Insecticidal Effect Of Bt Toxin Cry51Aa1 To Apolygus Lucorum And Adelphocoris Suturalis

Apolygus lucorum and Adelphocoris suturalis(Heteroptera:Miridae)are two main pests of the cotton mirid bugs.Before Bt cotton planting,the mirid bugs were a group of secondary insect pests in the cotton field and were controlled by insecticides to be used to kill the important pests.With the widespread adoption of Bt cotton,the target insect pest,Helicoverpa armigera,was effectively suppressed then the amount of insecticide used in the cotton field was decreased.Consequently the succession of insect pest complex in Bt cotton field is present.The mirid bugs become the important pests and cause serious damage to cotton and many other crops.But there is no other effective method to control the mirid bugs except the use of insecticide.It is obviously that the chemical control has the resistance and environment problem.So how to control the mirid bugs becomes the important task in the Bt cotton ecosystem.Cry51Aa1 is a novel aerolysin-type ?-pore-forming toxin(aerolysin-type ?-PFT)which was identified in a Bt strain F14-1 from northern China and has different structure with those 3-D Cry toxin.This toxin has been proven with the insecticidal toxicity against Leptinotarasa decemlineata.Cry51Aa2 with 97.7%sequence identity with Cry51Aa1 has insecticidal toxicity against L.decemlineata and the mirid bugs,Lygus hesperus and Lygus lineolaris.In this study,the toxicity of Cry51Aa1 against A.lucorum and A.suturalis was detected by the diet incorporation bioassay and the impacts on the development of nymph and the proteinase activity of adult were measured.The results indicate that Bt toxin Cry51Aa1 has relatively high insecticidal toxicity against the nymph of mirid bugs and could be used in the control of hemipteran insect pests in the future.1.Insecticidal toxicity of Cry51Aal to A.lucorum and A.suturalisTo evaluate the insecticidal toxicity of Cry51Aa1,the bioassay was conducted on neonates of susceptible A.lucorum and A.suturalis by a diet incorporation bioassay,using the new invented feeding facility for the small sucking pest.In this study,other Bt toxins(Cry1Ac and Cry2Ab)and chemical insecticides(Abamectin,Imidacloprid andBeta-cypermethrin)were used as control.The LC50 of Cry51Aal against A.lucorum and A.suturalis are 10.180 ?g·mL-1 and 8.760 ?g·mL-1,respectively.Cry1Ac and Cry2Ab have on insecticidal toxicity against the two mirid bugs.Compared with the LC50 of the chemical insecticides,Cry51Aal has little lower toxicity against the neonate of A.lucorum than Abamectin(LC50=8.793 ?g·mL-1)but higher toxicity than Imidacloprid(LC50=18.156?g·mL-1)and Beta-cypermethrin(LC50=26.372 ?g·mL-1);Cry51 Aal has little lower toxicity against the neonate of A.suturalis than Beta-cypermethrin(LC50=6.455 ?g·mL-1)but higher toxicity than Imidacloprid(LC50=18.964 ?g·mL-1)and Abamectin(LC50=29.377?g·mL-1).The results mean that Cry51Aa1 has relatively high insecticidal toxicity against the two mirid bugs2.Biological impact of Cry51Aa1 to A.lucorum and A.suturalisIn order to evaluate the biological impact of Cry51Aal on the development of A.lucorum and A.suturalis,the nymphs of A.lucorum and A.suturalis were fed on the diet with low concentration Cry51Aa1(LC10)from neonate to adult.Those developmental parameters such as the nymph developmental period of each instar,the survival rate of nymph,the eclosion rate and the average weight of adult are no difference with those nymphs fed on the normal diet.The whole developmental period of nymph has about 0.5 d extention and it is statistically different.Feeding on the low concentration Cry51Aa1 has litte effect on the nymph development of the two mirid bugs.3.Impact of Cry51Aa1 on the proteinase activity of A.lucorum and A.suturalisThe activities of three proteinases of the adults which emergenced from the nymphs fed on the low concentration Cry51Aal were measured.The trypsin-like activity in A.lucorum is no significantly different with the control.The activities of chymotrypsin-like and aminopeptidase N in A.lucorum are significantly lower than the control.Meanwhile the activities of trypsin-like,chymotrypsin-like and aminopeptidase N in A.suturalis are all significantly lower than the control.It could be found that feeding on the low concentration Cry51Aa1 will decrease the activities of proteinases which are associated with the digestion and absorption of nutrient substances and some physiological activities.4.Interaction between Cry51Aa1 and Cry2Ab against A.lucorum and A.suturalisIn order to assess the interaction between Bt toxins Cry51Aa1 and Cry2Ab against A.lucorum and A.suturalis,a series of bioassays of the two Bt toxins singly and in combinations with different concentrations were conducted.The expected and observed mortality of each combination were compared and analysed by Student's t-test.For the two mirid bugs no combination was synergism in the 25 combinations.In the 25 combinations of A.lucorum,16%was antagonism and 84%was independent.In the 25 combinations of A.suturalis,4%was antagonism and 96%was independent.The result means that there is no interaction between Cry51Aa1 and Cry2Ab against the two mirid bugs.and it suggests that the mode of action of the two toxins to mirid bugs may be independent.