A Study Of Superparasitism Behavior In Meteorus Pulchricornis (Hymenoptera:Braconidae)

Parasitoid deposits a clutch of eggs (the clutch may consist of a single egg) into a host that has already been parasitized by a member of the same species or itself, a behavior called "superparasitism", and the ensuing larval competition ends in the death of all but one. Superparasitism was not only a focus of theoretical study in evolutionary ecology to understand its adaptation, but also an issue in the practice of biological control using parasitoids to control pests in agriculture. Superparasitism is a common phenomenon in solitary parasitoids, but so far the mechanism of superparasitism is still open to debate. Using Meteorus pulchricornis (Wesmael)(Hymenoptera:Braconidae) as model system, this study was conducted to address four questions:i) whether the parasitoid can discriminate between healthy and parasitized hosts; ii) whether the fitness costs of superparasitism on parasitids vary with host quality; iii) whether potential factors interact to affect superparasitism; iv) whether time-associated behavioral traits might be utilized to evaluate the propensity of superparasitism.The main results and conclusions are summarized as follows.1A non-choice test was performed to examine the relationship between residence time on host patches and extent of superparasitism as measured by number of eggs per host. The results showed that the residence time had a significant effect on the number of eggs laid in a host. When the residence time was from1to3hours, an average of eggs in a host was0.77,0.9, and2.5respectively, but when the residence time was8hours, an average of5.8eggs was laid in a host. The study suggested that M. pulchricornis have an increasing tendency toward superparasitism with decreasing availability of high-quality hosts.2In order to examine effects of host quality attributes on fitness consequences of superparasitism for development of parasitoid progeny. A single choice test was used to observe influences of host species, instars, and plant species respectively on the host suitability in superparasitism for the development of parasitoid progeny in a solitary endoparasitoid, M. pulchricornis. Superparasitism prolonged development time and increased mortality of parasitoid progeny, reduced body size and longevity of the ensuing adults, compared to single parasitism. But such fitness costs of superparasitism can vary with host species, instars, and plants on which the host larvae fed. The development time of parasitoid progeny was shorter and adult size smaller thanin superparasitized hosts of Spodoptera exigua (Hiibner) than Prodenia litura (Fabricius) and Helicoverpa Armigera (Hiibner)(Lepidoptera:Noctuidae). The development time was extendedand adult body size and lifespan increased with increasing host instars at superparasitism, but the progeny survival was higher in middle instars than earlier and later instars. A significant interaction on body size was found between parasitic mode and host plants, where body size of parasitoid progeny was22.5%smaller in superparasitized hosts than in single parasitized larvae when the host larvae fed on cotton, but25.0%when the hosts fed on cabbage. The experiment results suggested that fitness costs of superparasitism to parasitoids can vary with host quality.3Experiments were carried out to evaluate effects of intraspecific competition, egg load and host encounter rate on the propensity of superparasitism. The analyses, using log-linear model with negative binomial distribution, showed that parasitoids with high host-encounter rate performed more superparasitism than those with low host-encounter rate. An interaction was found between competition and encounter rate with the effect on superparasitism, where parasitoids with high encounter rate performed more superparasitism when they experienced strong competition, but the contrary was true when they had an experience of no-competition. An interaction between competition and egg load was also exhibited. Time of host discrimination was used to evaluate hazard of host being superparasitized by COX proportional hazard model, which showed that host encounter rate and its interaction with egg load had significant impacts on instant probability of superparasitism. The results suggested that potential factors such as intra-specific competition, host abundance, and egg load may often be interacted to influence the propensity of superparasitism.4Dual-choice trials were performed to observe preferences of M. pulchricornis between healthy and parasitized hosts as affected by parasitization experience and interval time after parasitization. The choice trials showed significant effects of parasitization experience and interval time after parasitization. The parasitoids without an experience of parasitization showed high propensity to attack parasitized hosts than those with the experience of parasitizing healthy hosts. The probability of superparasitism decreased with extending of interval time after parasitization. The analysis of recognition time using COX proportional hazard model indicated diminishing hazards of being superparasitization with extending of interval time after single parasitism. The results suggested that M. pulchricornis have the ability of discrimination between healthy and parasitized hosts, which can obviously be improved by experiences of parasitization.5A release-recapture method was used to study superparastism as affected by host stages and body weight in the soybean field. The results showed an average parasitism of54.2%(168/310) and superparasitism of23.9%(74/310). The extent of superparasitism, as measured by the number of eggs laid in a host, was negatively influenced by host body weight. The probability of single parasitism decreased with host instars and body weight. But the probability of superparasitism increased with host instars and did not change with host body weight. The experiments suggested that superparasitism could happen in the filed, which may be associated with host life stages at the time of oviposition.6The recognition time is a prerequisite for evaluation of host quality prior to oviposition in parasitoids. It was measured and analyzed using the COX proportional hazard model for the study of superparasitism hazards (conditional probabilty) as affected by covariate facters. The study suggested that the recognition time used by parasitoids prior to superparasitism can a function of acceptance of the hosts, in that the longer the recognition time the more reluctant the parasitoids to superparasitize the hosts, which suggests a less hazard of being parasitized for the hosts. The current study suggests a noval chacteristic that can be useful for the assessment of superparasitism in parasitoids.