| Aphis craccivora Koch (Hemiptera: Aphididae), black bean aphid or peanut aphid, is a worldwide serious pest with numerous hosts. Besides feeding damage, A. craccivora spreads plant viruses, and results in heavily economic loss. Biological control is an important measure to control pests. Lysiphlebus japonicus Ashmead (Hymenoptera: Aphidiidae), a solitary endoparasitoid, is a useful parasitic natural enemy of A. craccivora. Although some field investigations and biological features have been done, the basic information of development and the mechanism of successful parasitism still lacking in L. japonicus.This paper studied the effects of some factors on parasitism, the determination of L. japonicus instars, the regulations on host development and metabolism. The results are as follows:1. In 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 L. japonicus on A. craccivora. The parasitization rate dereased with the increase of parasitoid age. The highest parasitization rate was obtained by parasitoids of 1d age. Exposure period affected the parasitization of L. japonicus. When the parasite:host ratio was 1:20, the parasitization rates did notincrease distinctively if the exposure period was more than 24hr.. While the ratio was 1:40, the parasitization rates kept on increasing significantly until the exposure period was more than 48hr.. The parasitoid density could affect the parasitization of L. japonicus too. When the parasitoid density was 1,5,10,15 female parasitoids/200 aphids, the parasitization rate increased readily with parasitoid density. However, when the parasitoid density was 20,30 female parasitoids/5 aphids, the parasitization decreased. Host density had significant effect on the parasitism of L. japonicus. The highest parasitization rate was more than 90%, which was gained at 50-100 aphids/5 female parasitoids. Both parasitization rate and superparasitism rate decreased with the increase of host density. The host instar could influence the parasitization rate. L. japonicus preferred to oviposit on blank bean aphid of younger instar and the parasitization rate for L2 was the highest, the second highest was for L1, but that for L4 was the lowest. Between apterous and alate aphid larvae, L. japonicus preferred for apterous larvae. All parasitization rates of apterous larvae were higher than those of alate host of the same instar.2. To make clear the larval development of L. japonicus and study the developmental interactions between L. japonicus and A. craccivora farther, we described and illustrated with scanning electron micrographs the chief diagnostic criteria of 4 instars of L. japonicus each. This is the first report on larval morph of L. japonicus. The 1st instar is mandibulate, caudate and has sclerotized spines. Supernumerary newly hatched larvae are eliminated by powerful mandibles of the prior hatched larva. The 2nd instar has degenerative mandibles and covered with small sclerotized bristles on abdominal segments; the cauda is short and blunt. The 3rd instar is mandibulate; its cuticle is smooth and the caudal segment retrogresses to a short salient. The 4th instar is strong and hymenopteriform; the mandibles aresmaller than those of 3rd instar, and various sensory organs are well developed, especially for the mouthpart. 3. In the laboratory conditions, host-instar selection of Lysiphlebus japonicus Ashmead and its development in Aphis craccivora Koch of different instars were studied. When served mixed instars aphids, The parasitoid showed a preference for small aphids. The relative parasitization rate of L2 aphids was the highest (26.4%), followed by L1 aphids, which was 20.6%, the others were lower. The devolopment of aphids was delayed after parasitized. At 25℃, the developmental period of parasitized L1 aphids was significantly prolonged at the 1st,2nd and the 3rd instar; and which of alate parasitized L3 aphids was eviden...
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