The Assay Method And Evaluation Criteira Of Chronic Toxicity Of Pesticides To Silkworm

Silkworm (Bombyx mori L.) is very sensitive to pesticides, which like many other pestsof agriculture and forestry and variety of mulberry pests, belongs to lepidopteron. When alarge number of pesticides are used in sericultural areas or farmland near mulberry orchard,the mulberry leaves will be polluted inevitably, which will cause a great threat to silkworm.Toxicity of pesticides to silkworm includes acute toxicity and chronic toxicity. At present, theassay method and evaluation criteria of chronic toxicity of pesticides to silkworm have not yetbeen established at home and abroad. In order to study and establish the chronic toxicitymethod and evaluation criteria of pesticides to silkworm, we carried out a systematicinvestigation about identifying administration phase, administration dose and evaluationcriteria on silkworm, using three different types of representative insecticides, lambda-cyhalothrin, emamectin benzoate and hexaflumuron, as the target pesticides. The main resultswere as follow:In the study, the silkworm was fed with mulberry leaves contaminated with the testedpesticides from second to third instar, and the results showed that: the growth and economiccharacteristics of silkworm suffered different degrees of impact after treatment. When theadministration phase was from second to third instar, the developmental duration and rate ofdead worm cocoon of silkworm showed positive correlation with concentration of pesticidesafter treatment, whereas the weight of molting silkworm, cocoon weight, cocoon shell weight,rate of cocoon shell, cocooning rate, pupation rate and rate of dead worm cocoon showednegative correlation with concentration of pesticides. When the administration phase was fifthinstar, although there was also a trend of the extension of developmental duration and thereduction in weight of molting silkworm, cocoon weight, etc. after treatment, the doseresponse relationship was not obvious. Moreover, the silkworm in the high concentrationgroup of lambda-cyhalothrin and emamectin benzoate died before mounting. It may be dueto intricate physiological and biochemical reaction in the body of fifth silkworm. If thesilkworm was treated with tested pesticides at this moment, it will cause serious impact onsilkworm and not be easy to get valid data. Therefore, when the administration phase is fromthe second to third instar silkworm as the administration phase were employedin the test method of chronic toxicity, which can reflect the actual situation As one of observedeconomic characteristics, only cocooning rate of the three pesticides exhibited significantdifference between each treatment and control. So we choose cocooning rate as the mainevaluation index.the assay method of chronic toxicity of pesticides to silkworm that we established is:disposing mulberry leaves using quantitative spraying method, confirming1/50,1/100,1/200,1/400,1/800of LC50of acute toxicity (96h) as administration dose by screening the series oftreatment dose, feeding the silkworm with mulberry leaves contaminated with pesticides fromsecond to third instar, feeding the silkworm with nontoxic mulberry leaves from fourth instarto cocooning, recording cocooning rate, the concentration when cocooning rate is40%is theconcentration under economic threshold, then evaluating the chronic toxicity of pesticides tosilkworm by grading standard of chronic toxicity. The grading standard is: ratio of highestconcentration field and concentration under economic threshold>10000, special high riskpesticides,1000<ratio≤10000, high risk pesticides,100<ratio≤1000middling risk pesticides,ratio≤100, low risk pesticides. According to the grading standard we formulated, the chronictoxicity of lambda–cyhalothrin, emamectin benzoate and hexaflumuron to silkworm isspecial high risk pesticides. We suggest the application of high risk pesticides should be faraway from mulberry orchard, and middling risk pesticides should avoid to being used in thegreat silkworm feeding stage, but in other stage they can be used conditionally. In conclusionif we want to control diseases and pests on mulberry, we should choose the pesticides that lowacute toxicity and low chronic toxicity.