Diatraea Grandiosella Research Articles

Toxicity of Transgenic Corn Tissues Expressing Insecticidal Protein of Bacillus thuringiensis Berliner to Southwestern Corn Borer (Lepidoptera: Crambidae) in the Laboratory

Laboratory studies were conducted to compare the response of Diatraea grandiosella (Dyar) larvae to lyophilized transgenic corn tissue expressing the Cry1Ab endotoxin protein of Bacillus thuringiensis (Berliner) (Bt) with conventional corn tissue incorporated into a BioServ™ artificial diet. Whorl leaf, stalk, shank, husk, silk or kernel tissues were tested independently in the diet. Larvae fed diet containing conventional corn weighed more and were longer in length than larvae fed diet containing Bt corn for all tissue types included in the study. The number of larvae that survived depended on the tissue type and age of the tissue. Larvae fed diet with kernel tissue expressing Bt toxin had greater weight and body length than larvae fed the other Bt tissue types. The negative effects of Bt corn tissues expressing the Cry1Ab endotoxin protein on growth of D. grandiosella was observed, even at the diluted concentrations of toxic tissues incorporated into the diet in this study.

Genetic relationships between resistance to stalk-tunneling by the European corn borer and cell-wall components in maize population B73×B52

The objective of this study was to assess the relationships among quantitative trait loci (QTL) detected for European corn borer (ECB) tunneling and cell-wall components (CWC) neutral detergent fiber (NDF), acid detergent fiber (ADF), and acid detergent lignin (ADL) content in leaf-sheath and stalk tissues in a maize recombinant inbred line population derived from inbred lines B73 and B52. Most of the QTL for ECB resistance (10/13) were at QTL positions for one or more CWC. Of the 12 QTL for NDF and ADF in leaf-sheaths, five for each trait were at or near QTL for ECB tunneling. Four of these five QTL for NDF and ADF mapped to common locations. Four of the eight leaf-sheath ADL QTL were detected in the same genomic regions as ECB QTL. For stalk tissue, four regions contained common/overlapping QTL for ECB tunneling, NDF, and ADF. Six such regions were observed for stalk ADL and ECB tunneling. Seven of the ten QTL associated with both CWC and ECB tunneling contributed to the negative correlations observed between these traits, while relatively few QTL effects were positively correlated. This suggests that while CWC contribute to ECB resistance in this population, other mechanisms and other genes also are involved. Several QTL contributing to the negative correlations between ECB tunneling and CWC in the leaf-sheaths mapped to similar positions as QTL detected in tropical maize populations for resistance to leaf-feeding by Diatraea grandiosella Dyar and Diatraea saccharalis Fabricus. These regions may contain genes involved in the synthesis of cellulose, hemicellulose, and lignin in the leaf-blades and leaf-sheaths of maize.

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.

Relative Concentration of Cry1A in Maize Leaves and Cotton Bolls with Diverse Chlorophyll Content and Corresponding Larval Development of Fall Armyworm (Lepidoptera: Noctuidae) and Southwestern Corn Borer (Lepidoptera: Crambidae) on Maize Whorl Leaf Profiles

To manage insect resistance to transgenic crops that express insecticidal proteins from Bacillus thuringiensis (Bt) Berliner, the U.S. Environmental Protection Agency recommends a refuge-based insect resistance management strategy where a percentage of non-Bt (refuge) crop is grown in proximity to a Bt-expressing crop. An important requirement for this strategy is that the toxin exists at a high effective dose for control of the target pest(s), so that heterozygous individuals in the population do not reach adulthood. Factors that cause reduced levels of toxin in the plant are a threat to this strategy. We quantified Cry1Ab from different areas of the maize, Zea mays L., leaf. In general, the distal tip of the V7 maize leaf had a higher concentration of Cry1Ab compared with the middle section of the V7 leaf, and the middle section of the developing V9 leaf had the lowest concentration of Cry1Ab. When these sections of maize tissue were fed to fall armyworm, Spodoptera frugiperda (J.E. Smith), and southwestern corn borer, Diatraea grandiosella Dyar, there was not a reduction in development or an increase in mortality with tissue that had higher concentrations of toxin. Another study tested the relative concentration of Cry1Ab between the white-yellow, yellow-green, and green portions of the developing ninth leaf within the maize whorl. There were differences in Cry1Ab concentration among these leaf areas. The green tissue had the highest concentration of toxin followed by the yellow-green and white-yellow tissues. Correlations between concentration of Cry1Ab and 5-d fall armyworm larval weights among the three leaf color profiles were all significant and negative, i.e., decreased concentration of Cry1Ab in the leaf tissue resulted in increased 5-d larval weights. There was 100% mortality to the southwestern corn borer larvae fed Cry1Ab maize leaf tissue. Differences in the amount of Cry1Ab in the developing V9 leaf profiles did not alter the absolute susceptibility of the southwestern corn borer to the toxin. In cotton, Gossypium hirsutum L., the amount of Cry1Ac was significantly lower in boll tips where flowers had remained attached compared with normal boll tips. Boll tips where the flowers remained attached are often the site where corn earworms, Helicoverpa zea (Boddie), penetrate Bt cotton bolls. This study demonstrated that, in two diverse plant species, tissue that has low chlorophyll content does not fully express Cry1A. Photosynthesis regulating factors related to mRNA transcription and translation should be studied for their effect on Cry1A production and insect control.

Plant-Incorporated <I>Bacillus thuringiensis</I> 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.

Southwestern corn borer damage and aflatoxin accumulation in conventional and transgenic corn hybrids

Southwestern corn borer ( Diatraea grandiosella Dyar) is a major pest of corn ( Zea mays L.) in the southern United States. In addition to the direct yield losses caused by southwestern corn borer, larval feeding on developing ears provides a site for fungi to enter the ear. Aspergillus flavus Link: Fries infection and the subsequent accumulation of aflatoxin in corn grain are major limitations to profitable corn production in the southern United States. This investigation was conducted to determine the effectiveness of transgenic corn hybrids expressing the δ-endotoxin insecticidal (CryIAb) proteins isolated from Bacillus thuringiensis ( Bt) in reducing southwestern corn borer damage and aflatoxin accumulation. Ear damage and aflatoxin accumulation were compared among 10 pairs of conventional non Bt and transgenic Bt corn hybrids following infestation with southwestern corn borer and inoculation with A. flavus using kernel-wounding and nonwounding techniques. Both non Bt and Bt hybrids exhibited high levels of aflatoxin accumulation when inoculated with a kernel-wounding technique. When inoculated with a non-wounding technique and infested with southwestern corn borer, aflatoxin accumulation was significantly higher in non Bt than Bt hybrids. Aflatoxin accumulation was also significantly higher for non Bt hybrids inoculated with A. flavus and infested with southwestern corn borer than for hybrids that were only inoculated with A. flavus. Southwestern corn borer larval establishment was significantly higher on non Bt hybrids than on Bt hybrids. Larval survival was extremely low on the Bt hybrids. The results of this investigation indicate that these Bt hybrids should be effective in reducing aflatoxin contamination in areas where high southwestern corn borer infestations occur. The reduced levels of aflatoxin accumulation associated with Bt hybrids are likely a consequence of reduced insect damage rather than resistance to A. flavus infection or aflatoxin accumulation per se.

Rubidium Chloride and Cesium Chloride Sprayed on Maize Plants and Evaluated for MarkingDiatraea grandiosella(Lepidoptera: Crambidae) in Mark–Recapture Dispersal Studies

Experiments were undertaken to determine the potential for using rubidium chloride (RbCl) or cesium chloride (CsCl) to mark southwestern corn borer, Diatraea grandiosella Dyar, for use in applied ecological studies. Maize, Zea mays L., plants were sprayed with aqueous solutions of RbCl or CsCl at rates of 100, 1000, or 10,000 μg/g and inoculated with D. grandiosella neonates. Rubidium and cesium were successfully absorbed and translocated in maize plants. There were only a few minor effects of the treatment on maize or on southwestern corn borers. Rb and Cs were detected in plants, but not in insects, by using flame atomic absorption spectrophotometry. Graphite furnace-atomic absorption spectrophotometry (GF-AAS) and neutron activation analysis (NAA) allowed identification of Rb and Cs in adults. Rb and Cs were detected by GF-AAS in feral unmarked adults, and they contained higher levels of Rb than Cs. Males and females contained similar amounts of Rb, but Cs levels were higher in males than in females. Adults recovered from field maize treated with 1000 μg/g Cs had higher levels of Cs than did those from untreated plants. Using NAA, neither Rb nor Cs was detected in adults recovered from greenhouse-grown untreated maize. Males and females recovered from maize treated with 1000 μg/g RbCl and CsCl contained similar amounts of Rb, but females contained more Rb than Cs. We conclude that application of 1000 μg/g RbCl or CsCl on plants is effective in marking adults of D. grandiosella with Rb or Cs and would be useful for mark–recapture dispersal studies.

Field Resistance of Bacillus thuringiensis Berliner Transformed Maize to Fall Armyworm (Lepidoptera: Noctuidae) and Southwestern Corn Borer (Lepidoptera: Crambidae) Leaf Feeding

The fall armyworm, Spodoptera frugiperda (J. E. Smith), and the southwestern corn borer, Diatraea grandiosella (Dyar), can cause economic damage to maize, Zea mays L., grown in the southeastern United States. Maize hybrids are commercially available that have been transformed to express insecticidal crystalline proteins from Bacillus thuringiensis (Bt) Berliner. The field efficacy of seven Bt hybrids were tested for control of leaf-feeding fall armyworm and southwestern corn borer. All Bt hybrids performed better than their conventional near-isolines for control of both insects. In general, the Bt hybrids provided intermediate resistance to the fall armyworm and near immunity to the southwestern corn borer. Based on larval establishment and weights, the fall armyworm was more tolerant of the insecticidal proteins expressed by the Bt hybrids than the southwestern corn borer. There was no difference in expression of insecticidal proteins among the Bt hybrids. Bt hybrids should be advantageous for the production of maize in areas that are affected by southwestern corn borer. The moderate level of resistance in the Bt hybrids to fall armyworm should be further examined to determine if amplifying the expression of insecticidal proteins or integrating other control methods along with the use of current Bt hybrid maize is needed to protect the crop from yield reduction by this pest.

Competition of Bacillus thuringiensis Cry1 toxins for midgut binding sites: a basis for the development and management of transgenic tropical maize resistant to several stemborers.

Binding and competition of five Bacillus thuringiensis toxins--Cry1Ab, Cry1Ac, Cry1Ba, Cry1Ca, and Cry1Ea--for midgut binding sites from three pests, Spodoptera frugiperda, Diatraea saccharalis, and Diatraea grandiosella, were investigated as part of a strategy to develop tropical transgenic maize resistant to several stemborers. On S. frugiperda, Cry1Ab and Cry1Ac compete for the same binding site; Cry1Ba and Cry1Ca compete for a second binding site. Cry1Ea recognizes a third specific binding site in S. frugiperda and does not compete with any of the other toxins. On D. grandiosella and D. saccharalis, Cry1Ac competes with Cry1Ab and not with Cry1Ba and Cry1Ca. Cry1Ba and Cry1Ca recognize each a specific binding site and do not compete with any of the other four toxins. Cry1Ea does not recognize any binding site on Diatraea species. Combinations of toxins are proposed to develop transgenic maize resistant to the three stemborers while allowing resistance management.

Oil-Soluble Dyes Incorporated in Meridic Diet of <I>Diatraea</I> g<I>randiosella</I> (Lepidoptera: Crambidae) as Markers for Adult Dispersal Studies

Mark-release-recapture experiments to study insect dispersal require the release of marked insects that can be easily identified among feral conspecifics. Oil-soluble dyes have been used successfully to mark various insect species. Two oil-soluble dyes, Sudan Red 7B (C.I. 26050) and Sudan Blue 670 (C.I. 61554), were added to diet of the southwestern corn borer, Diatraea grandiosella Dyar, and evaluated against an untreated control diet. Survival, diet consumption, larval and pupal weight, development time, fecundity, longevity, and dry weight of the adults were measured. Adults reared on the three diets were also tested for mating success. Some minor effects were observed for southwestern corn borers reared on the marked diets. Eggs, larvae, pupae, and adults were all reliably marked and readily identifiable. Adults retained color for their entire life span. Adults from each diet mated successfully with adults from the other diets. F1 progeny from the different mating combinations survived to the second instar but tended to lose the marker after 3-4 d on untreated diet. Both Sudan Red 7B and Sudan Blue 670 can be used to mark southwestern corn borer adults and thus should be useful for mark-release-recapture dispersal studies. The dyes will also be useful for short-term studies with marked larvae and oviposition behavior.

Explaining public resistance to genetically modified corn: an analysis of the distribution of benefits and risks.

Genetically modified (GM) crops have met with widespread approval among scientists and policy makers in the United States, but public approval of GM crops, both domestically and abroad, is progressing much more slowly. An underlying cause of public wariness may be that both nations and individual consumers do not perceive significant benefits to themselves from GM crops, while fearing the risks they may incur. In this study, an economic analysis is conducted to determine whether the benefits of one type of GM corn, Bt corn (genetically modified to resist damage from the ECB and Southwestern corn borer), outweigh the potential risks; and who the "winners" and "losers" are among stakeholder groups that may be affected by Bt corn. It is found that Bt corn growers, consumers, and industry all benefit from Bt corn adoption, though the purported health and environmental benefits of reducing chemical pesticide usage through Bt corn are negligible. Though the aggregated public benefit is large, the welfare gain to individual consumers is small and may not make up for perceived risks. While environmental and health risks of Bt corn are unlikely, the potential market risks-impacting both the organic corn market and total U.S. corn exports-are found to be significant. Currently, distributional analysis is not a part of regulatory decision making of Bt corn in the United States; yet it may help to explain why decision makers at both the government and individual-consumer levels have failed to embrace Bt corn and other GM crops.

Evaluation of Rubidium Chloride and Cesium Chloride Incorporated in a Meridic Diet to MarkDiatraea grandiosella(Lepidoptera: Crambidae) for Dispersal Studies

Southwestern corn borers, Diatraea grandiosella Dyar, were reared on a control meridic diet and diets that incorporated rubidium chloride (RbCl) or cesium chloride (CsCl) at the rate of 1000 μg/g (1 g/liter of wet diet) to evaluate the effects on biology of D. grandiosella and to determine whether the resulting adults are marked with the rubidium (Rb) and cesium (Cs) physiological markers. The effects of RbCl and CsCl on survival, diet consumption, larval and pupal weight, developmental time, adult deformity, adult longevity, fecundity, and adult dry weight were generally minor and seldom reached statistically significant proportions. Males and females mated successfully when paired in different combinations across treatments. Graphite furnace atomic absorption spectrophotometer and neutron activation analysis were both effective in detecting Rb and Cs in male and female adults reared on RbCl and CsCl diets. Rb and Cs concentrations of males and females reared on RbCl and CsCl diets were above the background levels found in adults reared on the control diet. Rb and Cs can be used as physiological markers to mark D. grandiosella in dispersal experiments.

Factors Influencing Population Dynamics of the Southwestern Corn Borer (Lepidoptera: Crambidae): A Reassessment

Mortality data from the literature on the southwestern corn borer, Diatraea grandiosella Dyar, were analyzed to determine which factors have the most impact on populations of this insect. Adult mortality, presumed to occur primarily as a result of dispersal, appears to have been the largest source of mortality, as well as the key factor in the annual variance of mortality. Nonpredation mortality during diapause was also an important factor. Of the density-dependent mortality factors, predation by the flicker, Colaptes aruaus (L.) appears to be the factor that limits the size of overwintering populations in the southeastern United States, while cannibalism among prediapausing larvae limits the size of overwintering populations in the western Great Plains. Based on the analysis of mortality rates and historical field data, the annual finite capacity for increase (multiplication rate) was estimated to be ≈10 in the central United States. This analysis suggests that to comprehend more clearly the factors that influence population variation in D. grandiosella, future research efforts should be focused on understanding adult dispersal, and the interaction between flicker predation and nonpredation mortality in the diapausing larvae stage.

The Value of Bt Corn in Southwest Kansas: A Monte Carlo Simulation Approach

While most Corn Belt farmers consider planting Bt corn to control European corn borer, southwestern Kansas farmers must also take into account an array of other insect pests, including corn rootworm, spider mites, and southwestern corn borer. This research uses a decision analysis framework to estimate the expected economic value of Bt corn in southwest Kansas. Mean per acre Bt values ranged from $12.49 to $34.60, well above the technology fee assumed to be $14 per unit, or $5.25 per acre at a seeding rate of 30,000 seeds per acre. The minimum value over all scenarios was $8.69 per acre. Using Monte Carlo simulation, it was shown that European and southwestern corn borer infestation probabilities, expected corn price, and expected pest-free yields are important determinants of the value of Bt corn.

Impact of a novel species of Nosema on the southwestern corn borer (Lepidoptera: Crambidae).

A study was undertaken to elucidate the impact of an undescribed Nosema sp. on the southwestern corn borer (SWCB; Diatraea grandiosella Dyar). The Nosema sp. (isolate 506) included in the study was isolated from an overwintering SWCB larva in Mississippi. It was highly infectious per os, with a median infective dose of 2.0 x 10(3) spores per larva. Even at the highest dosage tested (10(7) spores per larva), minimal mortality (< or = 3%) was observed in infected larvae, pupae, and adults reared in the laboratory on an artificial diet. However, infected pupae (0- and 7-d-old) were smaller, and the time to adult eclosion from pupation was slightly increased. Furthermore, the number of eggs produced by infected SWCB female moths substantially decreased (32%), and this effect was most pronounced on day 2, when the greatest number of eggs were oviposited by infected and noninfected moths. For eggs produced by infected females mated with infected males, hatch was slightly decreased by 16 and 15% for eggs laid on days 2 and 3, respectively. In addition, egg hatch was reduced in eggs oviposited by noninfected females mated with infected males on day 3. A low prevalence of infection (< 6%) was observed in the F1 generation originating from infected females mating with noninfected males, from noninfected females mating with infected males, and from infected females mating with infected males. Nosema 506 spores were observed in the proximity of reproductive tissues of infected female and male moths. Spores also were detected on the chorion surface and within eggs laid by infected females. Furthermore, 1-11% of larvae hatching from surface-sterilized eggs were infected by Nosema 506 indicating a transovarial mechanism of transmission.

Impact of a Novel Species of Nosema on the Southwestern Corn Borer (Lepidoptera: Crambidae)

A study was undertaken to elucidate the impact of an undescribed Nosema sp. on the southwestern corn borer (SWCB; Diatraea grandiosella Dyar). The Nosema sp. (isolate 506) included in the study was isolated from an overwintering SWCB larva in Mississippi. It was highly infectious per os, with a median infective dose of 2.0 × 103 spores per larva. Even at the highest dosage tested (107 spores per larva), minimal mortality (≤3%) was observed in infected larvae, pupae, and adults reared in the laboratory on an artificial diet. However, infected pupae (0- and 7-d-old) were smaller, and the time to adult eclosion from pupation was slightly increased. Furthermore, the number of eggs produced by infected SWCB female moths substantially decreased (32%), and this effect was most pronounced on day 2, when the greatest number of eggs were oviposited by infected and noninfected moths. For eggs produced by infected females mated with infected males, hatch was slightly decreased by 16 and 15% for eggs laid on days 2 and 3, respectively. In addition, egg hatch was reduced in eggs oviposited by noninfected females mated with infected males on day 3. A low prevalence of infection (<6%) was observed in the F1 generation originating from infected females mating with noninfected males, from noninfected females mating with infected males, and from infected females mating with infected males. Nosema 506 spores were observed in the proximity of reproductive tissues of infected female and male moths. Spores also were detected on the chorion surface and within eggs laid by infected females. Furthermore, 1–11% of larvae hatching from surface-sterilized eggs were infected by Nosema 506 indicating a transovarial mechanism of transmission.

Southwestern corn borer (Lepidoptera: Crambidae) damage and aflatoxin accumulation in maize.

Aflatoxin, a potent carcinogen, is produced by the fungus Aspergillus flavus Link: Fr. Drought, high temperatures, and insect damage contribute to increased levels of aflatoxin contamination in corn, Zea mays L. Plant resistance is widely considered a desirable method of reducing aflatoxin contamination. Germplasm lines with aflatoxin resistance have been developed. This investigation was undertaken to determine whether crosses among these lines exhibited resistance to southwestern corn borer, Diatraea grandiosella Dyar, and to assess the effects of southwestern corn borer feeding on aflatoxin accumulation. Differences in ear damage among southwestern corn borer infested hybrids were significant. Estimates of general combining ability effects indicated that the lines Mp80:04, Mp420, and Mp488 contributed to reduced ear damage, and SC213 and T165 contributed to greater damage when used in hybrids. Mean aflatoxin levels were 254 ng/g for hybrids infested with southwestern corn borer larvae and 164 ng/g for noninfested hybrids in 2000 when environmental conditions were conducive to aflatoxin production. In contrast, the overall mean aflatoxin level for southwestern corn borer infested hybrids was only 5 ng/g in 1999 when environmental conditions did not favor aflatoxin accumulation. Crosses that included lines selected for aflatoxin resistance as parents (Mp80:04 and Mp313E) exhibited lower levels of aflatoxin contamination both with and without southwestern corn borer infestation in 2000. Only the experimental line Mp80:04 contributed significantly to both reduced southwestern corn borer damage and reduced aflatoxin contamination.

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.

Selection for Resistance to Southwestern Corn Borer Using Marker‐Assisted and Conventional Backcrossing

Two maize (Zea mays L) lines, susceptible and resistant to first-generation southwestern corn borer (SWCB), Diatraea grandiosella Dyar, were hybridized then backcrossed to the susceptible parent to form a population that was selected over three backcross generations by either marker-assisted or conventional selection for resistance to first generation SWCB leaf feeding. Quantitative trait loci (QTL) mapping was done by BC1F1 genotyping and BC1F2 infestation with SWCB. Three putative QTL were identified on chromosomes 7, 9, and 10 accounting for 28% of the phenotypic variance. Marker-assisted selection (MAS) proceeded by selecting plants heterozygous at the QTL regions and homozygous for the recurrent parent genotype outside the QTL regions in the BC1F1 and BC2F1 generations. BC2F2 individuals were selected for the homozygous donor genotype in the QTL regions. Conventional selection initiated from the most resistant 30 BC1F2 lines. Conventional trials of BC2F2 and BC2F3 families were infested with SWCB and based on leaf damage ratings selected selfed progeny of the former generation formed the subsequent trial entries. A comparative trial of BC2F3 lines, selected by the two methods, was evaluated under SWCB infestation at three locations. Leaf damage ratings were taken at all locations and larvae weight was taken at one location. No significant differences for leaf damage ratings or larvae weight were found between lines selected by the two methods. Both methods produced lines significantly improved over the susceptible parent for SWCB leaf feeding damage indicating that the methods were equivalent as conducted in this experiment.

Susceptibility of field-collected populations of the southwestern corn borer, Diatraea grandiosella, to Bacillus thuringiensis.

The susceptibilitity of newly hatched larvae of laboratory-adapted and field-collected populations of the Southwestern corn borer, Diatraea grandiosella (Dyar) (Lepidoptera: Crambidae), to a Bacillus thuringiensis protein (Cry1Ab) was examined using a larval feeding bioassay. D grandiosella populations were collected from five states: Missouri, Kansas, Texas, Tennessee and Kentucky. Using larval mortality as the end-point of the bioassay, the magnitude of differences in the susceptibility of the laboratory-adapted and the field-collected populations to Cry1Ab protein varied from one to 46 times depending on the time of observation and the standard of comparison (LC50 or LC95). However, significant differences in susceptibility to Cry1Ab protein among these populations were not detected when the comparisons were based on growth inhibition (EC50 or EC95); the magnitude of differences was less than fourfold. Either using larval mortality or larval growth inhibition, the results indicated that the field-collected populations of D grandiosella were susceptible to Cry1Ac, and differences in susceptibility may reflect natural variation among populations. The bioassay using larval growth inhibition offers advantages over that using larval mortality, including giving more accurate representation of the toxicological effects of the toxin.