Host Selection And Development Strategies Of Parasitoid Meteorus Pulchricornis

Meteorus pulchricornis is a solitary endoparasitoid in Euphorinae (Hymenoptera: Braconidae). Its host range is broad, including most macro-Lepidopterous moth larvae such as the gypsy moth, Lymanstria dispar (L.), the cotton bollworm, Helicoverpa armigera (Hübner), and the beet armyworm, Spodoptera exigua Hübner. It is widely distributed in the Palaearctic Region. There are both biparental (sexual reproduction) and uniparental (parthenogenetic reproduction) strains. However, there have been limited studies on its biology and most of studies used the gypsy moth as host. Little is known about its host selection and developmental strategies using the noctuid larvae as hosts. In this dissertation, I first observed the parasitism and offspring developmental parameters of the uniparental parasitoid in attacking different instars of host larvae of the cotton bollworm and beet armyworm. Then I studied effects of state variables of the parasitoid, egg and time limitation, on selection of S. exigua larval instars. And finally I investigated effects of host plants and ambient temperatures on fitness-related developmental performances of the parasitoid offspring.1. A single choice test was performed to examine developmental strategies in the uniparental endoparasitoid M. pulchricornis and its host, the beet armyworm, S. exigua. The results support the hypothesis that a compromised developmental strategy is pursued by solitary koinobiont endoparasitoids in the Braconidae and Ichneumoidae, where a trade-offbetween offspring body size and development time was exhibited. All five larval instars of S. exigua were susceptible to parasitism by M. pulchricornis, but parasitism was higher in intermedia-aged hosts than in hosts at opposite ends of the age continuum. Conversely, parasitoid mortality from egreesion to adult ecolsion was higher in the youngest and oldest host larvae. Adult parasitoid size increased significantly in line with host size at time of parasitism, but development time from oviposition to adult eclosion was greater when intermediate-aged hosts were selected than in when hosts at opposite ends of the age continuum were used. Developmental flexibility and host regulation appeared to be important to the parasitoid, in that the host instar from which parasitoid larvae emerged varied depending on when parasitism took place in the L1 through L3 host instars. The duration of the final instar of parasitized host larvae was significantly prolonged compared with unparasitized larvae under most circumstances, but was shortened when the host was parasitized in instar L2 or L3, with parasitoids emerging from the L5 host larvae.2. In order to understand host instar preference of M. pulchricornis paired-choice experiments were conducted in the laboratory. Choices were made for the parasitoid between 1st and 3rd instar, 1st and 4th instar, 2nd and 4th instar, 3rd and 4th instar, 4th and 5th instar. M. pulchricornis preferred older hosts to the younger but the inverse occurred when the 5th instar host was offered, which was consistent with the conclusion from the non-choice experiment.3. A single choice test was performed to examine developmental strategies in the uniparental endoparasitoid M. pulchricornis and its host, the cotton bollworm Helicoverpa armigera. The results support the dome-shaped model in which the fitness functions are 'dome-shaped' relative to host size (and age) at parasitism. Older and hence larger host larvae were simply not better hosts for the developing parasitoid. Although parasitoid size (measured as cocoon weight and adult hind tibia length) was positively correlated with host instars at parasitism, parasitoids developing in larger hosts (L₅ and L₆) suffered much higher mortality than conspecifics developing in smaller hosts (L₂-L₄). Furthermore, egg-to-adult development time in M. pulchricornis was significantly longer in older larvae (L₄-L₆) than in the younger. From the performance of fitness-related traits in M. pulchricornis, the L3 host is deemed the most suitable for survival, growth, and development, followed by L₂ and L₄ hosts, whereas L₁, L₅, and L₆ are less favorable hosts. The oviposition tendency (represented by parasitism) of M. pulchricornis was not perfectly consistent with the performance of the offspring in that the same parasitism amongst L₂ to L₄ hosts resulted in differences in fitness-related performance in offspring parasitoids. Larval development in H. armigera was usually suspended, but occasionally advanced, in the final instar.4. Longevity, ovary structure, oviposition schedule and fecundity of M. pulchricornis were studied in relation to its host S. exigu, as a first step toward better understanding of its host selection and developmental strategies. The results showed that the adult longevity of females was 30 days and did not correlate with the body size. When offered 20 second to third instar larvae per day, a female ofM. pulchricornis deposited an average of 179 eggs. The egg load of the parasitoid significantly differed between different ages (6, 20, and 30 days after emergence), where it was highest at day 6 but lowest at eclosion. The ovigeny index of wasp was 0.08-0.11. There was a strong positive linear relationship between the body size (as measured by hind tibia length) and the egg load for any ages of the parasitoid.5. Effects of parasitism experience and state of egg load on host instar choices were investigated to understand how the learning and egg-limitation influence a female's assessment of host value. In order to obtain the parasitoid at same age with different egg loads, we provided host larvae to 6-day old parasitoids for oviposition. The result demonstrated that the female with oviposition experience used less time than that without it in the first host location, but experience did not affect the first host examination time and proportion of "correct" selection ( the 3rd instar larval). In contrast to the female with experience, the female without it had more in total host examination time, total times of probing and parasitism. It represented oviposition experience did not affect host instar selection of parasitoid. Whether the egg load was high or was low, the times of probing and parasitism were highest in the L4 host instar, and the host examination time was longest, followed by L3 and L5 host instar, In contrast to parasitoids with high egg load, that with low egg load did not prefer L3 host instar to L5 host instar. Our data support parasitoids with low egg load seemed to be restricted by egg limitation.6. Single choice and paired choice tests were performed to study effects of time- versus egg-limitation on host instar selection. In the single choice test, 6-day old parasitoids with high egg load preferred to attack L2-L4 host instar with parasitism of 70% and 20-day old parasitoids with low egg load preferred L3-L5 larvae with parasitism approaching to 80%, whereas 30-day old parasitoids with lower egg load attacked all host instar without preference with parasitism 50%. In the paired choice experiment, whether young or old, M. pulchricornis preferred older hosts to the younger in all instar pairs, except the 4th and 5th instar pair where no preference was exercised. When choices were made between the 1st and 3rd instar, the 1st and 4th instar, the total parasitism was significantly different with the parasitoid age. The preference for host instars was abating with the life expectancy declined. The preference index generally decreased with the increase of age, where the preference index of 30-day-old parasitoids for the 1st and 4th instar pair was significantly lower than 6-day and 20-day-old parasitoids. The 30-day-old parasitoids seemed to be restricted by time limitation.7. The effect of host plants on the weight of parasitized S. exigua larvae and on the fitness-related traits ofM. pulchricornis were studied, in order to gain insights into the influence of host plants on the parasitoid at higher trophic levels. Two experiments were exercised. In experiment A, the mean weight of larvae varied significantly with plant species. The body weight of S. exigua host larvae was allowed to vary with host plants in order to asses both the effect weight and quality of larvae, as affected by the host plants, on the fitness-related traits of the Meteorus pulchricornis. In experiment B, larvae with same weight across host plants were selected to separate the effect of weight from that of quality of the larvae. In both experiments, the parasitism was highest on the soybean, followed by that on the cotton, and was lowest on the cotton. The total fitness gains of M. pulchricornis were highest on the greengrocery as measured by body size, development time, and survival ofparasitoid offspring.8. Effects of temperature on host selection and developmental performances were studied to investigate whether the parasitoid changed preferences for host larvae reared under different temperatures and how developmental interactions between host larvae and the parasitoid offspring were affected. M. pulchricornis accepted the host S. exigua for oviposition and their progeny developed successfully in all host ages at three constant temperatures. The parasitism increased with the increments of temperature. Parasitoid mortality from egression to adult eclosion was lowest at 25℃followed by 20℃, it was highest at 30℃. The longevity of adult parasitoids was shortest at 25℃(25.52 days) followed by 30℃, it was longest at 20℃(47.42 days).The fitness-related traits of parasitoids did not increase linearly with the host age in which they developed at different temperature. Host quality was found to be optimal at intermediate host instars and the female preferred to parasitize these hosts. The parasitoid's larval development rate increased with temperature increments in a linear fashion up to 30℃. At 20℃, the mean larval development time was approximately 15 days and pupal development 9-10 days, whilst at 30℃development times were 8 days for larvae and 6 days for the pupae.