Cold stress responses help insects to survive under low temperatures that would be lethal otherwise. This phenomenon might contribute to the invasion of some Bemisia tabaci cryptic species from subtropical areas to temperate regions. However, the molecular mechanisms regulating cold stress responses in whitefly are yet unclear. Mitogen-activated protein kinases (MAPKs) which including p38, ERK, and JNK, are well known for their roles in regulating metabolic responses to cold stress in many insects. In this study, we explored the possible roles of the MAPKs in response to low temperature stresses in the Mediterranean cryptic species (the Q-biotype) of the B. tabaci species complex. First, we cloned the p38 and ERK genes from the whitefly cDNA library. Next, we analyzed the activation of MAPKs during cold stress in the Mediterranean cryptic species by immuno-blotting. After cold stress, the level of phospho-p38 increased but no significant change was observed in the phosphorylation of ERK and JNK, thus suggesting that the p38 might be responsible for the defense response to low temperature stress. Furthermore, we demonstrated that: i) 3 min chilling at 0°C was sufficient for the activation of p38 MAPK pathway in this whitefly; and ii) the amount of phosphorylated p38 increased significantly in the first 20 min of chilling, reversed by 60 min, and then returned to the original level by 120 min. Taken together, our results suggest that the p38 pathway is important during response to low temperature stress in the Mediterranean cryptic species of the B. tabaci species complex.

The whitefly Bemisia tabaci has risen to international prominence since the 1980s due to the rapid spread around the globe by the two species B and Q within this species complex. The invasion of B has often been associated with the displacement of indigenous whiteflies. As the genetic structure of B. tabaci is diverse, more case studies of the competitive relationships between B and indigenous species of the whitefly species complex will help to understand further the mechanisms underlying the invasion of B. We examined the competitive interactions between B and ZHJ2, a widely distributed indigenous whitefly in Asia, on host plants with differential levels of suitability to the two species in the laboratory, and also tested the effect of insecticide application on the competitive relationships. Three species of plants were tested including cotton, a plant showing similar levels of suitability to both species, squash, a plant showing higher suitability to B than to ZHJ2, and kidney bean, a plant showing higher suitability to ZHJ2 than to B. In the case of no insecticide application, B displaced ZHJ2 on cotton, squash, and kidney bean by the 6th, 3rd and 10th generation, respectively. With the application of imidacloprid, the displacement of ZHJ2 by B on cotton occurred by the 5th generation. As the displacement progressed, the proportion of B females increased, and the proportion of ZHJ2 females decreased on cotton and squash. In contrast, on kidney bean the proportion of B females remained unchanged while that of ZHJ2 increased. These results show the strong capacity of the invasive B to displace ZHJ2, and indicate that host plants with differential levels of suitability to the two species may affect the speed but not the trend of displacement and insecticide application may accelerate the process of displacement.

The worldwide distribution and extensive genetic diversity of the whitefly, Bemisia tabaci, has long been recognized. However, the levels of separation within B. tabaci and the nomenclature of the various genetic groups have been a subject of debate. Recent phylogenetic analyses indicate that B. tabaci is a complex composed of 28 morphologically indistinguishable species. In this article, we first review the debate and difficulties associated with B. tabaci’s taxonomy and systematics, and argue for the need to apply the biological species concept in order to elucidate B. tabaci’s systematics. We summarize the accumulated genetic and behavioural data on reproductive incompatibilities evident amongst phylogenetic mtCOI groups of B. tabaci. Crossing studies have been conducted with 14 of the 28 putative species covering 54 reciprocal inter-species pairs, and observations on mating behaviour have been conducted for seven species pairs. Data from both crossing trials and behavioural observations indicate a consistent pattern of reproductive isolation among the putative species. We then discuss the technical and conceptual complexities associated with crossing experiments and behavioural observations designed to reveal reproductive incompatibility. Finally, we elaborate on a strategy for further clarifying the pattern of reproductive isolation between B. tabaci groups and propose future research directions on the systematics of this complex.