Selection,effective dominance,and completeness of Cry1A.105/Cry2Ab2 dual-protein resistance in Helicoverpa zea(Boddie)(Lepidoptera:Noctuidae)

In the U.S.,Helicoverpa zea(Boddie)is a major pest targeted by both transgenic maize and cotton expressing Bacillus thuringiensis(Bt)proteins.Resistance of insect to Bt maize and cotton containing cry1A and cry2A genes has widely occurred in the U.S.In this study,two trials were performed to investigate larval survival and development of a Cry1A.105/Cry2Ab2 dual-protein resistant(VT2P-RR),a susceptible,and an F1 heterozygous(VT2P-RS)populations of H.zea on ears of nine Bt and three non-Bt maize hybrids.The Bt maize hybrids evaluated represent five common pyramided traits expressing two or three of the Cry1A.105,Cry1Ab,Cry1F,Cry2Ab2,and Vip3Aa20 proteins.In the laboratory,neonates of the three H.zea populations were inoculated on silks of ears collected from maize at R1-R2 plant stages;and larval survivorship was checked 10 d after neonate release.All three insect populations survived normally on non-Bt maize ears.Varied numbers of VT2P-RR and VT2P-RS survived on ears of Cry1A.105/Cry2Ab2 maize,while all larvae of the three populations died or could not develop on ears of Vip3Aa20-expressing maize.The results demonstrated that the dual-protein resistant H.zea was not cross-resistant to Vip3Aa20-expressing maize,and thus traits with vip3Aa20 gene should be effective to manage Cry1A.105/Cry2Ab2-resistant H.zea.The resistance in VT2P-RR was determined to be incomplete on Cry1A.105/Cry2Ab2 maize.The effective dominance levels varied greatly,from recessive to incompletely dominant,depending on maize hybrids and trials,suggesting that proper selection of maize hybrids could be important for mitigating the Cry1A.105/Cry2Ab2 resistance.The data generated should aid in modeling multiple-protein Bt resistance in H.zea.

Performance of Bt-susceptible and -heterozygous dual-gene resistant genotypes of Spodoptera frugiperda (J.E.Smith) (Lepidoptera:Noctuidae) in seed blends of non-Bt and pyramided Bt maize

A seed blend refuge has been implemented in the U.S.Corn Belt for Bt maize resistance management.The fall armyworm,Spodoptera frugiperda(J.E.Smith),is a target pest of Bt maize in the Americas.The larvae of this pest are mobile,which may affect the efficacy of seed blend refuges.In this study,field and greenhouse trials were conducted to determine the performance of Bt-susceptible(aabb)and-heterozygous dual-gene-resistant(AaBb)genotypes of S.frugiperda in seed blends of non-Bt and pyramided Bt maize.Three field trials evaluated larval survival,larval growth,and plant injury with aabb in seed blends of Bt maize expressing Cry1A.105/Cry2Ab2/Vip3A with 0–30%non-Bt seeds.Greenhouse tests investigated the performance of aabb and AaBb in seed blends of Cry1A.105/Cry2Ab2 with 0–30% non-Bt seeds.In pure non-Bt maize plots,after 9–13 d of neonates being released on the plants,0.39 and 0.65 larvae/plant survived with leaf injury ratings of 4.7 and 5.9(Davis's 1–9 scale)in the field and greenhouse,respectively.In contrast,live larvae and plant injury were virtually not observed on Bt plants across all planting patterns.Larval occurrence and plant injury by aabb on non-Bt plants were similar between seed blends and pure non-Bt plantings,suggesting that the blended refuges could provide an equivalent susceptible population as structured refuge under the test conditions.In the greenhouse,the two insect genotypes in seed blends performed similarly,indicating that the seed blends did not provide more favorable conditions for AaBb over aabb.The information generated from this study should be useful in managing S.frugiperda and evaluating if send blends could be suitable refuge options for Bt resistance management in the regions where the insect is a primary target pest.