Bemisia tabaci (Gennadius) is an important agricultural pest worldwide. The pest is a species complex composed of numerous biotypes, among which biotypes B and Q are the 2 most invasive and widely distributed. Our previous study found that the ratio of the biotype Q has been increasing and displacement of biotypes B by Q has been occurring on cotton and eggplant in Shandong Province of China during the past several years. To determine whether biotype Q has been increasing on other hosts and possible displacement of biotypes has been occurring in the province as a whole, we further surveyed B. tabaci biotypes B and Q on cultivated and wild host species near cotton or eggplant fields in 7 locations of Shandong Province during 2005–2008 with cleavage amplified polymorphic sequence (CAPS) of the mtCOI (mitochondrial cytochrome oxidase subunit I) marker. This research showed biotype Q has been increasing on all kinds of host plants and the displacement of biotypes B by Q has been occurring in the province as a whole. The displacement mechanism should be further researched and such knowledge might guide the application of the insecticides or adjustment of the crops to effectively control the pest.
Bemisia tabaci (Gennadius) is an important agricultural pest worldwide. It damages crops through direct feeding and vectors many plant viruses. The pest has been considered as a species complex that includes many genetic groups that are morphologically indistinguishable. Some are labeled as biotypes or host races because of differences in host range, geographical distribution, transmission ability of plant virus, and other biological characteristics. Biotype B is believed to originate from the Middle East-Northern Africa and have spread into many countries or regions over the past 2 decades. Biotype Q may have originated in the Mediterranean countries and circumstantial data shows that this biotype has been introduced into many non-Mediterranean countries or regions during the past several years (Chu et al. 2005; Ueda 2006; Brown et al. 2007).
Bemesia tabaci outbreaks in the mid-1990s in both Southern China and Northern China and subsequent research showed that the whitefly occurring in most of China was biotype B (Wu et al. 2002). In 2003, biotype Q was found in Kunming of Yunnan Province and then subsequently found in Beijing and Henan (Chu et al. 2006). Many populations of B. tabaci in Shandong Province, one of the most important agricultural provinces of China, have proved to be biotype B, but biotype Q was discovered in 2006 by use of mitochondrial cytochrome oxidase subunit I (mtCOI) sequence (Chu et al. 2007). It is important to monitor the spreading and the density of the biotype Q because it possesses greater resistance to many insecticides than biotype B in many countries (Dennehy et al. 2005; Horowitz et al. 2005). The biotypes of B. tabaci on cotton and eggplant in 6 locations within Shandong Province was determined with mtCOI sequences and biotype B-specific primers (Chu et al. 2010), which showed that the ratio of the biotype Q has been increasing and displacement of biotypes B by Q has been occurring during the past 4 years (2005–2008) on these two crops. To determine whether biotype Q has been increasing on other hosts and possible displacement of biotypes has been occurring in the province as a whole, we further surveyed B. tabaci biotypes on cultivated and wild host species near cotton or eggplant fields in 7 locations of Shandong Province during 2005–2008.
MATERIALS AND METHODS
Bemisia tabaci biotype was determined with the cleaved amplified polymorphic sequences (CAPS) of mtCOI amplified with new primers (C1-J-2195/R-BQ-2819). Adult whiteflies were collected from different plants including crops and weeds in 7 locations, DeZhou, ZiBo, Shou-Guang, JiNan, LiaoCheng, LinYi and ZaoZhuang in Shandong Province during 2005–2008 (Table 1). The adults were placed in tubes with 95% ethanol and stored at -20°C. Individual adults were ground and DNA was extracted. The mtCOI fragment (about 620bp) was first cleaved by the restriction endonucleases VspI (Khasdan et al. 2005) and then the uncut fragment was cleaved by the restriction endonucleases StuI (Ueda 2006). All of the mtCOI that could be cut by VspI should be biotype Q and mtCOI cut by StuI should be B.
RESULTS AND DISCUSSION
Our results shown in Fig. 1 and Table 1 revealed the following: In 2005, the biotype of B. tabaci populations in JiNan, LiaoCheng, ZaoZhuang were determined and biotype Q was only found in LiaoCheng in very low proportion (1.4%). In 2006, biotype Q was found in DeZhou (38.0%), ZiBo (38.9%), ShouGuang (62.6%), LiaoCheng (37.1%) and was absent in JiNan, LinYi and ZaoZhuang. By 2007, biotype Q dominated in most locations, DeZhou (71.8%), ZiBo (92.7%), ShouGuang (96.0%), LiaoCheng (92.4%), and biotype Q also was found in JiNan (61.7%) and LinYi (5.6%). In 2008, Q biotype comprised 100.0%, 93.1%, 100.0%, 93.1%, 97.7%, 89.1% and 100.0% of the B. tabaci population in DeZhou, ZiBo, ShouGuang, JiNan, LiaoCheng, LinYi and ZaoZhuang, respectively.
The present results are consistent with previous research on cotton and eggplant (Chu et al. 2010). These results suggest that the changes of B. tabaci biotypes occurred not only on the cotton and eggplant but also on the other plants including crops and weeds in Shandong Province during the past several years.
The displacement mechanism of biotypes Q and B remains uncertain, though the increase of biotype Q in many countries may be due to application of insecticides because biotype Q possesses greater resistance to insecticides than biotype B, but ecological and economic factors should be also considered. For example, the host plants that biotype Q preferred also might mediate the competition of B and Q. Multiple introduction of biotype Q from the other provinces or regions through human activities or natural sources should not be neglected.
Overall, our present result showed that the biotypes of B. tabaci changed greatly and B. tabaci biotype Q has been increasing on all kinds of hosts during the past several years. The displacement of biotypes B by Q has been occurring in the province as a whole. The speed of the displacement of biotypes B and Q was fast and essentially a cryptic invasion (Geller et al. 1997) because the biotypes are morphologically indistinguishable.The displacement mechanism should be further researched and such knowledge might guide the application of the insecticides or adjustment of the crops to effectively control the pest. Differentiation of B. tabaci biotypes is important, and molecular markers are important discrimination tools.
This work was funded by the Outstanding Youth Science Foundation of Shandong Province (JQ200811), National Basic Research and Development Program (2009CB119200), National Natural Science Foundation of China (30771410), Special Scientific Research Fund for Commonweal Trade of China (200803005) and Key Projects in the National Science & Technology Pillar Program in the Eleventh Five-year Plan Period (2006BAD08A18).