Effects Of Elevated CO₂and Nitrogen-Fixing Bacteria On Spodoptera Litura And On The Developmental Parameters Of Meteorus Pulchricorni（Hymenoptera:Braconidae）

Altered plant nutrition under elevated-CO2conditions may exercise influence of the third trophic level (host plant-herbivorous insect-carnivorous insect) through "bottom-up" effects. So far, most of researches have concentrated on interactions between host plants and herbivorous insects, while less focus have been placed on the higher trophic levels. In this study, we examined effects of elevated CO2on an herbivorous insect host and its endoparasitoid Meteorus pulchricorni. The following questions were addressed in this study:(1) how the elevated CO2and host generations under elevated CO2treatment affect developmental performances of M. pulchricornis?(2) do elevated CO2and nitrogen-fixing bacteria have combined effects indirectly on the food utilization efficiency of the host larva of Prodenia litura?(3) if elevated CO2and nitrogen-fixing bacteria indirectly influence developmental performances of M. pulchricornis. The main results and conclusions are summarized as follows.1. Indirect effects of elevated CO2and host generations under the elevated CO2on developmental parameters of M. pulchricornisLaboratory experiments were conducted using growth chambers by manipulating CO2concentration (390μL/L or780μL/L) and rearing Mythimna separate host larvae for multiple generations using wheat leaves cultured under aforementioned treatments, the host thus obtained were referred to as "elevated or ambient CO2-treated host". The host larvae at the3rd or4th stadium were exposed to parasitism and offspring developmental performances observed in the development time, body size, fecundity and longevity. The results showed that:(1) there was not significant differences in development time (egg-to-adult emergence time) both between CO2and host generation treatments;(2) the increase in body size, measured as adult hind tibia length, of emerging offspring adults from attacking the1st generation host was larger when the host was elevated-CO2-treated than ambient-CO2-treated; but not different when the host being parasitized grew to the9th generation;(3) larval production of offspring parasitoids decreased with host body weight at oviposition when the parasitoids emerged from ambient-CO2-treated hosts, but increased when they developed from elevated-CO2-treated hosts;(4) adult longevity of offspring parasitoids increased with host body weight at oviposition when the parasitoids developed in ambient-CO2-treated hosts at both first and9th generation, but decreased when they accomplished larval development in elevated-CO2-treated hosts at first generation. The result of this study suggested that effects of elevated CO2on tritrophic parasitoid in development may not be a simple function, showing a slight negative effect at the beginning when CO2as suddenly elevated but then no effect after a certain generations under elevated CO2.2. Effects of CO2and nitrogen-fixing bacteria on food utilization efficiency in Prodenia lituraThe "compensatory feeding hypothesis" proposes that herbivorous insects may increase their plant consumption to compensate for the deficiency of nitrogen in the plant tissue consumed. The empirical studies however are not overall in line with this prediction. In this study laboratory trails were conducted in growth chambers by manipulating CO2concentration (390μL/L,780μL/L) and the nitrogen-fixing bacteria (NFB)(with, without) for the common bean Phaseolus vulgaris in a factorial design. Spodoptera litura larvae at the4th stadium were fed with the bean plants cultured in aforementioned chambers, and tested for food consumption and utilization. ANCOVA results showed that CO2and NFB had significant effects both independently and in interaction on the4th instar larva in relative consumption (RCR) and relative growth rate (RGR). In ambient CO2there were no differences in RCR and RGR between NFB treatments, but in elevated CO2RCR and RGR increased in the absence as opposed to the presence of NFB. CO2and its interaction with NFB had significant influence on approximate digestibility (AD), where AD increased in elevated CO2as opposed to ambient CO2in the absence of NFB but slightly reduced in the presence of NFB. CO2and NFB treatments did not influence efficiency of conversion of ingested food (ECI). The results of this study lend support for the prediction of the compensatory feeding hypothesis.3-Indirect effects of CO2and nitrogen-fixing bacteria on life history of M. pulchricornis.To examine bottom-up effects in shaping legume-insect(Spodoptera litura)-enemy(Meteorus pulchricorni) tritrophic interactions under elevated CO2, laboratory experiments were conducted in growth chambers by manipulating CO2concentration (390μL/L, or780μL/L) and the nitrogen-fixing bacteria (NFB)(with, without) for the common bean Phaseolus vulgaris in a factorial design. P. litura larvae were fed with the bean plants cultured in aforementioned treatments until the third instar when exposed to parasitism by the endoparasitoid M. pulchricorni. The host larvae after parasitism were fed with the same plants as before, and followed by recording egression of offspring parasitoids from the host, pupation, adult emergence and longevity. The results below were obtained.(1) There was not a significant difference in development time between treatments.(2) CO2and NFB had significant effects in interaction on body size (pupal weight and adult hind tibia length) of offspring parasitoids. The body size of offspring decreased with host body weight at oviposition when the host was elevated-CO2-treated, but increased when ambient-CO2treated, in the absence of NFB. However, in the presence of NFB, the increase in pupal weight of offspring with host body weight was larger in elevated-CO2-treated hosts than in the ambient-CO2-treated, but the reverse pattern was detected in adult body size.(3) Adult longevity was influenced by CO2treatments, which was greater in elevated-CO2-treated hosts than in the ambient-CO2-treated. The results of this study suggested that NFB in legume plants may increase developmental fitness of parasitoids at elevated CO2.