Studies On The Ecological Mechanism Of Displacement Of Bemisia Tabaci B Biotype By Q Biotype

Biological invasions endanger national security, ecological security, and economic security.Bemisia tabaci, which has been regarded as a species complex consisting of many biotypes, is aninvasive and destructive pest of numerous protected and field crops worldwide. Among the B.tabaci biotypes, the B biotype and Q biotype are two damaging and invasive crop pests. In themid-1990s, the B biotype was introduced into China and caused tremendous losses to agriculturalproduction and the national economy. After the Q biotype was first detected in Yunnan Provincein2003, it spread quickly into East China and North China. The invasion and spread of the Qbiotype was accompanied by outbreaks of a B. tabaci-transmitted virus called Tomato yellow leafcurl virus (TYLCV). In this dissertation, a systematic monitoring of B. tabaci on many crops inmore than20provinces in China was conducted. The results indicated that over the last10yearsthe Q biotype has been displacing the previously well-established B biotype as the predominantbiotype in China. Studies on the influence of biotic factors (host plant, plant virus, and symbioticbacteria) and an abiotic factor (insecticide application) on the interactions between B and Qbiotypes showed that insecticide application is the key ecological factor contributing to thedisplacement of the B biotype by the Q biotype. Controlled laboratory experiments wereconducted to compare the virus acquisition capability of the two biotypes and to investigate themode of viral transmission (horizontal and vertical). The results showed that the displacement ofthe B biotype by the Q biotype contributed to TYLCV epidemics in China. The research resultswill help guide the sustainable control of B. tabaci and TYLCV.The main research contents and conclusions are as follows:1. The distribution and population dynamics of B and Q biotypes in ChinaPCR primers specific for mtCO1of biotypes B and Q and gene sequencing were used in anextensive survey (covering19provinces in2009and27provinces in2011) to determine thebiotype status of B. tabaci in China. The results in2009showed that the Q biotype was dominantin44locations while B biotype was dominant in17locations and that the Q biotype was mainlydistributed in the Yangtze River and the eastern coastal areas. Among98populations examined in2011,57were pure Q biotype,12were pure B biotype, and29were mixtures of B and Q biotypes(with the Q biotype representing>50%of the individuals in20of the29); the Q biotype waspredominant across China except in southeast provinces. Overall, the results showed that over thelast10years, the main damaging biotype of B. tabaci in China has changed from the B biotype tothe Q biotype.2. The impact of host plant on the competitive displacement between B and Q biotypesTo explore the impact of host plant on the competitive displacement between B and Qbiotypes, equal numbers of B and Q biotypes were added to the same poinsettia, pepper, tomato,and cabbage plants, and the biotypes of individuals in the developing populations wereperiodically determined. The results showed that the Q biotype could displace the B biotype on poinsettia and pepper after8and2successive generations, respectively. In contrast, the B biotypecould displace the Q biotype on tomato and cabbage after5and4successive generations,respectively. Different host plants had different effects on the interactions between B and Qbiotypes, resulting in the differences in the speed and direction of displacement of one biotype bythe other.3. The impact of insecticide on the competitive displacement between B and Q biotypesTo explore the impact of insecticide on the competitive displacement between B and Qbiotypes, equal numbers of both B and Q biotypes were added to the same cotton and cucumberplants. The plants were either treated or not treated with an insecticide, and the biotypes ofindividuals in the developing populations were periodically determined. The results showed thatthe Q biotype displaced the B biotype on cotton after6successive generations without insecticidebut after only4successive generations with insecticide (Thiamethoxam). On cotton, the use ofinsecticide significantly increased the rate at which the Q biotype displaced the B biotype.Although the B biotype displaced the Q biotype on cucumber after9successive generationswithout insecticide, the Q biotype replaced the B biotype when cucumber was treated withinsecticide. The Q biotype replaced the B biotype after5successive generations with theinsecticide Movento or Thiamethoxam, and after4successive generations with the insecticideBifenthrin. On cucumber, application of insecticide reversed the impact of host plant on theinteractions between B and Q biotypes, resulting in the displacement of the B biotype by the Qbiotype.Overall, the use of insecticide enhanced displacement of the B biotype by the Q biotype inone of two ways depending on the host: if the untreated host plant was more suitable to the Qbiotype (as is cotton), an insecticide increased the rate at which the Q biotype displaced the Bbiotype; if the untreated host plant was more suitable to the B biotype (as is cucumber), aninsecticide reversed the direction of displacement. These results demonstrate that insecticideapplication plays a key role in the displacement of the B biotype by the Q biotype in the field.4. Diversity of symbionts in Bemisia tabaci and its impact on the interactions between Band Q biotypesSymbiotic bacteria can influence the growth, development, and reproduction of insect hosts.In this study, gene-specific PCR was used to screen for the presence of the primary symbiontPortiera and the six secondary symbionts (Hamiltonella, Arsenophonus, Cardinium, Wolbachia,Rickettsia, and Fritschea) in61field populations (17B and44Q biotypes) collected fromdifferent plant species and locations in China and in5laboratory populations that had similargenetic background but that were cultured on5different host plant species for5years. The resultsindicated that the diversity and infection frequency of secondary symbionts in B. tabaci wasinfluenced by B. tabaci biotype, B. tabaci sex, host plant, and geographical location. Nocorrelation was detected between the diversity and infection frequency of symbionts in B. tabaci and the displacement of the B biotype by the Q biotype.An additional experiment was conducted to investigate the infection densities of symbionts in3laboratory populations that had similar genetic background but that were cultured on3differenthost plant species for6years of the B biotype and of a Thiamethoxam-resistant and-susceptible Bbiotype population. Overall, the results revealed that host plant and insecticide affected theinfection densities of symbionts in B. tabaci.5. The impact of TYLCV on the competitive displacement between B and Q biotypesThe direct effect of TYLCV on the growth and development of B. tabaci was investigated bycomparing the performance of viruliferous and non-viruliferous B and Q biotypeson cotton (anon-host for TYLCV). The results showed that TYLCV significantly decreased the survival,fecundity, and adult body size of viruliferous individuals of the B biotype but not of the Q biotypefeeding on cotton.The indirect effect of TYLCV on the growth and development of B. tabaci was investigatedby comparing the performance of non-viruliferous B and Q biotypes on healthy andTYLCV-infected tomato plants. The results showed that TYLCV-infected tomato plants improvedthe performance of the Q biotype but decreased the performance of the B biotype with respect tothe fecundity, survival rate, development time, and female body size.The impact of TYLCV on the competitive displacement between B and Q biotypes wasfurther examined by adding equal numbers of both B and Q biotypes to the same healthy andTYLCV-infected tomato plants, respectively and periodically determining the biotypes ofindividuals in the developing populations. The results showed that the B biotype displaced the Qbiotype after5successive generations on healthy tomato plants but after6successive generationson TYLCV-infected tomato plants. Although TYLCV infection did not prevent the displacementof the Q biotype by the B biotype, it significantly delayed the rate of displacement. The overallresults indicate that the mutualistic relationship between TYLCV and the Q biotype contributes tothe displacement of the B biotype by the Q biotype in China.6. The displacement of B biotype by Q biotype contribute to the epidemics of TYLCV inChinaA nation-wide field survey of18provinces that included55B. tabaci populations (12B and43Q biotypes) and2750individuals was conducted in2009to investigate the infection frequencyof TYLCV. The results showed that the percentage of TYLCV-infected individuals wassignificantly lower among B biotype (4.2%) than Q biotype (24.4%) individuals.Virus acquisition experiments indicated that the relative viral gene expression was greater inthe Q biotype than in the B biotype after each of five acquisition access periods (AAPs) and thatthe B and Q biotypes required AAPs of48and12h, respectively, to reach their maximum viralloads.Transmission experiments demonstrated that TYLCV was not transmitted vertically, i.e., was not transmitted to the next generation by the B and Q biotypes. TYLCV was detected in eggs andcrawlers but was not detected in the pupa or adult progeny from viruliferous B or Q whiteflies.TYLCV could be transmitted horizontally by both B and Q biotypes, and the overall percentage ofviruliferous individuals that transmitted TYLCV was52.2%for the Q biotype but only10.5%forthe B biotype, indicating that horizontal transmission of TYLCV was greater with the Q biotype.The epidemics of TYLCV are related to the displacement the B biotype by the Q biotype and thegreater horizontal transmission of TYLCV by the Q biotype than the B biotype.In summary, in this dissertation, we found that the predominant biotype of B. tabaci haschanged during the past decade, the ecological mechanisms of displacement of B biotype by Qbiotype and the outbreaks of TYLCV in China were fist revealed.