| In this paper, main parasitoids of diamondback moth (DBM), Plutella xylostella ( L.) and the bioactivity of Bacillus thuringensis (Bt) against DBM were studied. Investigations were also conducted on the selective toxicity of Bt to main parastitoids of DBM, the effect of Bt on host selection of Costesia plutellae Kurdjumov, sublethal effects of Bt on DBM and the larval parasitoid Costesia plutellae Kurdjumov, the independent and combined effects of Bacillus thuringiensis and Costesia plutellae Kurdjumov on DBM. And the total protein content in haemolymph of parasitized DBM larvae was determined, This research aims to explore the relationship between the two biological control agents to aid controling DBM.The species of the parasitoids on DBM in crucifer fields in the suburbs of Fuzhou were investigated. Four parasitic species, i.e., Macromalon orientate Kerrich, Cotesia plutellae Kurdjumov, Oomyzus sokolowskii Kurdjumov, Brachymerea sp., and three hyperparasitic species, i.e., Brachymeria secundaria Ruschka, Oomyzus sokowskii Kurdjumov, Trichomalopsis sp. were found. Investigation of adult emergence and hyperparasitizing of collected cocoons from field indicated that field parasitic adult emergence rate correlated to next field population, that Oomyzus sokowskii Kurdjumov can emergenced from the Costesia plutellae Kurdjumov pupa, and that the tow peaks of Costesia plutellae Kurdjumov fell on April-May and on the third period of September-the first period of October. During the peaks, Costesia plutellae Kurdjumov was often hyperparasitized and the rates of hyperparasitization reached 25%.The results of bioassay showed that toxicity of Bt preparations against the third larvae of DBM was as follows: luying (LC50=341.119mgL-1) > high effectual Bt/sha chong dang (LC50=406.92mg.L-1)> Bt soluble powder (LC5o=448.168mg.L-1) > sutai (LC50=525.318mg.L-1) (LC50=525.318mg.L-1) >nongtai (LC50=746.068mg.L-1) > ABG-6444(LC50=1989.803mg.L-1) >biliBt(8010) suspension concentration ( LC50=2037.164mg.L-1); The toxicity of Bt soluble powder to the different instar larvae of DBM decreased with the instar development, it indicated great difference among different instar.Bt fed by 4-instar larvae led to the reduction in both the ratio of pupating and adult emergence, which significantly increased with the increase of Bt concentration, When theadult were fed with Bt at concentrations recommended for field application, the longevity of the moth was not affected, but their egg production was reduced significantly.The sensitivity of the main parasitoids of DBM to field concentrations of seven Bt preparations was evaluated by residue film method. Luying is the highest toxic to Oomyzus sokolowskii Kurdjumov, and the linear regression of toxicity was y =0.814+1.9538 x (r=0.984), LC50=140.925079mg.L-1. According to the indexes proposed by Guoyujie (1995.), luying fell into deleterious preparation. Bili (8010) into low toxicity preparation and nongtai and Bt into innocuous preparations; Sutai of seven Bt preparations to Costesia plutellae Kurdjumov was the highest toxic and adult mortality reached 66.67%. After contacting film for one day, bili (8010) was the second highest toxic to Costesia plutellae Kurdjumov. Bt soluble powder, nongtai , Bt/sha chong dang were harmless to Costesia plutellae Kurdjumov.Studies on sublethal effects of Bt soluble powder on Costesia plutellae Kurdjumov suggested that the parasitoid could not distinguish between Bt-treated and untreated larvae of DBM and that Bt had no side-effects on adult emergence and longevity, mean egg production of per parasitoid and insect parasitized per female was not affected.The toxicity of Bt soluble powder on DBM was relatively high, while that to the main parasitoids of DBM was relatively low. Thus Bt soluble powder is selective bio-insecticides in practice.Studies on the independent and combined effects of Costesia plutellae Kurdjumov and Bt suggested that the two methods had synergetic effects.Finally, the total protein content of variey in haemolymph...
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