| The citrus whitefly, Dialeurodes citri(Ashmead) (Homoptera: Aleyrodidae), is a polyphagous citrus pest. The adults and larvals of citrus whitefly suck the tender leaves, make leaves chlorisis, defoliation, inhibit growing of the plants and fruits. And citrus whitefly secretes honeydew, causing the sooty mould fungus which develops on the honeydew and causes damage to the tree and its fruits. In recent years, as a result of abuse of chemical pesticide and change of orchard ecological condition, citrus whitefly leads serious damages to many citrus orchards in China. Therefore, the control of citrus whitefly becomes an important issue which needs to be resolved in agircutural industry. This situation indicates that adopting attracticide and reducing pesticide usage is a major research content. The population density of D. citri in 14 cultivars belonged to 7 groups of Citrus was surveyed. And behavioral responses of D. citri to those cultivars of Citrus were studied using Y-tube olfactometer. The volatiles of three cultivars were qualitatively and quantitatively analysed using solid phase micro extraction(SPME) and gas chromatography-mass spectrometry (GC-MS). In order to probe effect of plant volatile to D. citri host location, the relationship between D. citri and host plants was studied from behavioral responses and component analysis. And sequence analysis of mtDNA COI gene and molecular phylogeny of different geographical population of D. citri were studied as well. The main results are as follows:1. The field pest density of nymphs and eggs of D. citri in different Citrus cultivars The field population density of overwintering nymphs, nymphs and eggs of D. citri in 14 cultivars of 7 groups of Citrus young leaves was investigated. The result showed that there was a significant differences among the selectivities to those cultivars. Nanfeng Orange is the most attractive cultivar, while HB pomelo is the least one.2. Behavioral responses of D. citri to the leaves of Citrus The behavioral responses showed that Nanfeng Orange is the most attractive cultivar. A to Eureka lemon and Qianyang seedless were the second and the third, while HB pomelo is the least one. The attractive order of these 14 cultivars was Nanfeng Orange> A to Eureka lemon> Qianyang> London late navel orange> Jiangxi seedless> Dream umbilical> Early Gold Sweet Orange> Cara Cara Navel Orange> Newhall navel orange> Egan 2> Bright 13> Owari> Guoqing No.1> HB pomelo.3. Component analysis of host plant volatiles The leaves volatiles of Nanfeng orange, Early Gold Sweet Orange and Bright 13 were qualitatively and quantitatively analyzed using solid phase micro extraction(SPME) and gas chromatography-mass spectrometry (GC-MS).37 compounds were identified of which the contents exceed 1%. And most of them were terpenoid.9 compounds existed in three cultivars:myrcene, a- phellandrene, P-phellandrene, ocimene, y-terpinene, terpinolene,β-elemene, caryophyllene, a-farnesene. The proportion of caryophyllene was 14.06% and y-terpinene was 13.35% in Nanfeng orange which were higher than other cultivars. P-phellandrene was the richest ingredient in Bright 13 accounting for 34.20% of the total. The largest content of Early Gold Sweet Orange was ocimene accounting for 19.14% of the total.4. Behavioural responses of D. citri to the volatiles component of Citrus leaves (-)-a-Copaene showed attraction to D. citri at the dosages of 10-2-10-8g/ml(P<0.05), and the attraction is very significant at 10-2,10-5,10-6,10-7,10-8g/ml(P<0.01). Myrcene is attractive at 10-6g/ml(P<0.01). Component A showed attraction to D. citri at the dosages of 10-4-10"8g/ml(P<0.01), while it exhibited repellent action at higher dosages. D. citri showed a significant preference for component B at the dosages of 10-3-10-7g/ml(P<0.01), and a repellence at 10-1g/ml(P<0.05). Linalool,3-Carene, Tetradecane and Hexadecane showed attraction to D. citri at the dosages of 10-4,10-8g/ml(P<0.01),10-6g/ml(P<0.01), 10-4g/ml(P<0.01) and 10-4g/ml(P<0.05) respectively. Linalool,3-Carene, Thymol and Hexadecane exhibited repellent action at 10-1,10-2g/ml(P<0.01),10-2g/ml(P<0.05), 10-2-10-8g/ml(P<0.01) and 10-4g/ml(P<0.05) respectively. And component A and B showed strongest attraction to D. citri at the dosages of 10-8g/ml and 10-4g/ml respectively. Both of Nanfeng orange and Early gold sweet orange volatiles analogues could significantly attract D. citri adults, and the selectivities of them were 73% at 10-4g/ml and 63% at 10-5g/ml respectively.5. Genetic differentiation of different geographical population of D. citri The mtDNA COI gene of the 24 D. citri populations from different regions were collected and sequenced. Evolutionary distances among different populations were 0-0.028.9 polymorphic were detected in the 259bp mtDNA COI genes. The molecular phylogenetic tree indicated that there were genetic differentiation at a certain extent, but not obvious.
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