Parasitic Preference Of Encarsia Formosa On The Sweetpotato Whitefly, Bemisia Tabaci

Encarsia f ormosa Gahan is the most widely used parasitiod on the control of whiteflies. The sweetpotato whitefly Bemisia tabaci B biotype(cryptic species) has been replaced by Q in most areas in China and B. tabaci Q become the prominent species up to now. B. tabaci Q has wider host range, stronger ability of transmitting virus, and higher resistance to insecticides than B. tabaci B, therefore the chemical control can not surpress the population of whiteflies effectively and permanently. Biological control is the one of the promising and potential methods for controlling B. tabaci populations sustainably. In this present study, we compared the parasitic parameters of E. formosa on the B. tabaci Q rearing on different host plants firstly. Then, we explored the olfactory response of parasitoid E. formosa to B. tabaci B and Q rearing on the healthy tomato by the olfactory response experiment; The olfactory response of E. formosa to the B. tabaci B and Q rearing on TYLCV- infected tomato were also investigated; Finally, we carried out the parasitic effects of E. formosa on the B. tabaci B and Q with the same densities in insect-rearing cages. The main results are as follows.1. Biological characteristics of E. formosa parasitizing B. tabaci Q on five host plantsIn order to clarify the parasitic characteristics of E. formosa parasitizing the sweetpotato whitefly, B. tabaci Q biotype on different plants, the parasitism and development of E. f ormosa on B. tabaci Q biotype feeding on 5 host plants were investigated at 26±1°C, relative humidity of 60~80%, and the photoperiod of L: D=16 h: 8 h using the clip-cage method. The results showed that significantly higher parasitism rate was observed on pepper and eggplant(28.49% and 26.58%) than tomato and poinsettia(17.62% and 16.36%), while parasitism rate was intermediate on cotton(21.25%). Parasitoid- induced mortality of whitef lies was proved to be consistent with parasitism rate and the orders from high to low were eggplant(40.41%) > pepper(37.63%) > cotton(33.82%) > tomato(26.52%) > poinsettia(18.49%). The developmental time of E. formosa on whiteflies feeding on pepper, tomato, cotton, eggplant and poinsettia were 14.16 d、14.53 d、16.54 d、16.34 d and 16.89 d, respectively. The developmental duration on pepper and tomato were significantly shorter than other three host plants. The longevity of E. f ormosa differed among the 5 host plants, and the longevity on cotton and poinsettia with 17.00 d and 9.60 d, respectively, were significantly longer than the other three plants. However, no differences were observed on emergence rate of E. formosa among the tested 5 host plants. It is concluded that host plants could affect the biological characteristics of E. formosa on B. tabaci Q biotype, and the parasitism were higher when B. tabaci Q biotype reared on the pepper and eggplant. The results will provide theoretical and scientific guidance for the biological control of sweetpotato whiteflies.2. Olfactory response of E. formosa to the B. tabaci B and QThe olfactory response of E. formosa to the different developemtal stages of B. tabaci B and Q on the healthy tomato plant were investigated using the Y-tube experiment. The results showed that E. formosa is more likely to choose the whitefly-infected tomato compared to the control(tomato alone). Between the B. tabaci B and Q-infected tomato treatments, E. formosa chose more for the combination of B whitefly-infected tomato than Q whitefly- infected tomato, especially at the age of the 2nd-instar nymph,、 which was signif icant. The results implied the host volatiles with the whiteflies is more helpful for the host orientation and the tomato with B. tabaci B have stronger induction for E. formosa than the tomato with B. tabaci Q.3.Effects of TYLCV on the olfactory response of E. formosa to B. tabaci B and QThe olfactory experiment of E. formosa to B. tabaci B and Q on TYLCV-infected tomato were investigated. The results showed that there was no signif icant choice preference between the combinitation of whitefly and TYLCV-infected tomato vs non-whitefly infected healthy tomato, suggesting that TYLCV may suppress the B. tabaci-induced volatiles released by tomato that is helpful for host location of parasitiod. As for the different developmental stages of B. tabaci B and Q, E. f ormosa preferred the combination of B. tabaci Q and TYLCV tomato than B on the viruliferous tomato when the whiteflies are eggs and the 1?3th instar nymphs, but there were no significant differences between them. In the meanwhile, when the B. tabaci was the 4th-instar nymph, E. formosa preferred Q whiteflies on the viruliferous tomato significantly more than B whiteflies. It is concluded that TYLCV- infected tomato influenced the choice of E. formosa on the whitefly nymph, which varied with the different developmental stages of B. tabaci.4. Parasitic effect of E. formosa on B. tabaci B and Q coexisting in cageThe parasitic selection of E. formosa for 3rd-instar nymph B. tabaci B and Q on healthy and TYLCV- infected tomato respectively were investigated using the insect-rearing cage. The results showed that on the healthy tomato, the parasitism rate of E. formosa on B. tabaci B was higher than B. tabaci Q and same as mortality of B. tabaci by E. f ormosa. On the TYLCV-infected tomato, the parasitism rate and mortality of E. formosa on B. tabaci Q both increased, however, no signif icant differences have been found between B. tabaci B and Q. The peference of E. formosa on B. tabaci changed from B to Q on the viruliferous tomato, which was consistent with the results from olfactory response experiment, suggesting that TYLCV had influences on the choice preference of E. formosa to B. tabaci B and Q.