| The biological control values of natural enemies are strongly correlated to their ability to regulate the density of their host/prey.For parasitoids,they can kill their host by parasitism as well as destructive host feeding.The parasitism and host feeding strategies vary with physiological state of parasitoids or environmental conditions.Apart from parasitism and host feeding,nonreproductive effects was considered as another means by which parasitoids can also reduce their host populations.Here,we evaluated the interactions of an aphid parasitoid,Aphelinus asychis(Walker)with different instars of two aphid species,Myzus persicae(Sulzer)and Acyrthosiphon pisum(Harris),under controlled conditions in the laboratory.The main methods and results are as follows:In order to assess if hind tibia length accurately reflect body size of A.asychis,we tested if hind tibia is positively correlated with other characters of body size,such as pupal weight,head width and wing length.Hind tibia length was highly correlated with head width and wing length.The pupal weight and hind tibia length were logarithmic transformed before the linear regression analysis,and there was a significant linear correlation between the log-transformed weight and length.To assess host suitability of different instars of the host for A.asychis,we reared A.asychis on four nymphal instars of the pea aphid A.pisum.Females of A.asychis accepted all four nymphal instars of A.pisum for oviposition and their progeny could successfully develop in these hosts.The mortality of preadult stage of A.asychis differed significantly among host instars,and the lowest mortality was found in the second instar.Aborted parasitism was founder highest in the fourth instars.The progeny development time was shortest in the first instar,and more female-biased offspring sex ratio was found in the third instar of A.pisum.The body size of parasitoid progeny was significantly influenced by parasitized host instar,and the sons and daughters were found larger in third and fourth instars of A.pisum.Based on above,the optimal host suitability was found at intermediate host instars.To evaluate the effects of host density on patterns of parasitism and host feeding,egg load and host instar preference,we exposed A.asychis to a variable density of mixed instar of M.persicae.The number of eggs laid and the number of aphids parasitized by A.asychis were significantly influenced by host densities,and they were significantly lower at lower host densities.But the number of host-feeding and superparasitism were not significantly affected host densities.The residual eggs in the ovaries of A.asychis decreased with the increase in host densities.We used the number of eggs laid during the experiment and the residual eggs in the ovaries to estimate egg maturation during the experiment,and the results showed that host densities did not significantly affect egg maturation.The choice of host-instar for oviposition was significantly influenced by aphid densities,and parasitoid tended to parasitize predominantly younger instar of aphids(first-and second-instar)at higher host densities(75,100 aphids/leaf disk).Parasitoids showed no significant preference of host instars for oviposition at lower host density.Host-feeding preference was not affected by host densities.Parasitoid females preferred younger aphids to older at all host densities.We evaluated the effects of parasitoid age on egg load,fecundity and host instar preference of a honey-fed parasitoid,A.asychis.Host selective experiment was conducted to measure host preference of honey-fed A.asychis females at different ages,using the second and fourth instars of the green peach aphid M.persicae as their hosts.The results showed that honey consumption proved to have a significant effect on the longevity of males and females of A.asychis.In honey-fed groups,the linear regressions of hind tibia length and egg load were significant,and the egg load of parasitoids aged 20 days was considerably lower than that of other groups,while there was no significant difference between other groups.The total number of eggs laid by the honey-fed parasitoids within 24 h declined with the increase in the age of the parasitoids,thus highlighting a negative relationship between fecundity and aging.We used the regression equation of egg load and hind tibia length,the number of eggs laid during the experiment and the residual eggs in the ovaries to estimate egg maturation during the experiment,and the influence of parasitoid age on egg maturation was not significant.The choice of host-instar for oviposition was significantly influenced by the parasitoid age.Honey-fed parasitoids in the age groups of 1,5,10 and 20 days tended to parasitize predominantly second instar aphids,whereas 15-days old parasitoids showed no significant preference of host instars.On the other hand,host-feeding preference was not affected by parasitoid age.Parasitoid females of all ages preferred younger aphids to older aphids.We used the parasitoid A.asychis and its host A.pisum to investigate the influence of pseudoparasitism on host populations under laboratory conditions.Biological parameters of A.pisum were analyzed using an age stage,two-sex life table.The results of this study showed that pseudoparasitism was prevalent under laboratory conditions,and the frequency of this type of attack on third-and fourth-instar hosts was higher than the frequencies of parasitism and host feeding.Pseudoparasitism adversely impacted aphid populations,by decreasing aphid survival and reproduction,and impacts were greatest in hosts attacked at the first and fourth instars.These results indicate that pseudoparasitism enhances the biological control impact of A.asychis attacking A.pisum.To conclude,the parasitism and host feeding strategies of A.asychis changed under different circumstances,and pseudoparasitism could negatively affect host populations.These conclusions provide theoretical basis for the biological control of greenhouse aphids and the population dynamics of host and parasitoid. |
PDF Full Text Request