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Crops expressing Bacillus thuringiensis proteins have offered valuable crop protection benefits including reduced foliar insecticide use and crop yield protection. Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) has developed resistance to one or more Bt proteins, which consequently threatens these benefits. This has resulted in a renewed emphasis on the implementation of action thresholds for H. zea management decisions in cotton. Field experiments were conducted in College Station, TX; Stoneville, MS; and Blackville, SC, during 2020–2021 to evaluate current H. zea threshold recommendations for Texas, Mississippi, and South Carolina. Three Bt cotton technologies and five thresholds were evaluated. The Bt cotton technologies evaluated included non-Bt (NBT), Bollgard® 2 (BG2; Cry1Ac + Cry2Ab), and Bollgard® 3 (BG3; Cry1Ac + Cry2Ab + Vip3Aa). Across years and locations, within NBT cotton, all thresholds, including 20% oviposition, 6% fruiting forms injury, ≥3% large larvae, and preventive sprays, resulted in greater control of H. zea and higher yield and profitability relative to non-treated NBT cotton. The threshold using 6% injury to squares and bolls was the most effective in terms of yield relative to the preventive spray. Across years and locations, the BG2 cotton sprayed based on 20% oviposition or preventively resulted in greater control of H. zea , and all treatments resulted in a yield or profitability equivalent to that of the non-treated BG2 cotton. For BG3 cotton, spraying based on any of the thresholds was not beneficial. For NBT and BG2 cotton, thresholds of 20% oviposition, 6% injury to fruiting forms, or ≥3% large larvae provided a reduction in insecticide applications relative to the preventive spray treatment, suggesting that these thresholds were effective triggers for controlling H. zea. • NBT cotton sprayed based on the oviposition, injury, or larval threshold provided effective H. zea control. • BG2 cotton sprayed based on the oviposition threshold provided effective control of H. zea. • BG3 cotton sprayed based on either the oviposition, injury, or larval threshold was not beneficial. • NBT and BG2 cotton treated using the oviposition, injury, or larval threshold provided a reduction in insecticide applications. [ABSTRACT FROM AUTHOR]

Fall armyworm, Spodoptera frugiperda (J.E. Smith), is a major target species of Bt maize and Bt cotton in the United States. Field resistance to Cry1F maize in S. frugiperda has occurred in Puerto Rico, Brazil, and the southeast region of the United States. There is a great concern that the Cry1F-maize resistant S. frugiperda may move to the cotton fields and cause control problems to Bt cotton where both crops are planted. In this study, larval mortality and growth inhibition of Cry1F maize-susceptible (SS), -heterozygous (RS), and -resistant (RR) genotypes of S. frugiperda were evaluated using diet-incorporated bioassays with six Bt proteins that have been used in commercialized and future Bt cotton plants: Cry1F, Cry1Ab, Cry1Ac, Cry2Ab2, Cry2Ae, and Vip3A. Compared to the mortality data, the measuring of growth inhibition was more sensitive in determining Bt susceptibility of the insect to all six Bt proteins. Overall, RS had a similar performance as SS for the six Bt proteins with a resistance ratio of <5.5-fold. The resistance ratio of RR calculated based on larval growth inhibition was 930-fold for Cry1F, 50.5-fold for Cry1Ab, and >172-fold for Cry1Ac. In contrast, all three insect genotypes were susceptible to Cry2Ab2, Cry2Ae, or Vip3A. The results showed that the Cry1F-maize resistant S. frugiperda was cross-resistant to Cry1Ab or Cry1Ac, but not cross-resistant to Cry2Ab2, Cry2Ae, or Vip3A. The strong cross-resistance among Cry1 proteins suggest that it is necessary to plant pyramided cotton traits that contain at least two active Bt genes for Cry1F-resistant S. frugiperda to ensure the continued success of the Bt cotton technology, especially in the areas where Bt resistance has already occurred. [ABSTRACT FROM AUTHOR]

Fall armyworm, Spodoptera frugiperda (J.E. Smith), is a major pest of many crops and a cross-crop target of transgenic maize, cotton, and soybean containing Bacillus thuringiensis (Bt) genes. Some of the current Bt maize products for controlling lepidopteran species contain the Bt event MON 89034. The objective of this study was to determine the frequency of resistance alleles in field populations of S. frugiperda collected from Louisiana and Florida, U.S. to Cry1A.105, one of the two Bt genes in MON 89034. A total of 150 F 2 two-parent families of S. frugiperda were established using single-pair mating of field-collected individuals in 2011, which included 79 families from two locations in Louisiana and 71 families from one location in Florida. F 2 screen was conducted to detect resistance alleles in these families to Cry1A.105 protein in maize plants. Four out of the 79 Louisiana and 14 out of the 71 Florida families were identified to possess resistance alleles to the Cry1A.105 maize plants. Thus, the corresponding frequency of resistance alleles to Cry1A.105 maize was estimated to be 0.0158 with a 95% credibility interval (CI) of 0.0052–0.0323 for the Louisiana populations and 0.0559 with a 95% CI of 0.0319–0.0868 for the Florida populations. The resistant families survived on whole Cry1A.105 maize plants and demonstrated a significant level (>116-fold) of resistance to the Cry1A.105 protein in a diet-incorporated bioassay. These findings suggest that resistance allele frequency in S. frugiperda to single-gene Cry1A.105 maize in the U.S. southeast region apparently is not rare, most likely due to the selection of Cry1F resistance and its cross-resistance to Cry1A.105. [ABSTRACT FROM AUTHOR]

Corn earworm, Helicoverpa zea (Boddie), is a major target of pyramided Bacillus thuringiensis (Bt) corn. In the U.S. Corn Belt, a refuge-in-the-bag (RIB) strategy has been adopted to provide susceptible populations for insect resistance management (IRM) in planting pyramided Bt corn. In this study, eight field trials were conducted to evaluate occurrence and injury of H. zea in three planting patterns of non-Bt and Bt corn containing the Agrisure ® Viptera™ 3111 trait. Viptera 3111 corn possesses two Bt genes, Vip3A and Cry1Ab, targeting above-ground lepidopteran pests. Occurrence of H. zea eggs was similar between non-Bt and Bt corn ears. Eggs of H. zea were distributed either randomly or uniformly in all three planting patterns. Field trials consistently showed that Viptera 3111 corn was extremely effective in H. zea control in both structured and RIB plantings. Across all the trials, there were very little or no larval survival and damage on Bt corn ears. Ear damage and larval occurrence at the peak of 3rd to 4th instars on refuge ears in the RIB plantings were not less than those observed in the structured non-Bt corn plantings. However, larval development on RIB refuge ears was significantly delayed relative to that on ears of structured non-Bt corn plantings. Information generated from this study should provide useful information to improve IRM modeling for Bt corn. [ABSTRACT FROM AUTHOR]

The fall armyworm, Spodoptera frugiperda (J.E. Smith), is a major target of transgenic corn, Zea mays L., expressing Bacillus thuringiensis (Bt) proteins in both North and South America. A highly Cry1F-resistant strain of S. frugiperda was established from a field collection in Puerto Rico in 2011. In this study, three greenhouse trials were conducted to evaluate larval survival and leaf injury of Cry1F-susceptible, -resistant, and -heterozygous genotypes of S. frugiperda on whole plants of five non-Bt and eight Bt corn hybrids. The Bt corn products included two single-gene Bt corn hybrids containing Herculex®I (Cry1F) and YieldGard® (Cry1Ab) traits and six pyramided Bt corn hybrids representing four traits: Genuity® VT Double Pro™, Genuity®VT Triple Pro™, Genuity® SmartStax™, and Agrisure® Viptera™ 3111. In each trial, neonates of S. frugiperda were placed into the plant whorls at vegetative plant stages (V6–V10). Larvae of the three insect genotypes on non-Bt corn hybrids survived well and caused serious plant injury. Cry1Ab corn was ineffective against all three insect genotypes. On Cry1F corn plants, resistant larvae survived on 72.9% plants after 12–15 d and caused a leaf injury rating (Davis' 1 to 9 scales) of 5.7 after 7 d and 7.6 after 12–15 d. Both the larval survivorship and leaf injury rates of the resistant larvae on Cry1F corn plants were not significantly different from those observed on non-Bt corn hybrids. In contrast, no live larvae and little or no leaf injury were observed on the Cry1F corn plants that were infested with susceptible or heterozygous genotypes, or on the pyramided Bt plants. The results demonstrated that the Cry1F-resistant S. frugiperda was highly resistant to whole plants of Cry1F corn and the resistance was recessive. Hybrids that contained one of the four pyramided Bt traits were effective for managing the Cry1F resistance in S. frugiperda. [ABSTRACT FROM AUTHOR]

Protein contamination on refuge kernels due to cross-pollination from Bt corn to non-Bt corn ears is a major concern in the use of a seed mixture refuge strategy (“RIB”) for resistance management of ear-feeding pests. In this study, occurrence, distribution, and ear damage of the corn earworm, Helicoverpa zea (Boddie), were evaluated in three planting patterns of non-Bt and Bt corn plants containing Genuity® SmartStax™ traits. The three planting patterns were 1) pure stands of 27 Bt plants; 2) pure stands of 27 non-Bt plants; and 3) one non-Bt plant in the center surrounded by 26 Bt plants. A total of six trials were conducted in open field conditions with natural infestations in 2011 and 2012. Egg populations of H. zea were distributed randomly or uniformly, and the number of eggs laid was similar between Bt and non-Bt corn ears regardless of the planting patterns, suggesting that females of H. zea have no egg-laying preference between Bt and non-Bt plants. Bt corn plants containing Genuity® SmartStax™ traits were equally effective in the control of H. zea in pure stands of Bt corn and “RIB” plantings. Occurrence of larvae and ear damage on Bt corn were significantly lower than on non-Bt plants and there were no significant differences between pure stands of Bt and “RIB” plantings across all trials. However, the limited numbers of live larvae in the pure stands of Bt plants were distributed non-randomly, suggesting a possibility of uneven expression of Bt proteins or elevated larval movement in the pure stands of Bt plants. Larval occurrence (3rd–5th instars) and ear damage on the refuge ears in “RIB” plantings were similar to or greater than found on ears of pure stands of non-Bt plants. However, more studies are needed to understand the effect of pollen movement on the full life cycle of H. zea before a final conclusion on the refuge function of RIB planting can be made. [ABSTRACT FROM AUTHOR]

The fall armyworm, Spodoptera frugiperda (J.E. Smith), is an economically important crop pest and a major target of transgenic crops expressing Cry and/or Vip3Aa proteins. Widespread adoption of Bt crops has placed a strong selection pressure on the insect populations. Till now, seven lepidopteran and two coleopteran insect species have developed the field-evolved resistance to a Bt crop with practical consequences. Understanding the genetic basis of insect resistance to a Bt protein is essential for designing appropriate resistance management strategies. In this study, we firstly characterized the genetic basis of Vip3Aa resistance in S. frugiperda derived from Texas. Then, we compared the genetic basis of Vip3Aa resistance of this Texas population with a known Vip3Aa resistant population of S. frugiperda collected from Louisiana to determine whether the genetic basis of the resistance in these two resistant populations was the same. Finally, we evaluated the responses of the Vip3Aa-resistant population of S. frugiperda collected from Texas against Cry1F and Cry2Ab2 purified proteins. The results showed that the resistance to Vip3Aa protein in S. frugiperda collected from Texas was likely controlled by a single, functionally recessive and autosomal gene. In addition, the resistance to Vip3Aa in the Texas population of S. frugiperda was not cross-resistant to Cry1F and Cry2Ab2 proteins. The interstrain complementation tests between the Texas and Louisiana resistant populations indicated the genetic basis of the resistance to Vip3Aa in these two populations are different. Considering the rapid invasion of S. frugiperda and the adoption of Vip3Aa Bt crops in the United States and other countries, results generated from this study should provide important information for management of Vip3Aa resistance in S. frugiperda worldwide. • Vip3A resistance in S. frugiperda was autosomal, monogenic, and recessive. • Vip3A resistance in S. frugiperda was not cross-resistant to Cry1F and Cry2Ab2. • Genetic basis of Vip3A resistance in Texas and Louisiana populations are different. [ABSTRACT FROM AUTHOR]

Transgenic corn and cotton expressing Cry and/or Vip3Aa proteins from Bacillus thuringiensis (Bt) have been planted for managing lepidopteran and coleopteran insect pests for more than two decades in the U.S. Evolution of insect resistance to these Bt proteins is a major threat for the continued efficacy of Bt technology. Understanding patterns of cross-resistance and dominance of resistance in target insects is essential for resistance management and pest control. The bollworm, also called corn earworm, Helicoverpa zea (Boddie), is a major target pest of pyramided Bt crops in the U.S. In this study, we evaluated the responses of Cry2Ab2-resistant (RR), susceptible (SS), and heterozygous (RS) strains of H. zea to the Cry1Ac, Cry1A.105, Cry2Ab2 and Vip3Aa39 proteins using diet-overlay bioassays. The Cry2Ab2-resistant H. zea also possessed high levels of resistance to Cry1Ac and Cry1A.105, but were very susceptible to Vip3Aa. The resistance to Cry1 and Cry2 proteins in RR H. zea varied from incompletely recessive to completely dominant depending on the protein concentration. These data suggest that the RR H. zea possess multiple non-recessive resistance to Cry1/Cry2 proteins. These results have significant implications for understanding field-evolved resistance of H. zea to Cry1 and Cry2 proteins in U.S. Bt corn and cotton fields and should contribute to developing effective management strategies for control of H. zea. • Susceptibility of Cry2Ab2 resistant H. zea to different Bt proteins was evaluated. • Cry2Ab2-resistant H. zea showed non-recessive resistance to Cry1Ac and Cry1A.105 • Cry2Ab2-resistant H. zea was very susceptible to Vip3Aa protein. [ABSTRACT FROM AUTHOR]

In the United States, fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), is a target species of transgenic corn (Zea mays L.) expressing pyramided Bacillus thuringiensis proteins. In 2011, a total of 150 F2 two-parent families of S. frugiperda were established using single-pair matings of feral individuals collected from three locations in Louisiana and Florida. The objective of this study was to determine the susceptibility of these field derived families of S. frugiperda to a pyramided Bt corn hybrid containing Agrisure®Viptera™ 3111 traits. For each F2 family, 96 neonates were assayed on leaf tissue of Agrisure®Viptera™ 3111 corn in the laboratory. None of the 150 families survived for 7 days on leaf tissue of the Bt corn plants. The results demonstrate that the field populations of S. frugiperda collected from Louisiana and Florida were susceptible to the pyramided Bt corn product containing Agrisure®Viptera™ 3111 traits. The data generated in this study can be used as baseline data for resistance monitoring. [ABSTRACT FROM AUTHOR]

Fall armyworm, Spodoptera frugiperda , is a major target pest of pyramided Bt (Bacillus thuringiensis) corn and cotton in the U.S. In this study, we used a well-established F 2 screening method to detect resistance alleles to Vip3Aa51 protein in field populations of S. frugiperda in Texas. In 2018, a total of 95 F 2 two-parent families of S. frugiperda were established using single-pair mating of field individuals collected from two sorghum fields in two locations in Texas. For each F 2 family, 128 neonates were screened on a diagnostic concentration of 3.0 μg/cm2 Vip3Aa51 protein using diet-overlay bioassays. The results showed that two out of 95 two-parent families possessed a major resistance allele against Vip3Aa51 protein. The estimated frequency of major resistance alleles against Vip3Aa51 in S. frugiperda in Texas was 0.0078 with a 95% CI of 0.0016–0.0188. The Vip3Aa51-resistant strain (TX-RR) derived from the F 2 screening showed a high level of resistance to Vip3Aa51 protein, with a resistance ratio of >395.0-fold relative to a susceptible population (SS). Information generated from this study should be useful in studying mechanisms of Vip3Aa resistance in S. frugiperda , resistance monitoring, and resistance management of the Vip3Aa technology. • 95 F 2 families of S. frugiperda from Texas were screened against Vip3Aa51 in 2018. • Resistance allele frequency against Vip3Aa51 in S. frugiperda in Texas is low. • A Vip3Aa51-resistant strain of S. frugiperda is established using F 2 screen method. • The resistant strain has a resistance ratio of >395.0-fold against Vip3Aa51. [ABSTRACT FROM AUTHOR]

Seed blend, also called 'refuge-in-the-bag (RIB)', has been used as a refuge strategy to generate susceptible insect populations for Bt maize resistance management in the U.S. Corn Belt. A major concern related to the use of RIB is the cross-pollination of maize plants that cause Bt protein expression in the kernels of refuge ears. Bt protein expression in the refuge ears can negatively affect the insect populations on refuge plants, especially for ear feeders such as the corn earworm, Helicoverpa zea (Boddie). The current study investigated the reproduction of three populations of H. zea collected from a pure stand of non-Bt maize and the refuge ears of 90:10% (Bt:non-Bt seeds) and 80:20% RIBs, as well as the growth, development and reproduction of the F 1 offspring of the three populations. The biological parameters measured for the F 1 offspring included neonate-to-pupa developmental time, pupal weight, neonate-to-adult survival rate, egg production, and net reproductive rate of the adults on meridic diet. Reproduction of the parents and the fitness of the F 1 offspring were similar among the three populations and were not significantly affected by the seed blending patterns. Information generated from this study will be useful in assessing if RIB is a suitable method for Bt maize resistance management in the southern U.S. • Parental fecundity of H. zea populations from three seed mixes of non-Bt and Bt corn were evaluated. • Survival, development, and reproduction of the F 1 filial populations were measured on meridic diet. • Reproduction of the parents and the relative fitness of the F 1 offspring were not affected by Bt protein expression in refuge ears. • Information generated will improve our assessment of the suitability of seed mix for Bt corn IRM. [ABSTRACT FROM AUTHOR]