Paul M. Buckley

Genetic Basis of Resistance to Fall Armyworm (Lepidoptera: Noctuidae) and Southwestern Corn Borer (Lepidoptera: Crambidae) Leaf-Feeding Damage in Maize

Abstract Leaf-feeding damage by first generation larvae of fall armyworm, Spodopter frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), and southwestern corn borer, Diatraea grandiosella Dyar (Lepidoptera: Crambidae), cause major economic losses each year in maize, Zea mays L. A previous study identified quantitative trait loci (QTL) contributing to reduced leaf-feeding damage by these insects in the maize line Mp704. This study was initiated to identify QTL and their interactions associated with first generation leaf-feeding damage by fall armyworm and southwestern corn borer. QTL associated with fall armyworm and southwestern corn borer resistance in resistant line Mp708 were identified and compared with Mp704. Multiple trait analysis (MTA) of both data sets was then used to identify the most important genetic regions affecting resistance to fall armyworm and southwestern corn borer leaf-feeding damage. Genetic models containing four and seven QTL explained southwestern corn borer and fall armyworm resistance, respectively, in Mp708. Key genomic regions on chromosomes 1, 5, 7, and 9 were identified by MTA in Mp704 and Mp708 that confer resistance to both fall armyworm and southwestern corn borer. QTL regions on chromosomes 1, 5, 7, and 9 contained resistance to both insects and were present in both resistant lines. These regions correspond with previously identified QTL related to resistance to other lepidopteran insects, suggesting that broad-spectrum resistance to leaf feeding is primarily controlled by only a few genetic regions in this germplasm.

Inoculation techniques used to quantify aflatoxin resistance in corn

The development of Aspergillus flavus inoculation techniques has played an important part in developing corn (Zea mays L.) germplasm resistant to aflatoxin contamination. Corn genotypes evaluated for aflatoxin resistance in field studies must be artificially inoculated due to the sporadic nature of aflatoxin contamination from year to year. A number of different inoculation techniques are used by researchers in the South and Midwest. Field inoculation techniques either wound developing kernels or leave the kernels intact. Non‐wounding techniques apply A. flavus conidia to exposed silks or silks inside the husks without damaging kernels. Wounding techniques deliver A. flavus conidia onto kernels that have been mechanically damaged. Inoculation techniques utilizing ear feeding insects to vector conidia have also been used in field studies. Environmental conditions such as ambient temperature and drought stress appear to have a significant impact on artificial inoculations. Laboratory evaluation techniques have been developed to confirm aflatoxin resistance identified in corn genotypes in the field. Color mutants and transformants of Aspergillus spp. have been used in field and laboratory studies to identify resistant genotypes. More efficient, less labor intensive, and less costly inoculation techniques need to be developed to aid in the production of aflatoxin resistant corn hybrids.

Enhancing Maize Germplasm with Resistance to Aflatoxin Contamination

Preharvest kernel infection by Aspergillus flavus and the subsequent accumulation of aflatoxin in maize grain are chronic problems in the southeastern United States. Aflatoxin is a natural carcinogen, and its presence markedly reduces the value of grain. Losses to aflatoxin contamination reach devastating levels some years. Development and deployment of maize hybrids with resistance to aflatoxin contamination is generally considered the most feasible method of reducing or eliminating the problem. Research to address the aflatoxin problem was initiated by USDA–ARS at Mississippi State, MS, in the late 1970s. The goals of the research were to identify and develop aflatoxin‐resistant maize germplasm. First, reliable techniques for screening germplasm were developed. Then, germplasm from numerous sources was screened. The release of Mp313E in 1988 was the first release of maize germplasm with resistance to aflatoxin contamination. Two other germplasm lines, Mp420 and Mp715, were released in 1991 and 1999, respectively. Additional germplasm lines have been developed, but not yet released. Efforts are currently underway to identify other sources of resistance.When used in crosses with other lines, the aflatoxin‐resistant lines markedly reduce the level of aflatoxin contamination in the resulting hybrids. Analysis of a diallel cross indicated that general combining ability was a significant source of variation in the inheritance of resistance to aflatoxin contamination. Efforts to combine resistance to aflatoxin combination and agronomic qualities using both conventional breeding methods and molecular marker assisted selection have been initiated.

Roles of amino acids, protein, and fiber in leaf-feeding resistance of corn to the fall armyworm

The free amino acids have been shown by isolational work and choice bioassays to be more important than all other factors evaluated in defining leaf-feeding resistance of corn (Zea mays L.) to fall armyworm (FAW) [(Spodoptera frugiperda J.E. Smith)] larvae. 6-MBOA (6-methoxybenzoxazolinone) and maysin, toxins present in corn, were shown not to be significant factors for leaf-feeding resistance to first-instar FAW larvae because of their low concentrations in the whorl. Amino acid analysis showed that while the ratios of the essential amino acids in susceptible (S) and resistant (R) lines were similar, there were differences in the nonessential amino acids, particularly aspartic acid, which was higher in R lines. Also, the ratio of essential amino acids to nonessential amino acids was important, being too low in expressed whorl leaf juice (obtained from V8–V10 growth stage plants) to support larval growth, although juice was stimulatory in choice tests. The total protein content of whorls in S lines was about 15% higher than in R lines, but the significance of this difference is uncertain, because nutritional tests showed that larval growth increased with total protein only up to 12% protein. Sugars were only slightly stimulatory. Thus, the amino acids along with higher hemicellulose content of R lines, established by us earlier, appear to explain much of the basis of resistance in corn to larval leaf-feeding of the FAW.

Tissue culture and its use in investigations of insect resistance of maize

Abstract Southwestern corn borer ( Diatraeagrandiosella Dyar), sugarcane borer ( Diatraea saccharalis (Fabricius)), and European corn borer ( Ostrinia nubilalis Hubner) larvae were reared for 7 days on callus initiated from two maize ( Zea mays L.) hybrids with resistance to leaf feeding by the southwestern corn borer and two susceptible hybrids. Mean weights for larvae from resistant and susceptible hybrids were 4.8 and 7.0 mg for the southwestern corn borer, 5.5 and 7.6 mg for sugarcane borer, and 7.2 and 10.2 mg for European corn borer, respectively. When larvae were allowed to choose among calli of the four hybrids, significantly fewer of each species chose to feed on the southwestern corn borer-resistant hybrids. Both antibiosis and larval nonpreference appeared to be operating as mechanisms of resistance to southwestern corn borer, sugarcane borer, and European corn borer.

Enhancing Inherent Fall Armyworm (Lepidoptera: Noctuidae) Resistance of Corn With Bacillus thuringiensis Genes

Fall armyworm, Spodoptera frugiperda (J. E. Smith), is a serious pest of corn, Zea mays L., in the southern United States. Larvae feed extensively on leaves and other parts of the plant. Germplasm exhibiting a moderate level of resistance to leaf feeding damage has been identified and released. This germplasm has been used in breeding programs for developing corn hybrids with resistance to fall armyworm and other Lepidoptera. In recent years, much effort has also been devoted to developing corn hybrids with genes from Bacillus thuringiensis (Bt) that encode insecticidal proteins. Some of these hybrids have exhibited moderate resistance to fall armyworm damage. In this investigation hybrids with both native genetic resistance and genes from Bt encoding insecticidal proteins were evaluated for resistance to fall armyworm in field tests and laboratory bioassays. Hybrids with both types of resistance sustained less fall armyworm damage than hybrids that had only native genetic resistance or genes from Bt encoding insecticidal proteins alone. Larvae that fed on hybrids with both types of resistance were significantly smaller after feeding on plants in the field or on lyophilized whorl leaf tissue in a laboratory bioassay for 10 d than larvae fed on susceptible hybrids or hybrids with only one type of resistance. Both traditional host plant resistance and transformation of corn with genes from Bt provide hybrids with moderate levels of resistance, but when used together, they are complementary. Deployment of hybrids with both types of resistance should reduce losses to fall armyworm and also reduce the rate of buildups of resistance to Bt in fall armyworm populations.

Arrestant responses of southwestern corn borer larvae to free amino acids: Structure-activity relationships

The leaf-feeding resistance of corn or maizeZea mays L. to the southwestern corn borer, SWCB,Diatraea grandiosella Dyar has been attributed at least in part to decreased protein, increased crude fiber, and increased hemicellulose in the whorls of resistant genotypes. In this study, individual amino acids and sugars were evaluated as arrestants, with the objective of identifying those that gave weak or negative responses. Several structure-activity relationships were identified. Larvae responded to three-carbonn alkyl alpha amino acids more than to two-, four-, five-, and six-carbon compounds. Amino acids with terminal isopropyl functions gave decreased responses relative to theirn-alkyl counterparts. Dicarboxylic acids and their amides gave the lowest responses of all classes of amino acids. The normally occurring basic amino acids were all good arrestants. The guanido [HN:C(NH2)NH-] function was somewhat important to an arrestant response, as was the number of methylenes between the alpha and omega amino functions of diaminon-alkyl amino acids. Hydroxy amino acids were generally good arrestants unless the hydroxyl was located on a ring system. The two sugars present in expressed corn whorl juice, glucose and fructose, gave poor responses. However, two other sugars, mannose and arabinose, whose C-2 hydroxyls are conformationally in the axial position, were strongly arrestant. Formulated amino acid mixtures based on their content in whorl juice were as strong arrestants as whorl juice. However, the relative contributions of amino acids and sugars that are weak arrestants to the resistance of corn to SWCB larvae is uncertain because amino acid analyses did not reveal significantly higher contents of these amino acids in the whorl juices of resistant lines.

Fall Armyworm (Lepidoptera: Noctuidae) and Southwestern Corn Borer (Lepidoptera: Crambidae) Leaf Feeding Damage and Its Effect on Larval Growth on Diets Prepared from Lyophilized Corn Leaves 1

Abstract Fall armyworm, Spodoptera frugiperda (J.E. Smith), and Southwestern corn borer, Diatraea grandiosella Dyar, are economically important pests of maize (Zea mays L.) in the southeastern United States. These insects attack plants in both the vegetative and reproductive stages of growth. Plant resistance is widely considered a desirable means for reducing losses to both insects, and corn germplasm lines with resistance to leaf feeding damage have been developed and released. Fall armyworm and southwestern corn borer larvae feeding on resistant genotypes grow more slowly than those feeding on susceptible genotypes. The objectives of the investigation were to evaluate 20 single cross maize hybrids for leaf feeding damage by southwestern corn borer and fall armyworm, to compare larval growth on laboratory diets prepared from lyophilized leaf tissue of single cross hybrids with different levels of resistance to leaf feeding, and to determine whether larval growth differed between diets prepared from leaf tissue collected from plants previously infested with southwestern corn borer or fall armyworm and non-infested plants. Both fall armyworm and southwestern corn borer larvae weighed significantly less when fed on laboratory diets prepared from lyophilized leaf tissue of resistant single cross hybrids than susceptible hybrids. When tissue from either resistant or susceptible plants that had been infested with either insect in the field was used in bioassays, larval weights were further reduced. It appears that both constitutive and induced defensive mechanisms may be operating.

Diallel analysis for aflatoxin accumulation and fall armyworm leaf-feeding damage in maize

ABSTRACT Two major impediments to profitable maize, Zea mays L., production in the southern United States are from feeding by fall armyworm, Spodoptera frugiperda (J.E. Smith), and losses from the production and accumulation of aflatoxin in maize grain. A diallel cross was produced by making all possible crosses among five germplasm lines developed as sources of resistance to fall armyworm leaf feeding and five lines developed as sources of resistance to aflatoxin accumulation. For resistance to both leaf feeding and aflatoxin accumulation, general combining ability (GCA) was a significant source of variation. Specific combining ability (SCA) was significant for fall armyworm feeding only. Estimates of GCA effects for reduced aflatoxin accumulation were significant for Mp715 and Mp719, two lines selected for resistance to aflatoxin accumulation. The GCA effects for reduced fall armyworm damage were significant for all five lines selected for fall armyworm resistance: Mp707, Mp708, Mp713, Mp714, and Mp716.