Studies On The Laboratory Toxicity Effect Of Several Insecticides On Mythimna Separata(Walker) Eggs And Their Toxicity Mechanism

Mythimna separata?walker?is a historical and disastrous pest in agricultural production in China.Because of its migratory,explosive and disastrous characteristics,chemical control is still the main technique for its control.In this paper,seven kinds of insecticides were used to laboratory Toxicity on the eggs,and the effects on the subsequent growth and development of the eggs were observed.Finally,the cryostat section of the eggs were used to study the toxicity mechanism of different insecticides.The aim is to provide the basis for the effective prevention and control of the Armyworm.The main findings are as follows:From the laboratory Toxicity of 7 insecticides tested,the results showed that the chlorfenzoate benzoamide and hexavolon had the worst effect on the contact of eggs,with no contact effect on the eggs at different time,while the other five insecticides showed good effects.The LC₅₀ of these five insecticides when the eggs were treated after 24 hours:0.018mg/L for Matrine·Azadirachtin,0.183 mg/L for Emamectin benzoate,0.717 mg/L for Bacillus thuringiensis,5.4 mg/L for Methomyl and 12.54 mg/L for Cyhalothrin,respectively.After 48 hours postpartum,the LC₅₀ were 0.109 mg/L for Matrine·Azadirachtin,0.19 mg/L for Bacillus thuringiensis,1.25 mg/L for Methomyl,1.402 mg/L for Emamectin benzoate and1.562 mg/L for Cyhalothrin,respectively.After 72 hours postpartum,the LC₅₀ were 0.004mg/L for Bacillus thuringiensis,0.03 mg/L for Cyhalothrin,0.066 mg/L for Matrine·Azadirachtin,0.388 mg/L for Methomyl and 0.508 mg/L for Emamectin benzoate,respectively.Each insecticide also had obvious contact killing effect on the newly hatched larvae,among which the hexaflumuron had the worst efficiency.As the view of the comprehensive defense effect,all the other six insecticides had good control effect except hexaflumuron.But from the point of view of green control,Matrine·Azadirachtin and Bacillus thuringiensis,the two biological pesticides,the control effect on the egg of the armyworm are very well.The effects of various insecticides on the subsequent growth and development of the eggs were only significant in the larval survival rate and larval duration,but not in for the pupation rate,the weight of the pupa and the emergence rate.The Emamectin benzoate and the Bacillus thuringiensis effect on the larva above the second instar was better than that of other insecticides.The larval duration of each treatment was longer than that of the control group,but the difference was not significant.From the pupa rate,pupa weight and emergence rate,we can see that there is no significant difference between the treatment group and the control group,which indicates that the drug effect has no effect in the pupal stage after dipping the medicine in the egg stage.It was found that Methomyl slowed development of embryo,deformity of organ development,induced the body distortion,and eventually caused the death.Matrine·Azadirachtin made the embryo produce an abnormal embryo band,or broke and collapsed the formed embryo band.After dipping the Armyworm eggs into Matrine·Azadirachtin for 72 hours of postpartum,the body of the embryo curled up,the abdomen shrank,and finally the whole embryo dried up and died.The cause of embryo death which were treated by Bacillus thuringiensis after 24 h and 48 h was similar to that of Matrine·Azadirachtin.The effect of Cyhalothrin is mainly to delay the embryonic development time and to deform the formed insect body or break up the intestinal tract.The main effect of Emamectin benzoat is to affect the absorption of nutrients,resulting in malformation or atrophy of the insect body and making it not incubate normally.