Whitefly invasions in China
Since 2003, we have been conducting field surveys of B. tabaci in Zhejiang and throughout China, to monitor the spread of invasive whiteflies as well as changes of distribution of various whitefly cryptic species. The spread of the invasive B and Q whiteflies has been rapid, accompanied by rapid changes of patterns of distribution among the various whitefly species. By 2011, the invasive B and Q whiteflies were widespread throughout China, and the indigenous species were found only in the more remote mountainous areas. In many regions, the Q whitefly has been displacing the earlier invader B whitefly (Liu et al. 2007; Hu et al. 2011). We have been continuing with the field surveys every year in Zhejiang and 2-3 years throughout China.
Figure: Distribution of the most frequently encountered invasive B. tabaci B and Q whiteflies (MEAM1 and MED) in each province in China in 2003 and 2009/10 (Hu et al. 2011).
Competitive displacement between whiteflies
Laboratory population experiments demonstrate that the invasive B and Q whiteflies have the capacity to displace indigenous whiteflies very rapidly (Liu et al. 2007; Luan et al. 2012; Wang et al. 2012). The trends and velocity of displacement between whitefly species are affected by host plants, insecticide application, and the initial relative proportion between species (Luan et al. 2012; Sun et al. 2013). An integrated analysis of laboratory population experiments and field sampling indicate that insecticide application has played a major role in shifting the species competitive interaction effects in favour of the Q whitefly in the field (Sun et al. 2013). This is because field populations of Q whitefly have lower susceptibility than those of the B whitefly to nearly all commonly used insecticides including imidacloprid.
Figure: Changes of mean relative percentage of MEAM1 (B) and MED (Q), expressed as MEAM1/( MEAM1+MED), in mixed cohorts of the two cryptic species as affected by insecticide application. Five treatments were conducted on cotton plants with different concentrations of imidacloprid applied in the soil and/or different initial relative abundance between the two species. Error bars indicate standard errors. Treatments I. B alone; II, Q alone, III. B+Q, no insecticide; IV, B+Q, 12.5 mg L-1 imidacloprid; V; B+Q, 25.0 mg L-1 imidacloprid (Sun et al. 2013).
Behavioural interactions and Competitive displacement
Displacement between whiteflies has often been observed to be associated with differential changes of sex ratio in the interacting species. A simple but sophisticated video system was developed to observe and record continuously whitefly activities and movements on plant leaves for day and night (Ruan et al. 2007). Detailed behavioural observations show that mating behavioural interactions lead to increased or reduced frequency of copulation and female progeny production depending the on the species (Liu et al. 2007; Luan et al. 2012, 2013; Wang et al. 2012). Using simulation modeling, we linked our behavioural observations with population exclusion experiments to show that asymmetric reproductive interference between species play a key role in species exclusion (Crowder et al. 2010; Wang et al. 2012).
Figure: Equipment for observing and recording mating behaviour of Bemisia tabaci on plants. Whitefly adults were placed on the undersurface of a plant leaf enclosed by a ventilated clip-cage, which was firmly attached to a metal stand with the help of a small magnet. The front opening of the clip-cage was covered with a thin, clear plastic sheet. The video camera was placed on a tripod 4-5 cm away from cage (8-9 cm away from the leaf surface) so that the position of it can be adjusted to obtain a clear focus of the adults on the leaf (Ruan et al. 2007).
Figure: Observed and simulated data (with four models) of B “biotype” (%) over time in population cage experiments with B and ZHJ1 populations from China (Crowder et al. 2010).