Lethal And Sublethal Effects Of Pesticides On Third Instar Larvae Of Fall Armyworm Spodoptera Frugiperda(J.E.Smith)(Lepidoptera: Noctuidae)

Maize is a major crop in China,with a planting area and production of 16 million hectares and 216 million tons.It is one of the major cereal crops that plays an important role in increasing the overall production capacity of grain in China.Among the cereal crops grown in China,maize ranks first in the planting area.However,the invasion of the fall armyworm,Spodoptera frugiperda（J.E.Smith,1797）,belonging to the order of Lepidoptera and the Noctuidae family,has become the most devastating pests in many regions since 2019.This pest has become a major challenge to maize production,as well as other crops such as rice since it was discovered in China.Management of this pest in china has been highly dependent on insecticides,which could lead to overuse and abuse,by farmers,and thus bring in health hazard and environmental pollution.In the heat of the invasion,most of these insecticides were recommended without proper bio-efficacy testing against Spodoptera frugiperda.The current study sought to investigate the lethal and sublethal effects of methoxyfenozide,emamectin benzoate,and chlorantraniliprole on fall armyworm S.frugiperda in the laboratory to inform the proper use of the insecticide mentioned above in the field.The following are the main research contents and results:The toxicity of Methoxyfenozide,Emamectin benzoate,and Chlorantaniliprole on third instar larvae of S.frugiperda.The Leaf-dip bioassay results showed that methoxyfenozide was effective against third instar larvae of Spodoptera frugiperda,with the LC₅₀ 0.466μgm L^-1,the LC₁₀ and the LC₃₀ were 0.047 and 0.182μgm L^-1,respectively.The emamectin benzoate showed a high level of toxicity against third instar larvae of Spodoptera frugiperda.The LC₅₀ after 24 h exposure was 0.007μgm L^-1,and the LC₁₀and LC₃₀ values were 0.001 and 0.003μgm L^-1.Our results also showed that chlorantraniliprole was active on the third instar larvae of Spodoptera frugiperda.The LC₅₀ after 24h exposure was 1.217μgm L^-1H₂O,and the LC₁₀and LC₃₀ values were also 0.135 and 0.494μgm L^-1H₂O,respectively.Sublethal effects of Methoxyfenozide on the life cycle of Spodoptera frugiperdaThe LC₁₀and LC₃₀ obtained in the previous experiment were used to test the sublethal effects.In the study of sublethal effects using leaf dip bioassay,only 20%and 32%of the third instar treated with methoxyfenozide at LC₁₀ and LC₃₀ became pupae after 48h.The sublethal concentration of methoxyfenozide also induced changes in the life cycle.Specifically,Spodoptera frugiperda require more time to complete their development,with larval duration was prolonged to 12.38 days for LC₁₀ and even much longer for LC₃₀（17.11days）.The LC₁₀ did not have any effect on the pupae weight of both males and females.However,the LC₃₀ increased the male pupae weight to 0.11 and decreased the female pupae weight to 0.08g.Pupae duration was also increased to 14.05±0.07 by LC₃₀when compared to the control.There were 8%and 15%deformed pupae in the LC₁₀ and LC_30,and adult duration was decreased to 16.95 to 14.09 days for LC₁₀ and LC₃₀.The fecundity of FAW under LC₁₀and LC₃₀was 25.69 eggs/female,significantly decreased compared with the control（36.65 eggs/female）.The fertility was also decreased.LC₁₀was 38%,and LC₃₀was 29%when compared with the control group 98%.Sublethal effects of Emamectin benzoate on the life cycle of Spodoptera frugiperdaThe sublethal effects of emamectin benzoate after 48h exposure of third instar larvae of Spodoptera frugiperda were investigated.The larval mortality for LC₁₀ was 17%,and LC₃₀ was 43%,larval duration was increased to 15.74 days for LC₁₀ and 21.89 days for LC₃₀.The LC₃₀increase the male pupa weight to 0.11±0.02 and decrease the female pupa weight to 0.05±0.01 when compared with the control group and the LC₁₀.The pupa duration was increased to 10.02 days for LC₁₀ and 13.08 days for LC₃₀.Deformed pupae were 27%at LC₃₀,much higher than that at LC₁₀（15%）.The adult duration was reduced in the LC₃₀ to 12.05 days,and LC₁₀ was 13.94days compared to the control groups.Fecundity was also affected,the number of an egg laid per female are far lower in the LC₃₀ 105.98 egg/female and LC₁₀ was 144.55±6.91 egg/female.Emamectin benzoate also affected fertility.The number of viable larvae was fewer in the LC₃₀,50%,and LC₁₀,81%compared to the control group 98%.Sublethal effects of Chlorantaniliprole on the life cycle of Spodoptera frugiperdaThe larval mortality after 48h exposure of Spodoptera frugiperda was 15%and 38%for LC₁₀and LC_30,respectively.Treated larvae exhibited an increase in larval duration for LC₁₀ 12.98 days and LC₃₀ 16.34 days.The male pupal weight was not affected in both concentrations when compared with the control group.Still,the female pupal weight was higher 0.09±0.03 in the LC₁₀than the LC₃₀ 0.06±0.02,and the pupal duration was also increased in both concentration LC₁₀10.02±0.07 and LC₃₀ 13.02±0.09days when compared with the control group 7.99±0.06days.Deformed pupae were 14%and 26%for the LC₃₀ and LC₁₀.The adult duration was decreased to 12.96 days for LC₃₀ and 15.93days for LC₁₀ compared with the control group.The fecundity was decreased,with the females producing 126.20±9.56 eggs/female and 105.98±16.78 eggs/female at LC₁₀ and LC_30,respectively.Fertility was also affected in both concentrations.The number of viable larvae was 59%for LC₁₀ and 50%for LC_30,respectively.