| Spodoptera litura belongs to the family Noctuidae of order Lepidoptera.It has wide host range,wide distribution and strong capability of migration.It is one of the most important agricultural pests on chewing leaves worldwide.This species is mainly distributed in Southeast Asia,South Asia,America,Africa and many other places,among which tropical and subtropical regions in Asia are more popular.As widely distributed in various agricultural production areas in China,it has seriously reduced the production and income of cash crops,such as cabbage,cotton,sweet potato and tobacco.The control of Spodoptera litura by insect pathogenic fungi in biological control is the future research and development direction.This article mainly studied the pathogenicity of Cordyceps sp.strains on Spodoptera litura larvae,including selecte a strain with higher virulence,optimize its culture conditions,and explore the growth on Spodoptera litura larvae.This study also determined the changes in the activity of three protective enzymes in the body after being infected,to provide a scientific basis for future research and use of this strain for biological control.The main results of this study are listed as follows:1.Screening of highly virulent Cordyceps sp.strains of Spodoptera lituraIn order to screen highly virulent strains,9 strains of Cordyceps sp.:Fenggang Cordyceps cicadae,Leishan Cordyceps tenuipes,Cordyceps cicadae SLGY-2,oblique chain,Paecilomyces cateniannulatus 1,Paecilomyces cateniannulatus 2,Paecilomyces cateniannulatus 6.Paecilomyces cateniannulatus 8,and Paecilomyces cateniannulatus12,with 3 different concentrations:1×10~6/mL,1×10~7/mL,and 1×10~8/mL,were screened by dipping method.When the concentration of the spore suspension of the Cordyceps cicadae SLGY-2 was 1×10~8/mL,the lethal rate to Spodoptera litura was53.06%on 6 days and 65.62%on 7 days,which was higher than the lethal rates of others.2.Optimization of cultivation conditions of Cordyceps cicadae SLGY-2For further research,the spore production of Cordyceps cicadae SLGY-2,a strain with high virulence to Spodoptera litura,has been screened to improve the spore production activity.In the experiment,a single factor experiment was carried out on the nutrient content and pH value of the medium,temperature and light conditions of culture.And the optimal sporulation conditions were optimized by the response surface method with the amount of sporulation as an indicator.The results showed that the nutrient content of the medium was 38.3 g,pH 6.55,temperature 25.21?,and light18.04 h.Under this condition,the spore production could reach 5.70×10~8(±0.49)/mL.The culture material is easy to obtain,and the protocol is simple,which can be used for mass production of spores.3.Effects of Cordyceps cicadae SLGY-2 on the growth and development of Spodoptera lituraIn order to explore the effect of the growth of Spodoptera litura after being infected with the Cordyceps cicadae SLGY-2,the spore suspension of insect pathogenic fungi was configured with the following three concentrations:1×10~6/mL,1×10~7/mL,and 1×10~8/mL.The 2nd instar larvae of Spodoptera litura were treated by dipping method,and the changes of larval development period,pupal stage,pupal weight,pupation rate,eclosion rate,female ratio,adult period,egg production and hatching rate were observed and recorded.The results showed that:Cordyceps cicadae SLGY-2 at different concentrations had a delayed effect on the growth and development of Spodoptera litura larvae,i.e.,the higher the concentration,the longer the larval development period.At the same time,as the concentration of the spore suspension increased,the pupa duration also prolonged,but there was no significant effect on the pupa weight.The higher the concentration,the pupation rate and emergence rate showed a significant decreasing trend The spore suspension treatment had no significant effect on the lifespan of adults,but it significantly inhibited the number of single female eggs laid by females,and the peak oviposition period was 2 to 3 d.There was no significant difference in the effects of different concentrations of spore suspension treatment on egg hatching rate.4.Determination of protective enzyme activity in Spodoptera litura after infection with Cordyceps cicadae SLGY-2The second instar larvae of Spodoptera litura were infected with different concentrations of Cordyceps cicadae SLGY-2 spore suspension,and the changes of protective enzymes in the body were measured.The second instar larvae of Spodoptera litura were immersed in three spore suspensions of Cordyceps cicadae SLGY-2 at a concentration of 1×10~6/mL,1×10~7/mL,and 1×10~8/mL for different periods of time.Oxide dismutase(SOD),catalase(CAT),and peroxidase(POD)protect the enzyme activities.After treatment with different concentrations of spore suspension,the development period of Spodoptera litura larvae was significantly prolonged,the pupation rate and emergence rate were significantly reduced,and the number of eggs laid was also significantly reduced.There was no significant effect on the pupal weight and egg hatching rate.The second instar larvae of Spodoptera litura was infected with Cordyceps cicadae SLGY-2 within 72 hours.As the conidia of the three concentrations of spore suspension germinated and proliferated in the worm,the changes in enzyme activity showed that it first increased and then decreased,and the treatment.The higher the concentration,the enzyme activity reached its peak earlier.When the concentration of the spore suspension was 1×10~8/mL,the SOD activity in the worms reached the highest value at 75.98 U/mg at 16 h,the specific activity of the enzyme was 2.49 times that of the control group,and the CAT activity was 88.46 U/mg;When treated for 24 h,the POD activity was 7.56 U/mg,and the values of the three enzyme activities were greater than the first two concentrations,the results were significantly different from the control treatment results during the same period.The results showed that when Spodoptera litura resisted the infection process of Cordyceps cicadae SLGY-2,all three protective enzymes in the insects made a stress response,and as the spore concentration of the strain increased,the enzyme activity gradually increased,but finally showed a decline.To investigate the response mechanism and change law of protective enzymes in the body of fungal spore suspensions of different concentrations infecting second-instar larvae.The 2nd instar Spodoptera litura was treated with 3.1×10~6/mL,1×10~7/mL,and1×10~8/mL Cordyceps cicadae SLGY-2 spore suspensions at 8,16,24,32,and 40 hours.After 48,56,64 and 72 hours,the activities of three protective enzymes,superoxide dismutase(SOD),catalase(CAT)and peroxidase(POD),were measured.The results showed that within 72 hours of the treatment of the 2nd instar larvae of Spodoptera litura infected with Cordyceps cicadae SLGY-2,the conidia of germination and proliferation of conidia in the worm body,and the changes in SOD activity showed a pattern of increase following by decrease,at three concentrations of spore suspension.The POD activity showed an increasing trend after 3 concentrations,and the spore suspension concentration was 1×10~6/mL.After 16 hours of treatment,the CAT activity in the worms was 55.82 U/mg,and the POD activity was highest at 24 hours.At 4.00U/mg after 56 h,the highest SOD activity was 56.69 U/mg.At a concentration of1×10~7/mL,the CAT and POD activities of the insect reached the highest at 24 h,respectively 78.38 U/mg and 4.99 U/mg,and SOD activity was maximum 66.99 U/mg when treated for 48 h.When the concentration is 1×10~8/mL,the SOD activity of the worm reaches the highest when treated for 16 h,which wass 75.98 U/mgIt was 2.49times of the control group during the same period,and the CAT activity was 88.46 U/mg,and POD activity was 7.56 U/mg at 24 hours of treatment.The values of the three enzyme activities were all greater than the two previous concentrations,and the results were significantly different from the control of the same period.The results showed that during protection against the infestation of Cordyceps cicadae SLGY-2,Spodoptera litura responded to the three protective enzymes in the worm,and the enzyme activity gradually increased with the increase of the spore concentration of the strain,but eventually showed a decline. |
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