| Encarsia sophia (Girault&Dodd) is the dominant parasitoid of Bemisia tabaci (Gennadius) MEAM1. This species is a solitary, autoparasitoid. Fertilized eggs are laid internally in whitefly nymphs and develop as primary parasitoids, producing the female progenies, whereas unfertilized eggs are laid in a previously parasitized whitefly nymphs, by their own or other Encarsia and Eretmocerus species, and develop as hyperparasitoids, producing the male progenies. As a result, the specific reproductive pattern has leaded the application of E. sophia in biological control becoming the focus of controversy. In this study, we focued on the reproduction characteristic of E. sophia. We investigated the reproduction feature of E. sophia after mating, as wll as the effect of host density and host ratio on offspring sex ratio of E. sophia by means of reproductive biology study and ovary dissection. Also, we observed the host handling behavior of mated females under different host density. The main results of this research are as follows:1. The reproduction characteristic of E. sophia after matingThrough ovary dissection, we found that the reproductive system of E. sophia consisted of a pair of ovaries, oviducts and external genitals. The average number of mature eggs in newly emerged mated females’ovary was3.3±0.3. Newly emerged mated female was transferred on a leaf disc laid in a petri dish. The leaf disc contained15primary hosts (third-instar B. tabaci nymphs) and15secondary hosts (late larvae-prepupal of E. sophia). The female was transferred on a new leaf disc every24h till dead or escaped. The average lifespan of E. sophia female was26.6±2.0days, and the female could generated the same number of female and male offspring in its lifetime. The mated female could oviposit on secondary hosts and has the ability to lay unfertilized eggs in its lifetime. Furthermore, E. sophia female had sperm storage capacity. However, the number and/or the activity of sperm stored in females’ spermatheca after first mating was not available enough for E. sophia to fertilize eggs generating female offspring during its lifetime. The female could still ovposite in primary host at an average of15.0±1.2days after once mating. The ability of producing male offspring and sperm storage is an important prerequisite for E. sophia female to regulate the sex ratio of their progeny. Besides, E. sophia female had stable host-feeding capacity throughout her lifespan, she could feed on an average of2.8±0.2third-instar whitefly nymphs per day. The mean number of primary host fed by E. sophia female was68.3±5.6in its lifetime. Besides the parasitic ability, host-feeding ability is another important aspect of its biological control value.2. Regulation of offspring sex ratio in E. SophiaEncarsia sophia female was capable of adjusting its offspring sex ratio according to different host density and host ratio. When the host density was low, sex ratio reflected the relative abundance of primary and secondary hosts. The offspring sex ratio trended to1:1with the increasing of host density. Moreover, we measured the number of eggs laid by E. sophia mated female during the first5day after emergence. In most instances, under fixed host density, with the increasing proportion of primary hosts, more fertilized eggs and less unfertilized eggs would be laid by E. sophia female, and the total number of eggs laid increased. When the ratio of primary host and secondary host was4:1, with the increasing host density, female laid more unfertilized eggs while the amount of fetilized eggs was not changed. The mean number of host-feeding of E. sophia female during the first5day after emergence tended to increase with the increasing host density when the ratio of primary host and secondary host was4:1, but the number of whitefly nymphs fed by E. sophia female kept stabilization among days. However, E. sophia female tended to increase oviposition with increasing female age when measured by5days after emrgence.3. The effect of host density on host handling behavior of E. sophia femaleThe behavior of E. sophia female in patch included5types which were searching, host examination (primary host and secondary host), egg laying (fertilized eggs developing to female offspring and unfertilized eggs developing to male offspring), host-feeding, preening and resting. The process of egg laying could be divided into4steps:1) external host examine by antennae,2) internal host examine using ovipositor,3) oviposit,4) draw out ovipositor and leave. There were no significant differences between the external host examination time of primary host and secondary host. After external host examination, female would insert its ovipositor into the host for internal host examination. The time used in ovipositor insertion for rejecting laying fertilized egg was longer than that for laying unfertilized one. However, if the female decide to accept the host for oviposition, the ovipositor insertion time for laying fertilized egg was shorter than that for laying unfertilized ones. The process of host-feeding included5steps:1) external host examine using antennae,2) puncture host by ovipositor,3) preening,4) stand still,5) feed on the host. When E. sophia female entered the patch where the ratio of primary host and secondary host was4:1and the host density was30,50and70hosts/dish, the number of hosts treated, accepted as well as the probability of host acceptance by female within1h were not affected by the host density. Furthermore, it is easier for female to find secondary hosts than primary hosts in a patch, but the number and probability of primary hosts accepted was always higher than that for secondary hosts. When E. sophia female entered the patch in1h, with the increasing host density, it spent less time in searching and more time in host-feeding. Meanwhile, the time spend in other behavior didn’t change. In most cases, E. sophia female would take more time to producing female progeny than male one under the same host density and host ratio. |
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