Bt Corn Research Articles

Susceptibility of Fall Armyworms (Spodoptera frugiperda J.E.) from Mexico and Puerto Rico to Bt Proteins.

Simple SummaryFall armyworms from Mexico are susceptible to four Bacillus thuringiensis (Bt) proteins, while those from Puerto Rico have field-evolved resistance to Cry1F and Cry1Ac and are susceptible to Cry2Ab2 and Cry1A.105. Implications for fall armyworm management are discussed.Fall armyworm is one of the main pests of conventional and Bacillus thuringiensis (Bt) corn in many countries in the Americas, Africa, Asia and in Australia. We conducted diet-overlay bioassays to determine the status of susceptibility to four Bt proteins (Cry1A.105, Cry2Ab2, Cry1F and Cry1Ac) in three different populations of fall armyworm from Mexico, and one population from Puerto Rico. Bioassays showed that fall armyworms from Puerto Rico were resistant to Cry1F with a resistance ratio 50 (RR50) higher than 10,000 ng/cm2 and to Cry1Ac with a RR50 = 12.2 ng/cm2, displaying the highest median lethal concentration (LC50) values to all Bt proteins tested. The effective concentration 50 (EC50) values further confirmed the loss of susceptibility to Cry1F and Cry1Ac in this population. However, LC50 and EC50 results with Cry1A.105 and Cry2Ab2 revealed that fall armyworm from Puerto Rico remained largely susceptible to these two proteins. The Mexican populations were highly susceptible to all the Bt proteins tested and displayed the lowest LC50 and EC50 values to all Bt proteins. Our results suggest that Cry1F and Cry1Ac resistance is stable in fall armyworm from Puerto Rico. However, this population remains susceptible to Cry1A.105 and Cry2Ab2. Results with Mexican fall armyworms suggest that possible deployment of Bt corn in Mexico will not be immediately challenged by Bt-resistant genes in those regions.

What type of Bt corn is suitable for a region with diverse lepidopteran pests: A laboratory evaluation

ABSTRACT Transgenic crops that produce Bacillus thuringiensis (Bt) toxins are effective tools for controlling lepidopteran pests. However, the degree of susceptibility to Bt toxins differs among various pest species due to relatively narrow spectrum and high selectivity of such toxins. Bt corn hybrids for Chinese market were designed to target Asian corn borer Ostrinia furnacalis (Guenée), while their efficacy against other lepidopteran pests are not well defined, such as Conogethes punctiferalis (Guenée), Helicoverpa armigera (Hübner), Agrotis ypsilon (Rottemberg), and Mythimna separata (Walker), which are also important lepidopteran pests on corn in the Huang-Huai-Hai Summer Corn Region of China. To determine what type of Bt corn is suitable for this region, the efficacy of five Bt toxins, i.e., Cry1Ab, Cry1Ac, Cry1F, Cry2Ab, and Vip3A, to these five lepidopteran species was evaluated in laboratory. Both O. furnacalis and C. punctiferalis showed similar high susceptibility to all five Bt toxins. A. ypsilon and M. separate were less sensitive to Cry1Ab and Cry1Ac than the other species. H. armigera, A. ypsilon and M. separate were less sensitive to Cry1F than O. furnacalis and C. punctiferalis. H. armigera was more sensitive to Cry2Ab than other tested species. All five species were equally sensitive to Vip3A, though their LC50s were all relatively higher. These findings suggest that the first generation Bt corn expressing single Cry1 toxin should not be the first choice because of the potential risk of control failure or less efficacy against H. armigera, A. ypsilon or M. separate. The second-generation Bt corn expressing Cry1 and Cry2 toxins, or the third generation Bt corn expressing Cry1, Cry2 and Vip3A toxins might produce better protection of corn in the Huang-Huai-Hai Summer Corn Region of China.

Decline in Sublethal Effects of Bt Corn on Corn Earworm (Lepidoptera: Noctuidae) Linked to Increasing Levels of Resistance.

As part of an ongoing longitudinal study in North and South Carolina, this study reports the recovery of Helicoverpa zea (Boddie) pupae in field trials with genetically engineered corn, Zea mays L., hybrids that produce insecticidal toxins from Bacillus thuringiensis (Bt) in 2017-2019. In total, 10,400 corn ears were collected, which led to 3,927 H. zea pupae (2,215 in South Carolina and 1,712 in North Carolina). Late-planted corn led to a 3.39-fold increase in recovery of pupae compared to early-planted corn. Bt corn expressing Cry1F + Cry1Ab and Cry1A.105 + Cry2Ab2 had 1.67-fold and 2.51-fold fewer pupae than non-Bt near-isolines, respectively. Only six pupae were recovered from the hybrid expressing Cry1F + Cry1Ab + Vip3Aa20. Averaged across trials, Bt corn expressing either Cry1A.105 + Cry2Ab2 or Cry1F + Cry1Ab significantly reduced pupal weight compared to non-Bt near-isolines in North and South Carolina. Combining our data with a previous study at the same locations (Bilbo et al. 2018), reduction in pupal weight between Bt and non-Bt near-isolines significantly declined from 2014 to 2019 for Cry1Ab + Cry1F in North and South Carolina. This decline in levels of a sublethal effect of Bt corn expressing Cry1Ab + Cry1F on H. zea at both locations is likely correlated with resistance development.

Western corn rootworm abundance, injury to corn, and resistance to Cry3Bb1 in the local landscape of previous problem fields.

Western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), is a major pest of corn in the United States. Transgenic corn expressing insecticidal proteins derived from the bacterium Bacillus thuringiensis (Bt) is an important tool used to manage rootworm populations. However, field-evolved resistance to Bt threatens this technology. In areas where resistance is present, resistant individuals may travel from one field to a neighboring field, spreading resistance alleles. An important question that remains to be answered is the extent to which greater-than-expected root injury (i.e., >1 node of injury) to Cry3Bb1 corn from western corn rootworm is associated with rootworm abundance, root injury, and levels of resistance in neighboring fields. To address this question, fields with a history of greater-than-expected injury to Cry3Bb1 corn (focal fields) and surrounding fields (< 2.2 km from focal fields) were examined to quantify rootworm abundance, root injury, and resistance to Cry3Bb1 corn. Additionally, use of Bt corn and soil insecticide use for the previous six years were quantified for each field. Resistance to Cry3Bb1 was present in all fields assayed, even though focal fields had grown more Cry3 corn and less non-Bt corn than surrounding fields. This finding implies that some movement of resistance alleles had occurred between focal fields and surrounding fields. Overall, our data suggest that resistance to Cry3Bb1 in the landscape has been influenced by both local rootworm movement and field-level management tactics.

Baseline susceptibility and resistance allele frequency in Ostrinia furnacalis related to Cry1 toxins in the Huanghuaihai summer corn region of China

Ostrinia furnacalis (Guenée) is one of the most destructive pests of corn and is a major target of transgenic corn expressing Bt (Bacillus thuringiensis) toxins in the Huanghuaihai summer corn region of China. Prior to the widespread commercialization of transgenic Bt corn, it is necessary to estimate baseline susceptibility to Bt toxins and Bt toxin resistance allele frequencies in O. furnacalis. The median lethal concentration (LC50 ) values of the Bt toxins Cry1Ab, Cry1Ac and Cry1F for 15 different populations ranged from 0.887 to 1.617, 1.251 to 2.594 and 4.146 to 6.465 ng cm-2 , respectively. The LC99 values of 93, 45, and 197 ng cm-2 for Cry1Ab, Cry1Ac and Cry1F, respectively, killed > 99% of individuals of eight O. furnacalis populations collected in 2017 and were identified as diagnostic concentrations for monitoring susceptibility in O. furnacalis populations in this region. Using the F2 screening method with these diagnostic concentrations, the resistance allele frequencies related to Cry1Ab, Cry1Ac and Cry1F were found to be 0.002 (0.000283-0.006484), 0.001 (0.000030-0.004295) and 0.001 (0.000030-0.004295), respectively, in 2018. Fifteen populations of O. furnacalis collected in the Huanghuaihai summer corn region of China were all susceptible to Cry1Ab, Cry1Ac and Cry1F toxins, and the susceptibility showed no significant variation among these O. furnacalis populations. The estimated resistance allele frequency to Cry1Ab, Cry1Ac and Cry1F was rare in this region. This provided essential knowledge for making the decision to commercialize Bt maize, and monitoring resistance development and evaluating resistance management strategies in the future in the Huanghuaihai summer corn region of China. © 2020 Society of Chemical Industry.

Managing the Invasive Fall Armyworm through Biotech Crops: A Chinese Perspective

In late 2018, the highly destructive and polyphagous fall armyworm was first detected in China. It is now a major economic threat to corn production. In this article, the main control strategies that are available are reviewed and prospects to manage this pest with Bacillus thuringiensis (Bt) corn in China are discussed.

Reduced Mythimna separata infestation on Bt corn could benefit aphids.

Use of genetically engineered plants that express insecticidal Cry proteins derived from Bacillus thuringiensis (Bt) have been proven efficacious for managing lepidopteran pests. However, in some cases herbivores that are not targeted by the Bt trait have increased in importance. It has been suggested that reduced caterpillar damage to Bt crops could lead to decreased levels of induced plant defensive compounds which might benefit other non-target herbivores. Here we investigated the potential effect of reduced damage by larvae of Mythimna separata on aphid populations in Bt corn. We compared the performance of Rhopalosiphum maidis feeding on non-Bt corn plants that had been infested by M. separata larvae or were uninfested. The results showed that caterpillar-infested corn plants significantly reduced the fitness of R. maidis leading to a prolonged nymphal development time, reduced adult longevity and fecundity compared to uninfested plants. Consequently, the population growth rate of corn aphids feeding on caterpillar-infested corn plants was significantly lower than on uninfested plants. As expected, the aphids performed significantly better on Lepidoptera-resistant Bt corn than on non-Bt corn when plants were infested with M. separata, since the caterpillars caused very little damage to the Bt plants. The current findings indicate that reduced M. separata infestation could benefit aphid development in Bt corn. Bt corn has the potential to be commercialized in China in the near future and aphids and other non-target pests should be monitored in the farming fields.

Report on resistance of Portulaca oleracea to glyphosate application

During 2008, a confined field experiment was conducted on “Evaluation of glyphosate resistant Bt maize hybrids”. In this experiment, 900, 1800 and 3600 g ae of glyphosate was tried for standardizing the dose of glyphosate for glyphosate resistant Bt corn hybrids. We have observed that Portulaca oleracea was not killed with all the doses of glyphosate application and was reported earlier. To confirm this, a mini field experiment has been conducted during 2014 to study that is there any resistant mechanism with Portulaca oleracea against glyphosate. Portulaca oleracea seeds have been collected and sown in 12 microplots with a dimension of 1m x 1m and allowed to grow. Glyphosate has been applied with sub optimal (5ml/litre), optimal (10ml/litre), and higher dose (15 ml/litre) along with ammonium sulphate (2%) as an adjuvant. Drying symptom with glyphosate application at 10 ml and 15 ml was observed on 3 DAHS whereas no drying symptom was observed in Potulaca oleracea weeds which were applied with glyphosate at 5 ml/litre and this might be due to sub – optimal dose of glyphosate application. In glyphosate application of 10 ml and 15 ml/litre; Leaf alone showed drying symptom and the stem remains healthy pink and also glyphosate has not been translocated to the root portion of the Portulaca oleracea as it showed healthy root system. Rainfall was received on 5DAHS; by utilizing the moisture; regeneration was observed in glyphosate applied plots. This showed that moisture would facilitate for the regeneration or regrowth of the Portulaca oleracea and might be happened due to the poor translocation of the herbicides. There was no complete control of Portulaca with optimal and high dose of glyphosate application on 10 DAHS. Hence, after 30 DAHS a weed count was taken and it was showed that only 30% population was controlled with sub – optimal dose of glyphosate application (5ml/litre); 49% and 60% control with glyphosate application at recommended (10ml/litre) and 15ml/litre respectively.

Resistance of the fall armyworm, Spodoptera frugiperda, to transgenic Bacillus thuringiensis Cry1F corn in the Americas: lessons and implications for Bt corn IRM in China.

The fall armyworm, Spodoptera frugiperda (J.E. Smith), is a major target pest of Bt crops (e.g., corn, cotton, and soybean) in North and South America. This pest has recently invaded Africa and Asia including China and the invasion has placed a great threat to the food security in many countries of these two continents. Due to the extensive use of Bt crops, practical resistance of S. frugiperda to Cry1F corn (TC 1507) with field control problems has widely occurred in Puerto Rico, Brazil, Argentina, and the mainland United States. Analyzing data generated from decade-long studies showed that several factors might have contributed to the wide development of the resistance. These factors include (1) limited modes of action of Bt proteins used in Bt crops; (2) cross-resistance among Cry1 proteins; (3) use of nonhigh dose Bt crop traits; (4) that the resistance is complete on Bt corn plants; (5) abundant in initial Cry1F resistance alleles; and (6) lack of fitness costs/recessive fitness costs of the resistance. The long-term use of Bt crop technology in the Americas suggests that Bt corn can be an effective tool for controlling S. frugiperda in China. IRM programs for Bt corn in China should be as simple as possible to be easily adopted by small-scale growers. The following aspects may be considered in its Bt corn IRM programs: (1) use of only "high dose" traits for both S. frugiperda and stalk borers; (2) developing and implementing a combined resistance monitoring program; (3) use "gene pyramiding" as a primary IRM strategy; and (4) if possible, Bt corn may not be planted in the areas where S. frugiperda overwinters. Lessons and experience gained from the global long-term use of Bt crops should have values in improving IRM programs in the Americas, as well as for a sustainable use of Bt corn technology in China.

Gene Flow Between Bt and Non-Bt Plants in a Seed Mixture Increases Dominance of Resistance to Pyramided Bt Corn in Helicoverpa zea (Lepidoptera: Noctuidae).

For delaying evolution of pest resistance to transgenic corn producing Bacillus thuringiensis (Bt) toxins, limited data are available to compare the effectiveness of refuges of non-Bt corn planted in seed mixtures versus blocks. Here we addressed this issue in the ear-feeding pest Helicoverpa zea Boddie by measuring its survival and development in the laboratory on ears from field plots with 90% Cry1A.105 + Cry2Ab corn and 10% non-Bt corn planted in a seed mixture or blocks. We compared a strain of H. zea selected for resistance to Cry1Ac in the laboratory, its parent strain not selected in the laboratory, and their F1 progeny. The relative survival of the F1 progeny and dominance of resistance were higher on ears from Bt plants in the seed mixture than the block. Half of the kernels in ears from non-Bt plants in the seed mixture produced both Cry1A.105 and Cry2Ab. However, survival on ears from non-Bt plants did not differ between the block and seed mixture. In simulations based on the observed survival, resistance to Cry1A.105 + Cry2Ab corn evolved faster with the seed mixture than the blocks, because of the higher dominance of resistance in the seed mixture. Increasing the refuge percentage improved durability of Cry1A.105 + Cry2Ab corn more for the blocks than the seed mixture. These findings imply that, for a given percentage of non-Bt corn, resistance of H. zea and other ear-feeding pests to multi-toxin Bt corn is likely to evolve faster for seed mixtures than blocks.

The Impact of Bt Corn on Aflatoxin-Related Insurance Claims in the United States

Previous field studies have reached no collective consensus on whether Bt corn, one of the most commonly planted transgenic crops worldwide, has significantly lower aflatoxin levels than non-Bt isolines. Aflatoxin, a mycotoxin contaminating corn and other commodities, causes liver cancer in humans and can pose severe economic losses to farmers. We found that from 2001–2016, a significant inverse correlation existed between Bt corn planting and aflatoxin-related insurance claims in the United States, when controlling for temperature and drought. Estimated benefits of aflatoxin reduction resulting from Bt corn planting are about $120 million to $167 million per year over 16 states on average. These results suggest that Bt corn use is an important strategy in reducing aflatoxin risk, with corresponding economic benefits. If the same principles hold true in other world regions, then Bt corn hybrids adapted to diverse agronomic regions may have a role in reducing aflatoxin in areas prone to high aflatoxin contamination, and where corn is a dietary staple.

Inheritance of Bacillus thuringiensis Cry2Ab2 protein resistance in Helicoverpa zea (Lepidoptera: Noctuidae).

The corn earworm, Helicoverpa zea (Boddie), is a major target pest of pyramided Bt corn and cotton in the United States. Field-evolved practical resistance to Cry1 and Cry2 proteins in H. zea has been documented in multiple locations in the United States. Understanding the genetic basis of Bt resistance is essential in developing insect resistance management (IRM) strategies for the sustainable use of the Bt crop technology. In this study, we characterized the genetic bases of Cry2Ab2 resistance in H. zea using diet-overlay bioassays with two different forms of Cry2Ab2 protein. Laboratory bioassays using a Cry2Ab2-resistant (RR) strain, a susceptible (SS) strain, as well as cross and backcross strains, revealed that resistance to Cry2Ab2 was autosomally inherited and controlled by more than one locus. In diet bioassays, the dominance of Cry2Ab2 resistance in H. zea varied from incompletely recessive to incompletely dominant across all tested Cry2Ab2 concentrations of either Bt corn leaf powder or solubilized protein. On leaf tissue of TwinLink cotton (expressing Cry1Ab and Cry2Ae), Cry2Ab2 resistance in H. zea was completely dominant. These results have significant implications for understanding the widespread field-evolved resistance of H. zea against Cry1 and Cry2 proteins in Bt corn and cotton and should be useful in developing effective IRM strategies for H. zea. © 2020 Society of Chemical Industry.

DETERMINANTS OF Bt CORN (Zea mays L.) ADOPTION IN CAGAYAN VALLEY, PHILIPPINES

Corn production in the Philippines is important as many farmers depend on corn farming as their main livelihood. Corn production contributed over 100 billion pesos annually on the country’s economy. However, in the early 1990s, the production of corn in the country is threatened by Asian corn borer (Ostrinia furnaculis Guenee) which significantly reduce yield. To solve the problem, the use of Bacillus thuringiensis (Bt) corn gene which is resistant to stem borer was introduced thereby bringing sustainable increasing production. However, despite technology’s effectiveness, there are always factors which motivate or dismay farmers to adopt and these need to be known. The study utilized the 1,279 corn farmers in Cagayan Valley to determine the factors that influence their adoption of Bt corn technology. Using a logistic regression model, the results showed that a set of demographic profile and farm characteristics influenced the farmer’s decision to adopt the technology. The government should consider these factors to scale up the farmers’ adoption of the technology to ensure sustainable production to cater the rising demand on corn in the country and world today. Further study may be conducted to identify other factors contributory to the adoption of the technology.

Comparing Populations of Western Corn Rootworm (Coleoptera: Chrysomelidae) in Regions With and Without a History of Injury to Cry3 Corn.

Transgenic corn expressing insecticidal proteins derived from the bacterium Bacillus thuringiensis (Bt) is an important pest management tool. Western corn rootworm, Diabrotica virgifera virgifera LeConte, is a key pest of corn in the midwestern United States that has developed field-evolved resistance to all available Bt traits. The first Bt trait to be commercialized for management of rootworm was Cry3Bb1 in 2003, and field-evolved resistance appeared in 2009. In this study, we examined fields in counties where greater-than-expected injury to Cry3 (Cry3Bb1 or mCry3A) corn roots (>1 node) had previously been reported (problem counties) and counties where injury had not been reported (non-problem counties). Four to eight fields were sampled per county in 2015, 2016, and 2017 to quantify rootworm abundance, root injury, Cry3Bb1resistance, and rootworm management strategies. Rootworm abundance, root injury, and resistance to Cry3Bb1 did not differ between county types. Management tactics differed between county types, with problem counties growing more corn, using more soil insecticide, and growing more Cry34/35Ab1 corn. Additionally, a comparison of root injury to Bt and non-Bt corn within fields indicated that farmers derived an economic benefit from planting Bt corn to manage corn rootworm. Our results suggest that rootworm populations are similar between problem and non-problem counties in Iowa due to similar levels of selection pressure on Cry3 corn, but problem county fields have applied more management tactics due to previous rootworm issues in the area.

Mutations in a Novel Cadherin Gene Associated with Bt Resistance in Helicoverpa zea.

Transgenic corn and cotton produce crystalline (Cry) proteins derived from the soil bacterium Bacillus thuringiensis (Bt) that are toxic to lepidopteran larvae. Helicoverpa zea, a key pest of corn and cotton in the U.S., has evolved widespread resistance to these proteins produced in Bt corn and cotton. While the genomic targets of Cry selection and the mutations that produce resistant phenotypes are known in other lepidopteran species, little is known about how selection by Cry proteins shape the genome of H. zea. We scanned the genomes of Cry1Ac-selected and unselected H. zea lines, and identified twelve genes on five scaffolds that differed between lines, including cadherin-86C (cad-86C), a gene from a family that is involved in Cry1A resistance in other lepidopterans. Although this gene was expressed in the H. zea larval midgut, the protein it encodes has only 17 to 22% identity with cadherin proteins from other species previously reported to be involved in Bt resistance. An analysis of midgut-expressed cDNAs showed significant between-line differences in the frequencies of putative nonsynonymous substitutions (both SNPs and indels). Our results indicate that cad-86C is a likely target of Cry1Ac selection in H. zea. It remains unclear, however, whether genomic changes at this locus directly disrupt midgut binding of Cry1Ac and cause Bt resistance, or indirectly enhance fitness of H. zea in the presence of Cry1Ac by some other mechanism. Future work should investigate phenotypic effects of these nonsynonymous substitutions and their impact on fitness of H. zea larvae that ingest Cry1Ac.

First documentation of major Vip3Aa resistance alleles in field populations of Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) in Texas, USA

The corn earworm, Helicoverpa zea, is a major target pest of the insecticidal Vip3Aa protein used in pyramided transgenic Bt corn and cotton with Cry1 and Cry2 proteins in the U.S. The widespread resistance to Cry1 and Cry2 proteins in H. zea will challenge the long-term efficacy of Vip3Aa technology. Determining the frequency of resistant alleles to Vip3Aa in field populations of H. zea is critically important for resistance management. Here, we provided the first F2 screen study to estimate the resistance allele frequency for Vip3Aa in H. zea populations in Texas, U.S. In 2019, 128 H. zea neonates per isofamily for a total of 114 F2 families were screened with a diagnostic concentration of 3.0 μg/cm2 of Vip3Aa39 protein in diet-overlay bioassays. The F2 screen detected two families carrying a major Vip3Aa resistance allele. The estimated frequency of major resistance alleles against Vip3Aa39 in H. zea in Texas from this study was 0.0065 with a 95% CI of 0.0014–0.0157. A Vip3Aa-resistant strain (RR) derived from the F2 screen showed a high level of resistance to Vip3Aa39 protein, with a resistance ratio of >588.0-fold relative to a susceptible population (SS) based on diet-overlay bioassays. We provide the first documentation of a major resistance allele conferring high levels of Vip3Aa resistance in a field-derived strain of H. zea in the U.S. Data generated from this study contribute to development of management strategies for the sustainable use of the Vip3Aa technology to control H. zea in the U.S.

Plant Preferences of Z-Pheromone Race Ostrinia nubilalis (Lepidoptera: Crambidae) Based on Leaf Tissue Consumption Rates.

European corn borer, Ostrinia nubilalis Hübner (Lepidoptera: Crambidae), has been present in the United States for over 100 yr and documented on >200 plant species, including economically valuable crops. The reported preferred host of O. nubilalis is corn, Zea mays L. (Cyperales: Poaceae), although it is considered to be a generalist agricultural pest. Life cycles of the two pheromone races, E and Z, align with the seasonality of different agricultural plants. Since the introduction of Bt corn in 1996, overall O. nubilalis presence has declined and suggests that alternative crop plants might not be suitable hosts. We investigated plant vegetation preference of third-instar Z-race O. nubilalis for leaf disks of corn and a variety of other crops using 48 h no-choice and choice tests. Z-race larvae gained more mass on V6 non-Bt field corn leaf disks in comparison to other plant species options. Additionally, a preference for non-Bt field corn leaf disks was observed in most comparisons. Higher consumption of cucumber, Cucumis sativus L. (Cucurbitales: Cucurbitaceae), leaf disks as compared to non-Bt field corn leaf disks suggested an ability to feed on excised leaf tissues of a plant species that does not induce defenses to herbivory.

Evaluation of Insecticide Thresholds in Late-Planted Bt and Non-Bt Corn for Management of Fall Armyworm (Lepidoptera: Noctuidae).

The fall armyworm, Spodoptera frugiperda (J. E. Smith), is a major pest of corn in North and South America. It is managed primarily with transgenic corn-producing insecticidal proteins of Bacillus thuringiensis (Bt), but the development of resistance threatens their durability and necessitates the use of alternative management strategies. We conducted late-planted field trials during 2016 and 2017 in South Carolina using natural infestations. We evaluated the use of Bt and non-Bt corn hybrids in combination with foliar applications of chlorantraniliprole at varying infestation thresholds to protect field corn from infestation and damage and determine effects on grain yield. All Bt hybrids were more effective at reducing fall armyworm infestation rates and leaf injury than multiple insecticide sprays, and no Bt hybrid reached the lowest infestation threshold (20%) to require supplemental insecticide treatments, despite infestations in non-Bt corn reaching >68% in each year. The only Bt and/or insecticide treatment to significantly reduce ear feeding or the proportion of ears injured (mainly by Helicoverpa zea [Boddie]) was the Bt hybrid pyramid producing Vip3A. However, significant protection of yield was detected only in the Bt hybrids producing Cry1A.105 + Cry2Ab2. All Bt traits tested in this study were effective in reducing infestation and feeding damage from fall armyworm, although this did not always result in significant protection of yield. Our results demonstrate the potential and limitations of using chlorantraniliprole with Bt (when resistance is present) and non-Bt corn to manage this pest.

Management of Euxesta spp. in Sweet Corn with McPhail Traps.

Pests attacking the ear of sweet corn, such as Helicoverpa and Euxesta species, cause economic losses for the producer and the processing industry. Feeding on the style-stigmata preventing fertilization and on the developing grain and the association with pathogens are the main causes of product depreciation. The traditional control such as spraying with chemicals is not effective, even with several applications directed to the corn ear. Bacillus thuringiensis (Bt) corn also does not reach the fly. McPhail traps that have been used to monitor the pest can be a control strategy. This work evaluated the efficiency of food attractants placed inside McPhail traps to remove adult insects, in order to reduce ear damage. Twelve McPhail-type traps were installed in a randomized complete block design containing Bio Anastrepha® alone or combined with different doses of insecticide. Every 10 days, all the captured insects were counted and separated by species and sex. Only Euxesta eluta and Euxesta mazorca were found. The occurrence of insects was greater in the period between silk emergence and grain filling. The number of females was higher, probably due to the need to feed before oviposition. The number of E. mazorca females caught in the treatment containing only Bio Anastrepha® was higher compared with that of others. The mean ear damage was very low, and there was no interaction between the production parameters and the distance between the trap and the harvested plant. In short, the use of McPhail trap containing food attractants may be a viable alternative to control corn silk flies.

Extended evaluation of Bt protein cross-pollination in seed blend plantings on survival, growth, and development of Helicoverpa zea feeding on refuge ears.

A seed blend, also called refuge in a bag (RIB), is used as a strategy to provide refuge for planting pyramided Bt corn in the U.S. Corn Belt. A major concern for the RIB strategy is cross-pollination between Bt and refuge plants, which may reduce susceptible populations of ear-feeding insects like Helicoverpa zea and affect the evolution of resistance. Previous studies showed a 5:95% (Non-Bt: Bt) RIB would be unlikely to provide sufficient refuge populations of H. zea for resistance management. In this study, we extended our research and conducted multiple trials to determine if three RIB plantings with greater refuge sizes (10, 20, and 30% refuge) could provide sufficient H. zea susceptible populations to delay resistance development. Experimental results showed that cross-pollination in 10:90%, 20:80%, and 30:70% RIB plantings still significantly reduced larval survival, delayed larval development and decreased the pupal size of H. zea on refuge plants. Regression analysis showed that the percent reduction of neonate-to-adult survivorship, relative to the survivorship on pure non-Bt ears, was significantly negatively correlated with the refuge percentage. These findings suggest that, approximately a 15% non-Bt refuge plants in RIB plantings could produce a similar number of susceptible adult moths as a 5% structured refuge planting, while an approximately 30% non-Bt refuge plant in RIB plantings could provide a similar number of susceptible moths as a 20% structured refuge planting. Information generated from this study should be useful in refining resistance management strategies for Bt crop technologies. © 2019 Society of Chemical Industry.