| Due to the wide and intensive use of chemical insecticides in fields, insect pests have developed serious resistances to most varieties of insecticides, resulting in unsatisfied efficacies and even failures in pest control. Insect sex pheromone, as an alternative method to chemical insecticides, has many advantages such as high sensitivity, species-specificity and safety to environment, and therefore has been considered to be one of the prosperous green pest control methods. Obviously, thoroughly understanding the sex pheromone communication system between male and female insects would be very important not only for the clarification of the mechanisms of sex communication but also for the development of more efficient pest control method targeting on sex pheromone communication system. Here in the present study, some less-studied aspects of sex pheromone communication system in Spodoptera exigua and S. litura were explored, which included chemical identification of male sex pheromone, electrophysiological and behavioral responses of females to the conspecific female sex pheromone, and diel rhythm of electrophysiological response of males to female sex pheromone. The main results are as follows:1. Evidence for the presence of male sex pheromone in S. exigua and S. lituraThe copulation behaviors of S. exigua and S. litura were investigated by direct observation. For the two moth species, when male and female were placed together in a container, the copulation behavior of both sexes could be divided into four sequential periods:precalling period, calling period, precopulation period and copulation period. However, when male and female placed separately in different container, the females presented the same four periods, but males only presented the first two periods with the third and fourth periods hardly observed, indicating that the presence of female sex pheromone was crucial for males to display the hairpencils. Hairpencil and antennal excision assays were carried out to confirm the effect of the hairpencils on the mating success. Males with antennal excision resulted in almost no mating success; while females with no antennae caused a reduction in mating success rate by 37% in S. exigua and 28% in S. litura. In the case of males with hairpencil excision, the mating rate was decreased by 30% in S. exigua and 27% in S. litura. Taken together the results of antennal excision and hairpencil excision assays, it was suggested that the male sex pheromone was released from the hairpencil, and had a significant influence on mating success between female and male moths.2. Chemical identification of compounds produced by male hairpencil glands from S. exigua and S. lituraElectroantennogram (EAG) was conducted to investigate the effects of male hairpencil extracts on EAG responses of male and female S. exigua and S. litura. Hexane, Methylenechloride and methanol were used to extract the hairpencils, but only the extract by methanol could elicit a significant EAG response in both male and female moths, and a cross-response was found between two moth species for both hairpencils. The further GC-EAD assay led a finding of an active compound but the chemical identification of this compound failed by GC-MS possibly due to the heat instability and strong polarity of the compound. Moreover, hexane extract of hairpencils from male S. exigua were analyzed by GC-MS, and four chemical components were identified as palmitic(C16:0), linoleic(C18:2), oleic(C18:1), and stearic acid(C18:0). Hexane extract of hairpencils from male S. litura was considered to contain the same four acids based on comparing of gas chromatograms from the both moth species. Whether these four organic acids acted as male sex pheromone components or not needed to be further confirmed.3. Quantitative analysis of compounds produced by male hairpencil glands from S. exigua and S. lituraGC quantitative analysis of the hairpencil extracts with hexane from three old moths showed a similar titers and relative proportions of four components between the two moth species. The titers of palmitic, linoleic, oleic, and stearic acid were 27.80,24.58,15.40, and 5.10 ng/FM with a relative proportion of 38.14,33.73,21.13 and 7.00%, respectively, in S. exigua. While in S. litura, the corresponding titers were 24.36,20.02,15.87, and 6.02 ng/FM, and the relative proportion were 36.75,30.21,23.95%,9.08%, respectively. The dynamics measurements of extracts from different day old moths showed a sharp increase in the titers of the four acids from 0-day-old moth to two-day-old moth, and a stabilization of the titers was achieved since two-day-old moth on.4. Electrophysiological and behavioral responses of both sexes of S. exigua and S. litura to the conspecific female sex pheromoneElectroantennogram (EAG) recordings showed that female S. exigua and S. litura were indeed capable of perceiving their own sex pheromones (two single components and their mixture), and displayed a similar dose-response relation pattern to that of respective males, although intensities of female responses were much less than those of corresponding males. Furthermore, the female S. exigua calling behavior was apparently influenced by presence of the female sex pheromone. The proportions of calling females in the peak calling period were significantly reduced and the calling peak time postponed by 2.5h, but the calling duration was prolonged by 1.5h into 2h of photophase, compared with controls. However, the pheromone titers in the female moth glands were the same between treatment and the control, implicating a reduced pheromone biosynthesis in the glands of treated moths. In addition, olfactometer experiment exhibited no obvious tendency or escape behavior response of females to sex pheromone stimuli. Such modification in calling behavior in S. exigua was speculated to decrease the competition among female individuals and subsequently to obtain more chances to mate with males.5. Daily changes of EAG responses to the conspecific female sex pheromone in S. exigua and S. litura, and relation between EAG response and expression level of PBP in male S. lituraEAG response to conspecific female sex pheromone component by female and male S. litura and male S. exigua indicated no obvious diel rhythms in EAG responses. Males retained similar sensitivity between the scotophase and photophase. To explore the mechanisms of such high EAG sensitivity in photophase, the transcription levels of PBP genes in male S. litura antennae were measured by Real-time quantitive PCR in one scotophase time point and three photophase time points. The results showed a significant reduction from scotophase to photophase in the amounts of SlitPBPl and SlitPBP2 mRNA. As PBPs was thought to transport the sex pheromone molecules to the receptors localized on the dentrite membrane of olfactory neuron, the PBP expression level should positively correlate to the EAG responses. Therefore, such incongruity between the EAG sensitivity and PBP expression level might imply a more complicated situation regarding the PBP functions and sex pheromone transportations. |
PDF Full Text Request