Studies On The Invasion By Alien Bemisia Tabaci In Zhejiang And Comparison Of Biological Characteristics Between Biotypes Of The Whitefly

Zhejiang is a major transfer center for flowers and seedlings in China, and the invasive B biotype and the indigenous genotypes of Bemisia (abaci have been detected in this area. In order to determine the distribution of the different genotypes of B. tabaci through the years, and to clarify the process of spread of the B biotype and its displacement of indigenous genotypes in Zhejiang, from 2004 to 2008, a series of field sampling has been conducted on major host plants in 19 representative locations in Zhejiang. Along with the field sampling, this project also investigated the reproductive compatibility between the different biotypes found in Zhejiang, compared the host plant suitability and insecticide susceptibility between the B and ZHJ2 biotypes, and conducted competitive experiment on squash between the B and ZHJ2. These studies were conducted to clarify the systematic and phylogenetic relationships between different genotypes found in Zhejiang, and to examine the potential of the B biotype to displace the indigenous ZHJ2 biotype and some underlying mechanisms. The results are summarized as follows:From 2004 to 2008, field investigation was conducted in 19 locations which differ in terms of frequency of transport activities in Zhejiang, 11652 whitefly individuals collected from 154 plots were analyzed by RAPD PCR and mtCO1 sequencing to determine their genotypes. The results revealed the presence of at least 5 genotypes of B. tabaci in Zhejiang, and each genotype has its own distribution. In the locations with intensive transport activities in coastal areas, the percentage of B biotype increased rapidly from 2004 to 2006, and in the parallel with the invasion of the B biotype and its displacement of indigenous ZHJ1 genotype, there were significant increases in female ratio in the B biotype and decreases in female ratio in the ZHJ1 genotype. The Q biotype was detected in Zhejinag in 2005 for the first time, and since then its population has become a predominant in most coastal districts and Jinhua basin.Crossing experiments showed complete incompatibility in reproduction between the B and ZHJ2 biotypes, and very low levels of compatibility between the B and Q biotypes and between the ZHJ1 and ZHJ2 biotypes. Inter-biotype crosses produced significantly lower number of progeny than the intra-biotype crosses, indicating that there must be some mating interactions between the different biotypes, which gave a negative impact on egg-laying activity of female adults.The B biotype performed much better than the ZHJ2 biotype on squash, tomato and tobacco. The B and ZHJ2 biotypes preformed similarly on cotton and sweet potato. The ZHJ2 biotype performed better than the B biotype on kidney bean, and ZHJ2 also performed slightly better than B on pepper.In the mixed cohorts that were initiated with 13% B and 87% ZHJ2, the B biotype had totally displaced ZHJ2 by the 3rd generation. In the mixed cohorts, during the process of competition and displacement, the population increased rapidly, the female ratio of B increased while that of ZHJ2 decreased.After being treated by 12.5 ppm imidacloprid in egg, nymph or adult stage, the mortality of the B biotype in each stage was significant lower than that of the ZHJ2 biotype. Imidacloprid showed low ovicidal effects, but was highly toxic to the nymphs and adults.For the first time, this study conducted a series of field sampling in multiple locations over several years and demonstrated that the B and Q biotypes first invade areas with high frequency of transport through human movement of plant materials, and then rapidly spread into surrounding areas and displace indigenous genotypes, and in parallel with displacement significant changes in sex ratio occurred in the mixed populations. Crossing experiments demonstrated that in the B. tabaci species complex there are many genetic groups that are completely or nearly completely isolated in reproduction. Apart from behavioural mechanisms, difference in host plant suitability and insecticide susceptibility between whitefly biotypes may also play important roles in the displacement of indigenous whiteflies by alien whiteflies in Zhejiang.