Native Maize Resistance to Corn Earworm, Helicoverpa zea, and Fall Armyworm, Spodoptera frugiperda, with Notes on Aflatoxin Content
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Fifteen genotypes of maize, Zea mays L., varying from putatively resistant to susceptible to corn earworm, Helicoverpa zea (Boddie), and fall armyworm, Spodoptera frugiperda J.E. Smith (Lepidoptera: Noctuidae), based on visual observations in a breeding nursery, were evaluated for leaf and ear injury, yield, and aflatoxin content in an open field experiment. The genotypes were nine nontransgenic inbred lines and six hybrids including a Bt hybrid containing transgene events MON810 and MON88017. Larval development and progressive leaf injury were measured on selected genotypes in companion laboratory larval feeding and field-cage experiments. Resistance to corn earworm was confirmed for one nontransgenic inbred line in the open field as observed by significantly less ear injury with deepest penetration of larval feeding as low as 1.61 cm and total surface area as low as 1.44 cm2, compared to injury of susceptible genotypes (e.g., depth of penetration as high as 7.53 cm and surface ear injury as high as 14.16 cm2). The low injury of the putative resistant genotype approached the low ear injury seen in the transgene Bt hybrid. Fall armyworm was not abundant in the open field, and injury scores were not significant different across genotypes, ranging from 1.15 to 3.33 (using the 1-9 leaf injury scale). But putative resistance to fall armyworm was indicated in one genotype in field cages as observed by less leaf injury 7 to 24 days after artificial infestation compared with a susceptible genotype. Related to yield, kernel weight of the inbred genotypes (as low as 205 kg/ha) was significantly less than kernel weight of the hybrids (as high as 2599 kg/ha), but there was no apparent relationship of yield with plant injury. Few differences in aflatoxin content were observed, but variability was great, average 52 to 2838 ppb across genotypes. One genotype resistant to corn earworm was referred for advanced breeding and testing for resistance to multiple stresses, with the aim to include native resistance to the insects into the genetic background of high-yielding maize, including Bt hybrids, grown in the southern U.S. Further observations of three genotypes were recommended before consideration for advanced breeding.
