Safety Assessment Of Three Typical Microbial Pesticides On Silkworm And Honey Bee

Due to the severe threat of synthesized pesticides, the environmental friendly biopesticideis become more and more attractive to both farmers and scientists in the recent years. Particularly, the microbial pesticides, which belong to the biopesticide, have been manufactured for 225 products and applied in more than 30 countries worldwide, with over＄ 1 billion ourcomes per year and 10% of annual increase. Among the microbial pesiticides, more than 90% of them are insecticides, over 150 originated from bacteria,800 from fungi and 1600 from viruses so far. The representatives of commercial microbial insecticides include Bacillus thuringiersis (Bt), Beauveria bassiana (Bb) and Helicoverpa armigeranucleopolyhedrovirus (HaNPV).Microbial pesticides have a bright future in control of pests in the agricultural ecosystem due to the property of environemtal safety, although they occupy only 10～15% of the total pesticides up to date. In fact, no bodies can guarantee that the microbial pesticide is real safe for the envrionment, because no efficient methods or official roles have been applied for the environmental safety evaluation in the world. Based on literature survey and obtained experience in other countries, we are trying to set up a environmental safety evaluation system by determining the infectivity, toxicity and factors that might relate to the toxicity of the major microbial pesticides of Bt, Bb and HaNPV on economic insects silkworm (Minghu X Suju) and honey bee Apis mellifera and Apis cerana in China. Main results are as follows:1. The proliferation capacityof typical microbial pesticides on silkworms was monitored by real-time fluorescence quantitative PCR. Result showed that the copy numbers of Bb 18S rRNA gene in the silkworms (95%RH) increased 8,20 and 37 fold from the first day to the third day after the second instar insects were dipped of the Bb suspension at concentrations of 1×108, 1×109 and 1×1010 conidia/ml respectively for 5s. Meanwile, the HaNPV population in the 2nd instar silkworm increased about 3.4 fold in the dead insects but decreased 48.0 times in the alive insects than that in the silkworm without infection at 3 days after oral feeding the mulberry leaves that were immersed in 1×109 PIB/ml HaNPV suspension. Inoculating Bt to 5th instar silkworm by feeding mulberry leaves immersed in 1x108 spores/ml suspension, the copy number of Bt cry I gene in the insects decreased rapidly in the midgut, about one thirty-fourth as high as the virus in the leaves at 2 days after inoculation.2. Bt, Bb and HaNPV show both acute toxicity on the 2nd instar silkworm with LC50 of 1.44×108 spores/ml,4.64×109 conidia/ml and 2.62×108 PIB/ml respectively, and sublethal effect on silkworm, which can be identified according to the growth, development behavior and immune responses. Body weight, molting rate and phenol oxidase activity could be used as sensitive indicators for early diagnosis of sublethal effects. Adult silkworm doesn’t show distinct change in cocoon weight, cocoon shell weight, cocoon shell percentage and egg numbers under chronic concentration.3. Under the maximum hazard concentration, Bb shows acute toxicity to A. mellifera and A. cerana; while Bt and HaNPV hardly excert adverse effects on the given two species of bees.4. The sensitivity of silkworm and honey bee to microbial pesticides is also affected by instar ages of the insects and their living environments. Along with the increase of instar ages, the sensitivity of the insects to Bt, Bb and HaNPV decreased with a lower mortality rate, body weight and molting rate. As for the influence of environment:the motality of honey bees infected by Bb under 34℃ (temperature inside hive) was only 1/3 of the motality under 26℃, which indicates the temoerature; the LC50 of silkworm infected by Bb under 95%RH was only 1/30 of the LC50 under 80%RH, which indicates the impact of humidity.According to the above results, we suggested that environmental safety assessment of microbial pesticides on silkworm and honey bees should be carried out from aspects of infectivity and toxic effect. Firstly, establishing qPCR-based biosafety evaluation for microbial pesticides; secondly, adding sensitive indicators as body weight, molting rate and phenol oxidase activity for early diagnosis of microbial pesticides’infection on silkworm. Furthermore, environmental factors make a major impact on the infectivity and toxic effect of microbial pesticides on silkworm and honey bees, which implies the importance of selceting an appropriate condition for the safty assessment.