Host Selection And Offspring Developmental Performances In An Aphid Parasitoid Lysiphlebus Ambiguus As Affected By Endosymbiotic Bacteria Of Host Aphis Fabae

Numerous studies indicate that endosymbiontic bacteria Bushnera perform a pivotal function in nutritional physiology of host aphids. While close co-evolution between aphids and their endosymbionts, and between aphids and aphidiine parasitoids have been established, it is assumed that some association between endosymbionts of aphids and higher trophic aphid parasitoids should be susceptible to natural selection. This assumption has been supported by empirical studies that facultative endosymbiotic bacteria of aphids are linked to resistance of host aphids to their parasitoids. Though it was suggested that host-stage selection of parasitoids might be associated with state of endosymbiotic Bushnera of aphids, there has been no empirical studies to address the association between life history of parasitoids and primary endosymbiotic Buchnera aphidicola of host aphids. Accordingly this study was designed to compare parasitism of an aphid parasitoid Lysiphlebus ambiguus in relation to aposymbiotic and symbiotic host Aphis fabae, and developmental performances of progeny parasitoids from attacking the aphid cohorts. In addition, we studied effects of carbon dioxide enrichment on interactions between host A. fabae and its parasitoid L. ambiguus, which would provide us with insights into co-evolution between aphids and their parasitoids as affected by climate changes. The main results and conclusions are summarized as follows.1. Exploratory experiments were carried out to diagnose endosymbiotic bacteria of host aphids and to obtain the correction coefficient between mummies and effective stings exercised by mother parasitoids. The experimental stock aphids were screened for the presence of primary endosymbiotic Buchr.era aphidicola and of facultative bacterium Hamiltonella defensa with diagnostic polymerase chain reaction (PCR). The primary endosymbiotic B. aphidicola was detected, but the facultative endosymbiotic H. defense was not found in the A.fabae stock used in our experiments. Therefore, H. defense was excluded in the effects of endosymbions in our experiments. Survival of progeny parasitoids differed between symbiotic and aposymbiotc aphids and between stages, which was used to deduce effective stings made by mother parasitoids.2. Non-choice experiments were conducted to observe parasitism of host aphids, and performances of progeny parasitoids, as affected by host stages and endosymbiosis. The aphids were treated using antibiotic rifampicin to obtain aposymbiotic aphids.Aposymbiotic aphids were significantly smaller in body size than corresponding symbiotic aphids at all stages except the first in unparasitized aphids. However, the differences in body size between symbiotic and aposymbiotic aphids did not produce significant variation in parasitism as measured by number of mummies. Eclosion rates of parasitoid progeny from aposymbiotic hosts were significantly lower than from corresponding symbiotic aphids in the1st stage to adult except for the3rd stage. Though female progeny increased with host size of consecutive stages in symbiotic aphids, no significant variation in sex ratio was exhibited across host stages of aposymbiotic aphids. Development time from egg-to-adult of progeny was distinctly prolonged in aposymbiotic aphids, compared to that in the symbiotic. Parasitism of young nymphs and adults of symbiotic aphids produced a shorter development time than intermediate host stages, but development time of progeny decreased in a linear relationship with life stages of aposymbiotic aphids. Our study suggests that differences in mortality rates of male and female wasps during development is likely to have influenced secondary sex ratio in relation to aphid stage for aphid cohorts deprived of symbiotic bacteria, and that bacteria disruption in aposymbiotic aphids can produce deleterious effects on growth and development of progeny parasitoids.3. A series of paired-choice experiments was conducted to explore the preference of the parasitoid L. ambiguus for symbiotic and aposymbiotic A. fabae, and the suitability of these hosts for progeny parasitoid development. When given a choice between symbiotic and aposymbiotic aphids of the same stage, the parasitoid significantly preferred the symbiotic over aposymbiotic aphids only for the later stages (L4and adult). The suitability of aposymbiotic aphids for parasitoid development was only significantly lower than that of symbiotic aphids in L1and L4stages. When given a choice between similar-sized symbiotic L2and aposymbiotic L4aphids, the parasitoid preferred the former. No significant differences in preference were demonstrated when the parasitoid was given a choice between different stages of aposymbiotic aphids. For the latter two paired-choice experiments, there was no evidence of differential host suitability as exhibit by adult emergence rate and progeny sex ratio, although sample sizes were small. While lifetime fecundity increased with aphid stage at the time of oviposition, there was no significant influence of previous development from symbiotic versus aposymbiotic aphids. These results suggest that while L. ambiguus can discriminate between symbiotic and aposymbiotic A. fabae during later stages and when the aphids are of a similar size, there is little evidence that the primary symbiont is needed for successful parasitoid development.4. Foraging behaviors of L. ambiguus were studied in symbiotic and aposymbiotic black bean aphid patches on plant in a transparent cage. The results showed that symbiotic and aposymbiotic aphid patches were not significantly different in parasitism, number of visits, time of residence, and time of foraging. It suggests that L. ambiguus may not be able to discriminate between patches of symbiotic and aposymbiotic aphids.5. Effects of elevated CO2(750μl l-1) on the development of the black bean aphid A. fabae, and on performances of parasitoids L. ambiguus, were studied in incubators. Aphids under elevated CO2were significantly smaller in body size as measured by hind tibia length than corresponding aphids under nomal conditions at all stages.Non-choice experiments showed that L. ambiguus parasitized all stages of A. fabae cohort from the elevated CO2, and mummy production rates decreased with host aphid stages. In comparison with progeny parasitoids attacking aphids from normal CO2, progeny parasitoids in aphids from the elevated CO2performed as that:(i) parasitization rate for the4th stage and adult aphids was significantly low;(ii) wasp emergence rate in the third stage host was low;(iii) female proportion was low in the first stage host;(iv) body size was large;(v) development time in the second to forth stage hosts was reduced. The results suggest that elevated CO2should influence L. ambiguus in parasitism with Aphis fabae and development of progeny parasittoids.6. This study has produced innovative findings in following aspects,(i) Association between primary endosymbitoic bacteria of host aphids and parasitoids has being revealed for the first time, which is that Lysiphlebus ambiguus is able to discriminate against aposymbiotic hosts Aphis fabae at higher stages and such preference is inagreement with offspring performances,(ii) Elevated CO2can influence L. ambiguus in parasitism with Aphis fabae and development of progeny parasitoids, which contributes to our understanding of ecological interactions between parasitoids and their hosts, as affected by CO2enrichment with the climate change.