| 1. The effects of some factors of Aphidius gifuensis and Aphidius avenea on parasitismIn the laboratory condition, parasitoid stage, exposure period, female parasitoid density, host density, host instar and morph of host wings were found to affect the parasitization of A. gifuensis and A. avenea on Sitobion avenae. The research shows that the parasitoid density has an evident influence on both of them when the temperatures at (22±1)℃. The highest parasitization rate was obtained when the aphids female was 20:1. the parasitization rate of Aphidius gifuensis is 71%,the parasitization rate of Aphidius avenea is 77%. The parasitoid rate. decreased with the increase of parasitoid density. The highest parasitization rate of Aphidius gifuensis was obtained by parasitoids of 2d age while Aphidius avenea at the 3rd age. Between apterae and alate on 3rd or 4th hosts, they prefer apterae. The parasitoid rate of apterae is higher than any other alate at the same age. The parasitoid density could affect the parasitization of Aphidiidae too. Aphidiidae increased readily itself. However, when the parasitoid density reach a certain extend, the parasitoid rate decreased. When the parasitic aphidiidae density is too high, they may disturb with each other when paratsitized. The age of female aphidiidae could influence the parasitization rate. The parasitoid rate decreases as the day increases. Female of one day age has the highest parasitoid rate. The parasitoid rate of A.gifuensis and A. avenea are 73.2% and 75.2% respectively.2. Host-stage selection of Sitobion avenae by Aphidius gifuensis and Aphidius avenea, and its effect on development Host-stage selection of Aphidius gifuensis, a parasitoid of Sitobion avenae and the effect of host stage being parasitized on the parasitoid development and fecundity were investigated at (22±1)℃. When aphids of mixed stages were provided. Aphidius gifuensis and Sitobion avenae are parasitic to aphids in all instars , but the two have difference in the choice of parasitism. Aphidius gifuensis showed a preference for younger aphids to parasitize. The relative parasitism on the 2nd instar aphids was the highest(34.0%),while Aphidius avenea showed a preference for middle aphids to parasitize, The relative parasitism on the 3rd instar and 4th instar aphids are 37.2% and 21.6%. The parasitoid rate of Aphidius gifuensis(12.7%)to adults is higher then that of Sitobion avenae(5.5%), the two all have low parasitoid rate to alate. The highest rate of Aphidius gifuensis is 4.1% and Sitobion avenae is 5.0%. The parasitoid of Aphidius gifuensis Ashmead prolongs the development of Sitobion avenae. 2nd and the 3rd instar of the hosts significantly prolonged when 1st instar aphids were parasitized. However,no obvious influence on development of 3rd and 4th was observed after they were parasitized. The life span of each aphids parasitized at various nymphal stages sharply shortened.All of the parasitized 1st instar aphids turned to mummies before the adults. The fecundity of the adults from younger aphids parasitized distinctly declined in comparison with the control adults.Immature stage of the parasitoid in 2nd instar of the hosts developed the fastest with duration of 9.4d,and the slowest of 12.1d in 1st instar of the hosts.3. Effect of host density and temperature on the fitness of Aphidius gifuensis and Aphidius aveneaThe adult's life-span of Aphidius gifuensis and Sitobion avenae was examined at the different temperatures (14,18,22 and 26℃). The results showed that the number of A. gifuensis increased with the increasing of the temperature, but the parasitization number between each female differed significantly. At the temperatures (14,18,22℃) , the parasitic number of A.gifuensis Ashmead are all very small, but larger than that of Sitobion avenae. The two reach their largest number at 22℃. At the same time, the research of their function to their parasitism shows that they belong to HollingⅡfunctional model. Under the same temperature, the parasitic number of A. gifuensis and S. avenae increase as the grow the density of S. avenae . The parasitic functional response of A. gifuensis Ashmead and S. avenae has strong disturbing influence inside the seeds. As its density increases, the number of parasites decreases gradually. Hassell (1969)model can show the relationship between the searching efficiency and its density of A. gifuensis Ashmead and S.avenae, and the results are E = 1.889P-0.0242and E = 1.971P-0.0265respectively.
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