Studies On Competition And Mating Behavioural Interactions Between The B Biotype And The Non-B ZHJ-2 Of Bemisia Tabaci (Homoptera: Aleyrodidae) In Zhejiang, China

The B biotype of the whitefly Bemisia tabaci invaded into China in mid to late 1990s and outbreaks of this pest have occurred in some areas. The B biotype of B. tabaci has caused severe losses to crops, and there has been evidence to indicate that it has been replacing the native non-B biotypes of whitefly. Zang et al. (2005b) have studied the competitive displacement between the B biotype and a non-B ZHJ-1 of B. tabaci collected from Zhejiang. In order to further understand the competitive potential of the B biotype and the mechanisms of its displacement of native non-B biotypes, we studied the competition and mating behavioural interactions between the B biotype and another non-B biotype (ZHJ-2) of the whitefly collected from Zhejiang. We investigated the numerical changes of each of the two biotypes in their mix populations on cotton in the laboratory, with or without the intervention of an insecticide. We also observed the mating interactions between the two biotypes as an effort to examine the behavioural mechanisms for the competition and displacement. The results are summarized as follows:(1) Competition between the B biotype and the non-B biotype ZHJ-2 on cotton.When the two biotypes started their coexistence on cotton with 87% of ZHJ-2 and 13% of B biotype without the intervention of an insecticide, the trend of increase in proportion of females was similar to that of the ZHJ-2 living on cotton alone. The female: male ratio was 1.2 after 25 days, increased gradually with time and reached the maximum of about 2.0. When the mixed population was sprayed with imidacloprid at 102 days after initiation, the female: male ratio increased quickly from 1.6 to 3.0 following the insecticide application, but soon decreased to about 2.0. Numerical estimates of the single and mixed populations indicated that the ZHJ-2 population increased rapidly on cotton, and the trend of population increase was similar to that of the mixed population without insecticide intervention. Following the spray of imidacloprid, the mixed population decreased quickly at first, and thenincreased gradually. The result of RAPD-PCR indicated that the proportion of the B biotype in the mixed population without insecticide intervention increased slightly during some periods of time but did not show the trend of displacing the ZHJ-2 population. In contrast, following the intervention of imidacloprid, the proportion of the B biotype increased from 20% to 50%, continued to increase thereafter, and reached 95% in another 125 days, and thus showed a trend of ZHJ-2 being replaced by the B biotype.(2) Reproductive interference and behavioural mechanisms. Data of continuous video recording showed that when males and females of the two biotypes were placed together, they exhibited courtship behaviour, but no copulation occurred. Both the B biotype and ZHJ-2 could mate more than once. In various inter- or intra-biotype treatments, they all had their first mating within 12h of emergence. The average number of copulation events in the first 72h after emergence was 3.5 and 2.8 for the B biotype and ZHJ-2 respectively when one male and one female were placed together, and the female: male ratios of the offspring produced by the B biotype and ZHJ-2 were 1.2:1 and 1.6:1 respectively. When a pair (a male and a female) of B biotype were supplemented with one or three males of the same biotype, the timing of first mating was unaffected by the addition of one male, but occurred 8 h earlier with addition of three males, and the average numbers of copulation events increased to 6.6 and 5.5 respectively. When a pair of B biotype were supplemented with one or three males of ZHJ-2, their first mating occurred 4-8 h earlier, and the average number of copulation events increased to 6.3 and 4.8 respectively. For the ZHJ-2, when a pair were supplemented with one or three males of the same biotype, the first mating occurred 5-10 h earlier, and the average number of copulation events increased marginally to 4.1 and 4.8 respectively. However, when a pair of ZHJ-2 was supplemented with one or three males of the B biotype, the average number of copulation events declined to 2.7 and 1.8 respectively, although their first mating occurred earlier. When a pair of B biotype were supplemented with one male of B biotype or one or three males of ZHJ-2, the percentage of females in the progenyincreased from 55% to 75°/ck 69.2% and 65.5% respectively. For the ZHJ-2 population, when one pair of adults were supplemented with one male of ZHJ-2, the percentage of females in the progeny increased from 61.9% to 70.4. However, the percentage of females in the progeny was unaffected by the addition of one or three males of the B biotype.The major new findings from this study include:(1) In some host plant conditions, the B biotype has the potential capacity to displace the native non-B biotype ZHJ-2, but such a displacement can occur only when the proportion of B biotype reaches over 50%.(2) The differences in insecticide susceptibility between the B biotype and non-B biotypes of B. tabaci may offer some advantage for the B biotype to increase its proportion, and consequently promote its displacement of non-B biotypes.(3) The mating interactions between the two biotypes favours the copulation and insemination of the B biotype, resulting in an increase of proportion of females in the progeny. On the contrary, the mating interactions between the two biotypes produce disadvantages to ZHJ-2, but the female: male ratio in its progeny is unaffected. Therefore, this asymmertric competition between the two biotypes may be an important factor in the relatively higher potential of competition of B biotype.(4) Comparison and integration of the competitive interference between B biotype and ZHJ-2 population (this study) and that between B biotype and ZHJ-1 population (Zang et ah, 2005b) indicate that the competition and displacement between the B biotype and various non-B biotypes share certain features but differ in others. Detailed examinations of these common features and variations between different pairs of the B biotype and non-B biotypes may provide a valuable approach to unravel the invasion biology of the B biotype of B. tabaci.