistance to pymetrozine in MED was estimate to be 0.1158,and was assumed as 0.0579 in field population?a half value of indoor estimation?.The risk predicting of resistance to pymetrozine for field population showed that it takes 13²0 generations to achieve 10-fold resistance at the selection pressure of 70%⁹0%.Therefore,Bemisia tabaci have a risk of resistance to pymetrozine.6.The biochemical mechanisms of pymetrozine resistance in B.tabaci MED were studied by cross-resistance analysis and detoxifying enzyme activity test.The results showed that compared with the SS-Q,the selected R-F₁₈8 strain showed clear cross-resistance to thiamethoxam?10.43-fold?,acetamiprid?6.22-fold?,nitenpyram?4.64-fold?and imidacloprid?3.61-fold?.No cross-resistance was seen to pyriproxyfen,chlorpyrifos and avermectin.The individuals from resistant B.tabaci strains(R-F₁₁1 and R-F₁₈)have a higher level of tolerance to pymetrozine treatment,and this difference was reflected in the metabolic enzyme activity assays.For example,the MFO activities of the two resistant R-F₁₁1 and R-F₁₈ strains were 1.88and 2.77-fold,respectively,greater than that of the susceptible SS-Q strain.In contrast,both CarE and GSTs activities between three strains showed no significant difference.The results showed that MFO in B.tabaci to pymetrozine resistance played a leading role in the formation,and it may participate in the resistance formation.7.This study researched three different processes to effectively reduce intrusion by B.tabaci and control damage from tomato chlorosis virus?ToCV?,The treatments included the use of a regular solar greenhouse as a check?CK?;greenhouses I and II with 60-and80-mesh insect-proof nets,respectively,that were installed in both houses at the front and upper ventilations;and greenhouse III with the addition of not only the 80-mesh insect-proof nets as in greenhouse II but also a ventilating pipeline to the back wall.The effects of mesh size and back wall ventilation on the greenhouse temperature and humidity,the number of B.tabaci,and the level of ToCV infection were studied.The results indicate that the 80-mesh insect-proof netting reduced the number of B.tabaci by 98.4%while providing better ventilation than the 100-mesh netting;therefore,the 80-mesh size is recommended for field use.During 2014,the number of B.tabaci and the ToCV infection level were lower in greenhouse II than in the others,but the presence of netting elevated the internal temperature of the greenhouses;this temperature elevation was more apparent when using the 80-mesh netting.During 2015-2016,the temperature of the greenhouses with installed netting was obviously reduced by drilling holes through the back walls.From August 25 to October 29 in2015,the average temperature of greenhouse III between 10:00 and 16:00 hours was 26.55°C,which was 7.27°C lower than that of greenhouse II.From August 20 to October 29 in 2016,the average temperature of greenhouse III during the same time of day was 27.11°C,which was 5.63°C lower than that of greenhouse II.No significant difference existed in the average relative humidity between greenhouses II and III.Importantly,the number of B.tabaci and the ToCV infection rate were effectively controlled in greenhouse III,where the ToCV infection rate was as low as 6.67%.Thus,installing 80-mesh insect-proof netting at the front and upper ventilation areas as well as adding a ventilating pipeline to the back wall could effectively reduce the number of B.tabaci and prevent ToCV damage.This netting technique is recommended for implementation in solar greenhouses.