Genetic basis of resistance to fall armyworm (Lepidoptera: Noctuidae) and southwestern corn borer (Lepidoptera: Crambidae) leaf-feeding damage in maize

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.     

Impact of plant resistance on southwestern corn borer (Lepidoptera: Crambidae) biology and plant damage

Southwestern corn borer, Diatraea grandiosella Dyar (Lepidoptera: Crambidae), is a major insect pest of corn, Zea mays L., in the southern United States. Germplasm lines with resistance to southwestern corn borer have been developed and released by the USDA-ARS. Two single-cross hybrids produced by crossing germplasm lines with resistance to southwestern corn borer and a susceptible single-cross hybrid were infested with southwestern corn borer larvae in a 2-yr field test conducted in Mississippi. The susceptible hybrid sustained significantly more leaf damage and stalk tunneling than either resistant hybrid. The number of tunnels and the length of tunneling were significantly lower on the resistant hybrids. In 2003, up to 15 times more tunneling was observed on the susceptible hybrid. Larvae feeding on the resistant hybrids were delayed in their movement from the whorl to the stalk and larval survival was 50% lower on the resistant hybrids than on the susceptible hybrid. Larvae recovered from the susceptible hybrid 7-14 d after infestation weighed twice as much as those recovered from the resistant hybrids. Similar differences in larval weight were observed in the laboratory when larvae were reared on diets prepared from lyophilized tissue from the three hybrids. These results provide a foundation for other investigations designed to identify and determine the roles of specific genes and gene families associated with southwestern corn borer resistance in corn.     

Management of feeding damage and survival of southwestern corn borer and sugarcane borer (Lepidoptera: Crambidae) with Bacillus thuringiensis transgenic field corn

The efficacy of transgenic corn hybrids expressing an insecticidal Bacillus thuringiensis (Bt) delta-endotoxin from different transformation events was evaluated in field corn, Zea mays L., against the southwestern corn borer, Diatraea grandiosella Dyar, and sugarcane borer, Diatraea saccharalis (F.). Susceptibilities of neonates and third instars were determined on Bt and non-Bt corn plants (V6 and R1 stages) in field plots and corn leaf tissue feeding exposure in laboratory bioassays. Bt corn hybrids associated with MON810 and CBH351 transformation events sustained significantly less injury by southwestern corn borer and sugarcane borer during mid-whorl stage infestations compared with their respective non-Bt hybrid equivalents. Southwestern corn borer and sugarcane borer feeding injury to ear leaf-sheath and husk tissues during the silking stage of corn was significantly reduced in MON810 and CBH351 Bt corn compared with their respective non-Bt hybrids. However, resistance levels to feeding injury in Bt hybrids associated with the MON810 event were significantly higher than that in the hybrid associated with the CBH351 event. Southwestern corn borer and sugarcane borer caused more feeding injury to husk tissue than to ear leaf-sheath tissue in both Bt and non-Bt hybrids infested during the silking stage. Laboratory performance of the MON810 event against southwestern corn borer and sugarcane borer varied among hybrids associated with the same event. Third instars of southwestern corn borer were highly susceptible to MON810 Bt corn hybrids in leaf tissue experiments. However, sugarcane borer larvae were susceptible to the MON810 event only in one of the Bt hybrids evaluated. Sugarcane borer mortality was significantly lower after 96 h of feeding exposure on CBH351 Bt corn leaf tissue than on MON810 Bt corn leaf tissue. Plant resistance to southwestern corn borer and sugarcane borer increased as plants matured, independent of the presence of a Bt construct. These results are essential to estimate the importance of Bt transgenic corn in areas of southern United States and other areas where mixed populations of southwestern corn borer and sugarcane borer are predominant and cause severe damage to corn production.     

Breeding for resistance to aflatoxin accumulation in maize

Contamination of maize,Zea mays, grain with aflatoxin, a naturally occurring toxin produced byAspergillus flavus, frequently reduces the value and marketability of maize produced in the southern USA. Drought, high temperatures, and insect damage are often associated with high levels of maize aflatoxin contamination. Growing resistant maize hybrids is generally considered the most feasible method of reducing or eliminatingA. flavus infection and subsequent accumulation of aflatoxin. Developing appropriate screening techniques and identifying maize germplasm with resistance to aflatoxin contamination provides the foundation for a breeding program. Only a few sources of aflatoxin resistance have been identified. Four germplasm lines (Mp313E, Mp420, Mp715, and Mp717) have been developed and released by USDA-ARS at Mississippi State University. NC 388, developed at North Carolina State University, is reported as another putative source of aflatoxin resistance. Conventional phenotypic selection was used to successfully combine resistance to aflatoxin contamination from two of these lines, Mp313E and Mp715, with desirable agronomic qualities from Va35. The identification of quantitative trait loci (QTL) associated with resistance to aflatoxin contamination will also permit the use of marker assisted selection in transferring resistance into elite germplasm lines. Development of parental inbreds that combine aflatoxin resistance with superior agronomic quality is an essential component of a hybrid maize breeding program designed to reduce or eliminate aflatoxin contamination.     

Plant resistance and its effect on the peritrophic membrane of southwestern corn borer (Lepidoptera: Crambidae) larvae

The southwestern corn borer, Diatraea grandiosella Dyar (Lepidoptera: Crambidae), is a serious pest of corn, Zea mays L., in the southern United States. Corn germplasm lines with conventional genetic leaf-feeding resistance to this pest, the fall armyworm, Spodoptera frugiperda (J.E. Smith), and other lepidopterans have been released to the public by USDA-ARS scientists located in Mississippi. Recent studies suggest the insect resistant lines disrupt the integrity of the peritrophic membrane of the fall armyworm. The objectives of this study were to investigate any morphological differences in the structure of the peritrophic membrane of southwestern corn borer larvae feeding on resistant and susceptible corn hybrids and to quantify the damage. Larvae were reared under field and laboratory conditions on three corn hybrids (two resistant and one susceptible). Scanning electron microscopy was used to examine the peritrophic membrane for abnormalities such as holes or tears and to count the holes or tears in the membrane. Differences in the degree of damage to peritrophic membrane of larvae fed on resistant and susceptible plants were not detected. Up to five distinct layers of the membrane were observed in each larva. Variation in the amounts of damage to the peritrophic membrane observed from larvae feeding on all plant material was high. Plant resistance adversely affects growth and development of southwestern corn borer larvae, and further investigations are needed to explain the role of plant resistance and its relation to peritrophic membrane in southwestern corn borer larvae.     

Plant-incorporated Bacillus thuringiensis resistance for control of fall armyworm and corn earworm (Lepidoptera: Noctuidae) in corn

Fall armyworm, Spodoptera frugiperda (J.E. Smith), and corn earworm, Helicoverpa zea (Boddie), perennially cause leaf and ear damage to corn, Zea mays L., in the southeastern United States. Transgenic Bacillus thuringiensis (Bt) hybrids with the Bt11, MON810, or 176 events expressing the Cry1Ab insecticidal endotoxin from were evaluated for control fall armyworm and corn earworm at seven locations in Georgia during 1999 and 2000. Corn was planted at the recommended time for each location and 1 and 2 mo later in the southern locations. All Bt events consistently reduced whorl infestation and damage, although event 176 did not prevent whorl damage in the later plantings in the southern locations in both years. All events also reduced seedling damage by the lesser cornstalk borer, Elasmopalpus lignosellus (Zeller), in one trial and stalk infestations and tunnel length by southwestern corn borers, Diatraea grandiosella Dyar, in another trial. Hybrids containing Bt11 and MON810 events reduced ear infestations in all trials, although reductions were small in later plantings. Nevertheless, both events reduced grain damage from earworms and armyworms by an average +/- SE of 52.5 +/- 5.1% in all trials. The hybrid containing event 176 did not reduce ear infestations and damage. Total grain aflatoxin concentrations were not significantly affected by Bt resistance in any trial (N = 17). Yield responses were variable with the prevention of yield loss being proportional to the severity of insect damage. Although plantings made after the recommended time did not consistently benefit from Bt resistance, Bt11 and MON810 events were effective in reducing damage to field corn when large infestations occurred. The Bt11 and MON810 events mitigated the risk of severe lepidopteran damage to corn, thereby making later plantings of corn feasible in double-cropping systems.     

Performance of transgenic corn hybrids in Missouri for insect control and yield

The efficacy of Bacillus thuringiensis-transformed corn (Zea mays L.) hybrids compared with comparable nontransformed corn hybrids for controlling first- and second-generation European corn borer, Ostrinia nubilalis (Hübner), and second-generation southwestern corn borer, Diatraea grandiosella Dyar, was determined. Yield comparisons were obtained from the same plots of corn hybrids. Both generations of European and the second-generation of southwestern corn borer were effectively controlled, but the Bt hybrids varied in degree of control. Hybrids from Ciba Seeds, DEKALB, and Mycogen had more European corn borer tunneling than those from Novartis or Cargill, and this was generally ascribed to different transgenic events. The Bt-transformed hybrids had virtually no leaf-feeding damage and less tunneling than the non-Bt corn hybrids. Some Bt corn hybrids had no tunneling, whereas other Bt hybrids had a small amount of tunneling. All of the non-Bt hybrids had significant leaf-feeding damage and stalk tunneling from both insects. Only three live European corn borer larvae (stunted) were found in the Bt corn hybrids while splitting stalks to assess tunnel length. When insect damage was significant, and in some evaluations where damage was not significant, differences in yields among hybrids were observed. No significant insect population differences were observed for five genera of beneficial insects for Bt versus non-Bt corn hybrids. Corn hybrids that have been transformed with the Bt gene provide an effective means of control for corn borers and efforts to reduce the likelihood of development of borer resistance are warranted.     

Occurrence of aflatoxin in three maize (<Emphasis Type="Italic">Zea mays</Emphasis> L.) hybrids over 5 years in Northern Mississippi

Aflatoxins are produced as secondary metabolites under conducive climatic conditions by Aspergillus flavus. The incidence of aflatoxin varies with environmental conditions, genotype, and location. An expanded understanding of the interaction of the plant, fungus, and weather conditions is needed to further elucidate the field infection process of maize by A. flavus and subsequent aflatoxin contamination. One of the problems in evaluating maize hybrids for resistance to kernel infection and aflatoxin contamination is identifying a time period and environmental conditions that are most advantageous. Three maize genotypes (Pioneer Brand 3223, Mo18W × Mp313E, and Mp313E × Mp420) were evaluated from 1998 to 2002 in response to A. flavus inoculation and aflatoxin contamination and weather conditions favorable for aflatoxin contamination were identified. The highest aflatoxin levels were observed in 1998 and 2000 (1186 and 901 ng g⁻¹; P < 0.0001); while the lowest levels were detected in 1999 (39 ng g⁻¹). Pioneer 3223 had significantly higher levels (1198 ng g⁻¹) than Mp313E × Mp420 (205 ng g⁻¹), and Mo18W ×Mp313E (161 ng g⁻¹; P < 0.0001). The hybrids had six weather-related variables in common that were positively correlated with aflatoxin accumulation. Four of these occurred during 65–85 days after planting and were temperature-related. These results suggest that regardless of the hybrid’s maturity or physiological development, the time from 65 to 85 days after planting may be indicative of a period of stress which leads to greater aflatoxin accumulation at harvest. The U.S. Government's right to retain a non-exclusive, royalty-free license in and to any copyright is acknowledged.     

Quantitative trait loci (QTL) for reducing aflatoxin accumulation in corn

Aflatoxin produced by Aspergillus flavus in corn poses significant health risks to both humans and livestock. Exploitation of host-plant resistance in breeding programs is a sustainable way to minimize aflatoxin contamination. Identification of quantitative trait loci (QTL) associated with resistance to aflatoxin accumulation in kernels can accelerate development of aflatoxin-resistant corn using marker-assisted selection. An F_2:3 mapping population, developed from a cross involving a resistant inbred Mp715 and a susceptible inbred B73, was evaluated in replicated field trials with developing ears artificially inoculated with A. flavus for 2 years to identify QTL for reduced aflatoxin accumulation. Using composite interval mapping, 6 to 7 QTL for aflatoxin content were identified in both years with contribution of individual QTL ranging from <1 to 10% of phenotypic variation. More QTL were detected for husk coverage with phenotypic variance range of <1 to 16% explained by individual QTL. Both B73 and Mp715 alleles at these QTL loci contributed toward resistance. Husk coverage and aflatoxin levels were significantly correlated in both years. Our findings were further supported by overlapping of QTL for husk coverage ratings in four genomic regions on chromosomes 4, 8, and 10, where aflatoxin resistance QTL were reported in previous studies. Since most of the QTL were of low to moderate effects, pyramiding of these QTL may lead to enhanced resistance to aflatoxin accumulation in corn.     

Combining ability in maize for fall armyworm and southwestern corn borer resistance based on a laboratory bioassay for larval growth

The fall armyworm, Spodoptera frugiperda (J. E. Smith), and southwestern corn borer, Diatraea grandiosella Dyar, are major insect pests of maize, Zea mays L., in the southern USA. Both insects feed extensively on leaves of plants in the whorl stage of growth. A diallel cross of seven inbred lines with different levels of susceptibility to leaf feeding damage in the field was evaluated in a laboratory bioassay for fall armyworm and southwestern corn borer larval growth. Diets were prepared from lyophilized leaf tissue of field-grown plants of the inbred lines and their 21 F₁ hybrids. One inbred line, Tx601, exhibited heavy leaf damage in field tests but showed moderate resistance in the laboratory bioassay. Both general and specific combining ability were highly significant sources of variation in the inheritance of fall armyworm and south-western corn borer larval growth in the laboratory bioassay. Tx601 showed excellent general combining ability for reduced larval growth of both species.This article is a contribution of the United States Department of Agriculture, Agricultural Research Service, in cooperation with the Mississippi Agricultural and Forestry Experiment Station. Journal No. J-8525     

Survival of kansas dipel-resistant European corn borer (Lepidoptera: Crambidae) on Bt and non-Bt corn hybrids

The Kansas Dipel-resistant and susceptible European corn borer, Ostrinia nubilalis (Hübner), were evaluated in the greenhouse on different Bt transgenic events expressed in corn hybrids. There were important differences in the resistance offered by the different Bt event corn hybrids. Hybrid comparison tests indicate that these Dipel-resistant first-instar European corn borer were not able to survive to adulthood on whorl-stage MON810, Bt11, or 176 Bt event corn plants. Third instars did not survive to adulthood on whorl-stage MON810 or Bt11 event corn plants but a small number of fifth instars were found on whorl-stage DBT418 plants infested with Dipel-resistant larvae. First and third instars of these Dipel-resistant European corn borers caused more leaf-feeding damage and more tunneling on whorl-stage Bt-corn plants than did the Dipel-susceptible European corn borers. However, in the single Bt corn hybrid test, there was no survival of the Dipel-resistant European corn borers on DK580BtX or MAX454 Bt plants 35 to 42 d after they had been infested with first instars. These results demonstrate that the current Kansas selection of Dipel-resistant European corn borer strain cannot establish reproducing populations in the tested Bt corn lines and hybrids.     

Aflatoxin Contamination of Preharvest Corn as Influenced by Timing and Method of Inoculation

Four corn (Zea mays L.) hybrids were grown in 1977 and 1978 and inoculated with Aspergillus flavus Link 20 or 40 days after silking. Inoculation methods included needle, knife, and multiple-puncture injury to the kernels. The level of aflatoxin contamination, insect damage to the ear, and the percentage of ears having visible greenish A. flavus Link-type mold were determined. Differences among hybrids were not significant for any of the three characteristics measured, although aflatoxin levels of the early-maturing, loose-husked hybrids were approximately twice as high as those of two later-maturing, tight-husked types. Differences among treatments for insect damage rating were not statistically significant. Delaying inoculation until 40 days after silking significantly reduced the aflatoxin contamination level of samples harvested at maturity. Fewer than one-half the ears inoculated at 40 days after silking (35.3%) exhibited visible signs of infection compared with ears inoculated 20 days after silking (82.9%). The needle inoculations were less effective in eliciting aflatoxin production (163 μg/kg and 45.1% visibly infected ears) than were knife (202 μg/kg and 61.8% visibly infected ears) and multiple puncture (305 μg/kg and 70.4% visibly infected ears) methods of inoculation.     

Frequency and severity of western bean cutworm (Lepidoptera: Noctuidae) ear damage in transgenic corn hybrids expressing different Bacillus thuringiensis cry toxins

The frequency and severity of corn ear damage caused by western bean cutworm, Striacosta albicosta (Smith), were measured on transgenic corn, Zea mays L., hybrids expressing two different insecticidal Bacillus thuringiensis (Bt) (Berliner) Cry toxins (Bt) selected to protect against damage caused by larval European corn borer, Ostrinia nubilalis (Hübner). A field cage experiment deliberately infested with western bean cutworm egg masses resulted in less damage in the hybrid expressing the Cry1F protein and supported fewer western bean cutworm larvae than its non-Bt isoline. Corn hybrids expressing Cry1F, grown in small plot field experiments at three locations over two separate years and exposed to natural western bean cutworm infestations suffered less damage than non-Bt or Bt-hybrids expressing a Cry1Ab protein. Later maturing hybrids suffered more damage than shorter-season hybrids. Finally, corn ears observed in strip trials for several years in diverse agronomic conditions in farmer-cooperator fields corroborated the in-plant protection conferred by corn hybrids expressing the Cry1F protein in small plot field trials.     

Establishment of Cry9C susceptibility baselines for European corn borer and southwestern corn borer (Lepidoptera: Crambidae)

In 1997 and 1998, Cry9C susceptibility baselines were established for field-collected populations of European corn borer, Osrinia nubilalis (Hubner), and southwestern corn borer, Diatraea grandiosella Dyar. Bioassay of neonate European corn borer larvae of 16 colonies collected from the midwestern United States indicated LC50 values ranging from 13.2 to 65.1 ng of Cry9C protein per square centimeter. Neonate European corn borer LC50 values ranged from 46.5 to 214 ng/cm2. Neonate larvae of three colonies of southwestern corn borer collected from the southern and southwestern United States exhibited LC50 values from 16.9 to 39.9 ng of Cry9C protein per square centimeter. Southwestern corn borer neonate LC90 confidence limit values ranged from 40.3 to 157 ng of Cry9C protein per centimeter. The most sensitive southwestern corn borer colony was collected from the Mississippi delta exhibiting an LC50 value of 22.6 ng of Cry9C per cm2 and also displayed the widest LC0 confidence limits of 40.3-94.8 ng of Cry9C per cm2. Geographic baseline susceptibility data establishes the natural genetic variation and provides the foundation for future testing of insect populations exposed to increased use of Bacillus thuringiensis-based crops. Insect resistance management and stewardship of Cry9C will rely upon baseline data for the validation of discriminating dose assays for European corn borer and southwestern corn borer.     

Use of transgenic Bacillus thuringiensis Berliner corn hybrids to determine the direct economic impact of the European corn borer (Lepidoptera: Crambidae) on field corn in eastern Canada

Transgenic corn expressing Bacillus thuringiensis Berliner (Bt corn) (Maximizer and Yieldgard hybrids, Novartis Seeds), non-Bt isolines and high-performance (check) hybrids were evaluated for European corn borer, Ostrinia nubilalis (Hübner), damage and grain yield in commercial strip plots across Ontario in 1996 and 1997. Bt corn hybrids reduced stalk tunneling damage by 88-100%. In 1996, minimal damage was found in locations where only one generation of European corn borer occurred per year. Bt corn proved its greatest potential for reducing the number and length of cavities below the primary ear in locations where two generations of European corn borer were present. A yield response to using Bt hybrids only occurred when levels of tunneling damage exceeded 6 cm in length. European corn borer infestations resulted in a 6 and 2.4% reduction in yield for 1996 and 1997, respectively, when Bt hybrids were compared with their non-Bt isolines. A linear relationship was found between tunnel length per plant in centimeters (x) and yield protection (%) obtained from using Bt corn (y) (y = 1.02 + 0.005x, r2 = 0.7217). At a premium of $34.58 Canadian (CDN) perhectare for Bt corn seed, an infestation of at least 6 cm of corn borer tunneling per plant was required to break even at a market price for corn of $2.50 per bushel CDN. During the period of study, low infestations (0-2 cm) of European corn borer occurred at 25% of the locations assessed, moderate infestations (4-6 cm) occurred at 42% of the locations, and high infestations (>6 cm) occurred at 33% of the locations. At a corn price of $3.00 per bushel CDN and seed premiums of $34.58 per hectare CDN, 5 cm of tunneling was required for a return on investment in Bt seed, comprising only 55% of the growers in the study. With infestations of more than 6 cm of tunneling occurring only 33% of the time, a return on seed investment would be realized in only one of three growing seasons. At a seed premium of $24.70 per hectare CDN per year, at least $74 per hectare CDN in the year of infestation would be required to make up for the two years of no return. In this study, a $74 per hectare CDN return at a corn price of $9.26 per hectare CDN with >16 cm of tunneling damage would have occurred only 7.3% of the time.     

Production of transgenic tropical maize with cryIAb and cryIAc genes via microprojectile bombardment of immature embryos

To enhance the level of resistance to insects in tropical maize germplasm we have developed techniques to successfully transform elite tropical maize inbred based on the activity of specific cryI proteins against four major maize pests – corn earworm, fall armyworm, southwestern corn borer and sugarcane borer. Constructs containing cryIAb or cryIAc synthetic genes were used. To generate transgenic plants we have established methods for biolistic bombardment and the selection and regeneration of immature embryos and calli from the elite tropical lines CML72, CML216, CML323, CML327 and hybrids. Transgenic plants resistant to the herbicide Basta^TM contained the bands for the cry, bar and gus genes as detected by Southern blot analyses. A simple leaf bioassay presented varying levels of resistance to Southwestern corn borer of transgenic tropical maize carrying the cryIAc gene. Analyses of the progenies confirmed the sexual transmission of the introduced genes and their stable expression. Received: 25 September 1998 / Accepted: 27 October 1998     

Systemic infection of stalks and ears of corn hybrids by <Emphasis Type="Italic">Aspergillus parasiticus</Emphasis>

This study was conducted to explore systemic infection by the Aspergillus flavus group into corn ears via the stalk. An A. parasiticus mutant which produces norsolorinic (NOR) acid (a visible orange intermediate of the aflatoxin biosynthetic pathway) was used in field studies to monitor systemic infection of corn stalk and ear tissues. Corn hybrids resistant and susceptible to aflatoxin contamination were grown in the field and inoculated prior to tasseling by inserting A. parasiticus infested toothpicks into stalks between the 5th and 6th node below the lowest ear shoot. Beginning 2 weeks after inoculation, systemic infection by the NOR mutant was assessed weekly by collecting ear shank tissue and stalk tissue from the nodes between the infection sites and the developing ears. Ears were collected at the end of the growing season to determine the level of kernel infection by the NOR mutant. In two separate studies, the A. parasiticus NOR mutant was isolated from stalk tissues at all of node positions and ear shank tissue from several susceptible corn hybrid plants at the first harvest date 2 weeks after inoculation. The NOR mutant was also isolated from stalk and ear tissue of a resistant hybrid. The NOR mutant was only isolated from kernels of susceptible hybrids in 2003 and 2004. Infection rates of kernels in infected ears were very low (<1%). In 2005, the fungus was found in only one kernel from an ear of the resistant hybrid. The NOR mutant was not isolated from stalks, ears, or kernels from control (uninoculated) plants grown in the plots with inoculated plants. Although infection levels of corn kernels were low, systemic movement of the A. parasiticus up the stalk appears to be another possible route to infection of developing corn ears.     

Modeling the development of resistance by stalk-boring lepidopteran insects (Crambidae) in areas with transgenic corn and frequent insecticide use

We simulated the population dynamics and population genetics of two bivoltine species of corn borers, the European corn borer, Ostrinia nubilalis (Hübner), and the southwestern corn borer, Diatraea grandiosella Dyar, in a hypothetical region of irrigated transgenic and nontransgenic corn where insecticide was applied only to the nontransgenic refuge crop. Over the 100-yr time horizon, resistance developed quickly in both species and to both transgenic corn and the insecticide when the allele for resistance to the respective toxin was dominant. When the allele for transgenic resistance was not dominant and the refuge location was constant over the time horizon, spraying the refuge to control southwestern corn borer had no effect on how quickly resistance to the transgenic corn developed. In contrast, the European corn borer developed resistance to transgenic corn much sooner when the refuge was sprayed once per year, and the time to 3% resistance allele frequency decreased as efficacy of the insecticide increased. Only when the refuge was treated less than once every 5 yr (10 generations) did the frequency of application decline enough to permit resistance management for the European corn borer to approximate the effectiveness of an unsprayed refuge. A consistently sprayed refuge <40% of the corn acreage was an inadequate resistance management strategy for the European corn borer even when a low efficacy insecticide (70% mortality) was used. When assumptions about European corn borer adult behavior were changed and the adults behaved similarly to adult southwestern corn borer, the development of resistance to the transgenic crop was slowed significantly.     

Conventional resistance of experimental maize lines to corn earworm (Lepidoptera: Noctuidae), fall armyworm (Lepidoptera: Noctuidae), southwestern corn borer (Lepidoptera: Crambidae), and sugarcane borer (Lepidoptera: Crambidae)

Plant resistance is a useful component of integrated pest management for several insects that are economically damaging to maize, Zea mays L. In this study, 15 experimental lines of maize derived from a backcross breeding program were evaluated for resistance to corn earworm, Helicoverpa zea (Boddie); fall armyworm, Spodoptera frugiperda (J. E. Smith); southwestern corn borer, Diatraea grandiosella Dyar; and sugarcane borer, Diatraea saccharalis (F.). Experimental line 100-R-3 was resistant in the field to leaf feeding by fall armyworm and line 116-B-10 was resistant in the field to leaf feeding by fall armyworm and leaf and stalk feeding by southwestern corn borer. When corn earworm larvae were fed field harvested silks from experimental line 81-9-B in the laboratory, their pupal weights were significantly lower than the pupal weights of larvae that were fed silks from the resistant control, Zapalote Chico. Maysin levels lower than those commonly associated with corn earworm resistance were present in the resistant experimental line, 107-8-7, indicating a new basis confers resistance to corn earworm in this line. These resistant experimental lines will provide plant breeders with new sources of resistance to lepidopterous insects for the development of improved maize breeding populations.     

Biocontrol of aflatoxin in corn by inoculation with non-aflatoxigenic Aspergillus flavus isolates

The ability of two non-aflatoxigenic Aspergillus flavus Link isolates (CT3 and K49) to reduce aflatoxin contamination of corn was assessed in a 4-year field study (2001–2004). Soil was treated with six wheat inoculant treatments: aflatoxigenic isolate F3W4; two non-aflatoxigenic isolates (CT3 and K49); two mixtures of CT3 or K49 with F3W4; and an autoclaved wheat control, applied at 20 kg ha^?1. In 2001, inoculation with the aflatoxigenic isolate increased corn grain aflatoxin levels by 188% compared to the non-inoculated control, while CT3 and K49 inoculation reduced aflatoxin levels in corn grain by 86 and 60%, respectively. In 2002, the non-toxigenic CT3 and K49 reduced aflatoxin levels by 61 and 76% compared to non-inoculated controls, respectively. In 2001, mixtures of aflatoxigenic and non-aflatoxigenic isolates had little effect on aflatoxin levels, but in 2002, inoculation with mixtures of K49 and CT3 reduced aflatoxin levels 68 and 37% compared to non-inoculated controls, respectively. In 2003 and 2004, a low level of natural aflatoxin contamination was observed (8 ng g^?1). However, inoculation with mixtures of K49?+?F3W4 and CT3?+?F3W4, reduced levels of aflatoxin 65–94% compared to the aflatoxigenic strain alone. Compared to the non-sclerotia producing CT3, strain K49 produces large sclerotia, has more rapid in vitro radial growth, and a greater ability to colonize corn when artificially inoculated, perhaps indicating greater ecological competence. Results indicate that non-aflatoxigenic, indigenous A. flavus isolates, such as strain K49, have potential use for biocontrol of aflatoxin contamination in southern US corn.