Seeds of change: corn seed mixtures for resistance management and integrated pest management

Inheritance and fitness cost of laboratory-selected resistance to Vip3Aa in Helicoverpa zea (Lepidoptera: Noctuidae)

The polyphagous pest Helicoverpa zea (Lepidoptera: Noctuidae) has evolved practical resistance to transgenic corn and cotton producing Cry1 and Cry2 crystal proteins from Bacillus thuringiensis (Bt) in several regions of the United States. However, the Bt vegetative insecticidal protein Vip3Aa produced by Bt corn and cotton remains effective against this pest. To advance knowledge of resistance to Vip3Aa, we selected a strain of H. zea for resistance to Vip3Aa in the laboratory. After 28 generations of continuous selection, the resistance ratio was 267 for the selected strain (GA-R3) relative to a strain not selected with Vip3Aa (GA). Resistance was autosomal and almost completely recessive at a concentration killing all individuals from GA. Declines in resistance in heterogeneous strains containing a mixture of susceptible and resistant individuals reared in the absence of Vip3Aa indicate a fitness cost was associated with resistance. Previously reported cases of laboratory-selected resistance to Vip3Aa in lepidopteran pests often show partially or completely recessive resistance at high concentrations and fitness costs. Abundant refuges of non-Bt host plants can maximize the benefits of such costs for sustaining the efficacy of Vip3Aa against target pests.

Impact of seed blend and structured maize refuge on Helicoverpa zea (Lepidoptera: Noctuidae) potential phenological resistance development parameters in pupae and adults

BACKGROUND

Helicoverpa zea, an economic pest in the south-eastern United States, has evolved practical resistance to Bacillus thuringiensis (Bt) Cry toxins in maize and cotton. Insect resistance management (IRM) programs have historically required planting of structured non-Bt maize, but because of its low adoption, the use of seed blends has been considered. To generate knowledge on target pest biology and ecology to help improve IRM strategies, nine field trials were conducted in 2019 and 2020 in Florida, Georgia, North Carolina, and South Carolina to evaluate the impact of Bt (Cry1Ab + Cry1F or Cry1Ab + Cry1F + Vip3A) and non-Bt maize plants in blended and structured refuge treatments on H. zea pupal survival, weight, soil pupation depth, adult flight parameters, and adult time to eclosion.

RESULTS

From a very large sample size and geography, we found a significant difference in pupal mortality and weight among treatments in seed blends with Vip3A, implying that cross-pollination occurred between Bt and non-Bt maize ears. There was no treatment effect for pupation depth, adult flight distance, and eclosion time.

CONCLUSION

Results of this study demonstrate the potential impact of different refuge strategies on phenological development and survival of an important pest species of regulatory concern. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Monitoring resistance of Ostrinia nubilalis (Lepidoptera: Crambidae) in Canada to Cry toxins produced by Bt corn

The first case of field-evolved resistance in European corn borer, Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae) to transgenic corn (Zea mays L.) producing a Bacillus thuringiensis (Bt) Berliner toxin was discovered in Nova Scotia, Canada in 2018. This case involved resistance to Bt corn producing Cry1Fa toxin. As a mitigation response, Bt corn hybrids producing only Cry1Fa were replaced in that region with hybrids producing two or three Bt toxins targeting O. nubilalis. In this study, we collected O. nubilalis in several corn-growing regions of Canada during 2018 to 2020 and tested their progeny for susceptibility to four Bt toxins produced by currently available Bt corn that targets O. nubilalis: Cry1Fa, Cry1Ab, Cry1A.105, and Cry2Ab. Based on toxin concentrations killing 50% of larvae from 23 field-derived strains relative to two susceptible laboratory strains, the resistance ratio was at least 10 for Cry1Fa for 12 strains (52% of strains) consisting of 10 strains from Nova Scotia, as well as strains from near Montreal, Quebec and Roseisle, Manitoba. We found low but statistically significant resistance relative to at least one of two susceptible strains for Cry1Ab (23% of strains), Cry1A.105 (45% of strains), and Cry2Ab (14% of strains), with maximum resistance ratios of 3.9, 5.8, and 2.0, respectively. These results provide key information for addressing O. nubilalis resistance to Bt corn in Canada.

Global Patterns of Insect Resistance to Transgenic Bt Crops: The First 25 Years

Crops genetically engineered to produce insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) have improved pest management and reduced reliance on insecticide sprays. However, evolution of practical resistance by some pests has reduced the efficacy of Bt crops. We analyzed global resistance monitoring data for 24 pest species based on the first 25 yr of cultivation of Bt crops including corn, cotton, soybean, and sugarcane. Each of the 73 cases examined represents the response of one pest species in one country to one Bt toxin produced by one or more Bt crops. The cases of practical resistance rose from 3 in 2005 to 26 in 2020. Practical resistance has been documented in some populations of 11 pest species (nine lepidopterans and two coleopterans), collectively affecting nine widely used crystalline (Cry) Bt toxins in seven countries. Conversely, 30 cases reflect no decrease in susceptibility to Bt crops in populations of 16 pest species in 10 countries. The remaining 17 cases provide early warnings of resistance, which entail genetically based decreases in susceptibility without evidence of reduced field efficacy. The early warnings involve four Cry toxins and the Bt vegetative insecticidal protein Vip3Aa. Factors expected to favor sustained susceptibility include abundant refuges of non-Bt host plants, recessive inheritance of resistance, low resistance allele frequency, fitness costs, incomplete resistance, and redundant killing by multi-toxin Bt crops. Also, sufficiently abundant refuges can overcome some unfavorable conditions for other factors. These insights may help to increase the sustainability of current and future transgenic insecticidal crops.

Impact of agronomic practices on Fusarium mycotoxin accumulation in maize grain

In the Great Lakes region of North America, Gibberella ear rot (GER), caused by Fusarium graminearum, affects grain quality due to the accumulation of mycotoxins. GER severity is strongly influenced by environmental conditions; however, agronomic practices can also influence disease severity and mycotoxin accumulation. In this study, three separate small-plot experiments were conducted at Ridgetown, ON, Canada during 2019 and 2020 under an inoculated-misted system to determine Fusarium mycotoxin accumulation as affected by: (1) plant population density; (2) in-row-plant developmental variability; and (3) the effect of integrated Bt refuge genetics. In this study, DON concentrations were at least 49% higher in maize at 113,600 plants/ha compared to 79,000 plants/ha. Moreover, mycotoxin accumulation was higher in plants that were delayed developmentally in the crop row; total DON concentrations were at least 310% higher in late silked plants adjacent to early silked plants. Results of the plant population density and in-row-plant developmental variability suggest that the main driver for mycotoxin accumulation was stress induced by plant competition rather than environmental conditions; this highlights the importance of avoiding plant competitive stress as a strategy to reduce the risks of mycotoxin accumulation. In this study, there was no statistical difference in DON accumulation between the Bt component and the non-Bt component in each of the four hybrids tested; however, there was evidence that hybrids varied in susceptibility, including the Bt and non-Bt components that were paired commercially in a bag of seed maize. Reducing mycotoxins in maize requires integrated management, which includes agronomic considerations. These results indicate that mycotoxins are favoured with high plant populations and plant-to-plant variability in the row, especially in susceptible hybrids.

Best Management Practices to Delay the Evolution of Bt Resistance in Lepidopteran Pests Without High Susceptibility to Bt Toxins in North America

Canadian and United States (US) insect resistance management (IRM) programs for lepidopteran pests in Bacillus thuriengiensis (Bt)-expressing crops are optimally designed for Ostrinia nubilalis Hübner in corn (Zea mays L.) and Chloridea virescens Fabricius in cotton (Gossypium hirsutum L.). Both Bt corn and cotton express a high dose for these pests; however, there are many other target pests for which Bt crops do not express high doses (commonly referred to as nonhigh dose pests). Two important lepidopteran nonhigh dose (low susceptibility) pests are Helicoverpa zea Boddie (Lepidoptera: Noctuidae) and Striacosta albicosta Smith (Lepidoptera: Noctuidae). We highlight both pests as cautionary examples of exposure to nonhigh dose levels of Bt toxins when the IRM plan was not followed. Moreover, IRM practices to delay Bt resistance that are designed for these two ecologically challenging and important pests should apply to species that are more susceptible to Bt toxins. The purpose of this article is to propose five best management practices to delay the evolution of Bt resistance in lepidopteran pests with low susceptibility to Bt toxins in Canada and the US: 1) better understand resistance potential before commercialization, 2) strengthen IRM based on regional pest pressure by restricting Bt usage where it is of little benefit, 3) require and incentivize planting of structured corn refuge everywhere for single toxin cultivars and in the southern US for pyramids, 4) integrate field and laboratory resistance monitoring programs, and 5) effectively use unexpected injury thresholds.

Managing Fall Armyworm in Africa: Can Bt Maize Sustainably Improve Control?

The recent invasion of Africa by fall armyworm, Spodoptera frugiperda, a lepidopteran pest of maize and other crops, has heightened concerns about food security for millions of smallholder farmers. Maize genetically engineered to produce insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) is a potentially useful tool for controlling fall armyworm and other lepidopteran pests of maize in Africa. In the Americas, however, fall armyworm rapidly evolved practical resistance to maize producing one Bt toxin (Cry1Ab or Cry1Fa). Also, aside from South Africa, Bt maize has not been approved for cultivation in Africa, where stakeholders in each nation will make decisions about its deployment. In the context of Africa, we address maize production and use; fall armyworm distribution, host range, and impact; fall armyworm control tactics other than Bt maize; and strategies to make Bt maize more sustainable and accessible to smallholders. We recommend mandated refuges of non-Bt maize or other non-Bt host plants of at least 50% of total maize hectares for single-toxin Bt maize and 20% for Bt maize producing two or more distinct toxins that are each highly effective against fall armyworm. The smallholder practices of planting more than one maize cultivar and intercropping maize with other fall armyworm host plants could facilitate compliance. We also propose creating and providing smallholder farmers access to Bt maize that produces four distinct Bt toxins encoded by linked genes in a single transgene cassette. Using this novel Bt maize as one component of integrated pest management could sustainably improve control of lepidopteran pests including fall armyworm.

Quantifying the Contribution of Seed Blended Refugia in Field Corn to Helicoverpa zea (Lepidoptera: Noctuidae) Populations

Helicoverpa zea (Boddie), a pest of cotton that also occurs in field corn, is commonly controlled through the use of foliar-applied insecticides or transgenic crops expressing Bacillus thuringiensis (Berliner) (Bt) genes. To minimize the risk of Bt resistance in pest populations, refuge systems have been implemented for sustainable agroecosystem management. Historically, structured refuge compliance among growers has been low, leading to the commercialization of seed blended refugia. To test the viability of seed blended refugia in southern U.S. field corn, field studies were conducted in Mississippi and Georgia during 2016, 2017, and 2018 growing seasons. To quantify adult H. zea emergence from structured (non-Bt corn) and seed blended refuge options, emergence traps were utilized. Kernel damage among seed blended refuge and structured refuge corn ears were recorded and compared. The timing of moth emergence was recorded. When compared to a structured refuge, H. zea adult moth emergence from seed blended refugia did not significantly differ. Kernel damage of non-Bt plants in the seed blended treatments was not significantly different than non-Bt plants in the structured refuge treatments. Moth emergence timing was not significantly delayed between the structured refuge and seed blended refuge treatments. Results of this study suggest that a seed blended refuge may provide an effective insecticide resistance management alternative for H. zea in areas where structured refuge compliance is low.

Performance of Bt-susceptible and -heterozygous dual-gene resistant genotypes of Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) in seed blends of non-Bt and pyramided Bt maize

A seed blend refuge has been implemented in the U.S. Corn Belt for Bt maize resistance management. The fall armyworm, Spodoptera frugiperda (J.E. Smith), is a target pest of Bt maize in the Americas. The larvae of this pest are mobile, which may affect the efficacy of seed blend refuges. In this study, field and greenhouse trials were conducted to determine the performance of Bt-susceptible (aabb) and -heterozygous dual-gene-resistant (AaBb) genotypes of S. frugiperda in seed blends of non-Bt and pyramided Bt maize. Three field trials evaluated larval survival, larval growth, and plant injury with aabb in seed blends of Bt maize expressing Cry1A.105/Cry2Ab2/Vip3A with 0-30% non-Bt seeds. Greenhouse tests investigated the performance of aabb and AaBb in seed blends of Cry1A.105/Cry2Ab2 with 0-30% non-Bt seeds. In pure non-Bt maize plots, after 9-13 d of neonates being released on the plants, 0.39 and 0.65 larvae/plant survived with leaf injury ratings of 4.7 and 5.9 (Davis's 1-9 scale) in the field and greenhouse, respectively. In contrast, live larvae and plant injury were virtually not observed on Bt plants across all planting patterns. Larval occurrence and plant injury by aabb on non-Bt plants were similar between seed blends and pure non-Bt plantings, suggesting that the blended refuges could provide an equivalent susceptible population as structured refuge under the test conditions. In the greenhouse, the two insect genotypes in seed blends performed similarly, indicating that the seed blends did not provide more favorable conditions for AaBb over aabb. The information generated from this study should be useful in managing S. frugiperda and evaluating if send blends could be suitable refuge options for Bt resistance management in the regions where the insect is a primary target pest.

Effects of gene flow between Bt and non-Bt plants in a seed mixture of Cry1A.105 + Cry2Ab corn on performance of corn earworm in Arizona

BACKGROUND

Using natural populations of Helicoverpa zea from Arizona, we tested the hypotheses that gene flow between Bacillus thuringiensis (Bt) plants and non-Bt plants in a seed mixture of 10% non-Bt corn and 90% Bt corn producing Cry1A.105 and Cry2Ab reduces larval performance on ears from non-Bt plants, or increases performance on ears from Bt plants.

RESULTS

Gene flow was not detected in blocks of non-Bt or Bt corn but was extensive in seed mixtures. Analyses of larval weight and abundance over a period of 3 to 4 weeks did not indicate consistent effects of gene flow on development rate and survival. However for non-Bt plants, the ear area damaged and percentage of ears with exit holes were significantly lower in the seed mixtures than blocks. By contrast, the percentage of ears with exit holes and ear damage did not differ significantly between the seed mixtures and blocks for Bt plants. Nearly 100% of the ears were damaged and the damaged area was substantial, showing that H. zea is a major ear-feeding pest in Arizona. Relative to non-Bt corn, the pyramided Bt corn did not significantly reduce the percentage of damaged ears and only reduced the ear area damaged by 21 to 39%, indicating that H. zea may have evolved resistance to Cry1A.105, Cry2Ab, or both.

CONCLUSIONS

Our results indicate that gene flow between Bt and non-Bt plants in seed mixtures reduced effective refuge size, and that block refuges may be needed to manage the evolution of H. zea resistance to Bt corn in Arizona. © 2020 Society of Chemical Industry.

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.

Geographic Distribution of Bacillus thuringiensis Cry1F Toxin Resistance in Western Bean Cutworm (Lepidoptera: Noctuidae) Populations in the United States

The western bean cutworm (WBC), Striacosta albicosta (Lepidoptera: Noctuidae), can be a severe pest of transgenic corn in the western Plains and Great Lakes regions of North America, including on hybrids expressing the Bacillus thuringiensis (Bt) Cry1F toxin. The level and geographic distribution of Cry1F resistance are not completely known. Neonate S. albicosta from 10 locations between Nebraska and New York state were subjected to dose-response trypsin-activated native Cry1F toxin overlay bioassays. In 2017, the mean estimated lethal concentration causing 50% larval mortality (LC50) ranged from 15.1 to 18.4 µg Cry1F cm-2, and were not significantly different among locations. In 2018, LC50 estimates at Scottsbluff, NE (22.0 µg Cry1F cm-2) and Watertown, NY (21.7 µg Cry1F cm-2) were significantly higher when compared to locations in Michigan (15.8 µg Cry1F cm-2). Significantly lower 14-day larval weight among survivors was correlated with higher Cry1F dose. Results from this study indicate that S. albicosta survivorship on purified Bt Cry1F toxin shows a relatively even distribution across the native and range expansion areas where seasonal field infestations typically occur.

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

BACKGROUND

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.

RESULTS

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.

CONCLUSION

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.

Plant Abandonment by Busseola fusca (Lepidoptera: Noctuidae) Larvae: Do Bt Toxins Have an Effect?

Busseola fusca (Fuller; Lepidoptera: Noctuidae) is an important pest of maize in Africa and can be effectively controlled by Bt maize. However, the sustainability of this technology is threatened by resistance evolution, which necessitates the implementation of the high-dose/refuge insect resistance management (IRM) strategy. Despite the success of this IRM strategy, it is based on several assumptions about insect-hostplant interactions that are not always valid for different pest species. In this study, the plant abandonment behavior of Cry1Ab-resistant and susceptible B. fusca larvae were evaluated on a non-Bt, single toxin (Cry1Ab), and a pyramid event (Cry1.105 + Cry2Ab2) of maize over a four-day period. The aim was to determine if larvae are more likely to abandon maize plants that contain Bt-toxins than conventional non-Bt plants, and if resistance to the Cry1Ab-toxin affects this behavior. This study found that both Bt-resistant and susceptible B. fusca neonate larvae show feeding avoidance behavior and increased plant abandonment rates when exposed to Bt maize leaf tissue. The implications of these findings for the design of IRM strategies and choice of refuge structures are discussed in the context of Bt maize in Africa.

Larval Migration Behaviour of Busseola fusca (Lepidoptera: Noctuidae) on Bt and Non-Bt Maize under Semi-Field and Field Conditions

Busseola fusca (Fuller) (Lepidoptera: Noctuidae) is a destructive pest of maize throughout the African continent. Bt maize is an effective control measure for this pest, however, selection pressure for resistance evolution is high. This necessitates the implementation of insect resistance management (IRM) strategies such as the high-dose/refuge strategy. This IRM strategy relies on the validity of several assumptions about the behaviour of pests during insect-hostplant interactions. In this study, the migration behaviour of B. fusca larvae was evaluated in a semi-field (greenhouse) and field setting. The effect of factors such as different Cry proteins, plant growth stage at infestation, and plant density on the rate and distance of larval migration were investigated over four and five week periods. Migration of the larvae were recorded by using both a leaf feeding damage rating scale and destructive sampling at the end of the trials. Results indicated that B. fusca larval migration success was significantly affected by plant growth stage and plant density-while limited larval migration was recorded in plots inoculated with larvae at a late growth stage (V10), higher plant density facilitated increased interplant migration. The results also suggest that B. fusca larvae do not migrate extensively (rarely further than two plants from the natal plant) and that larval mortality is high. Implications for IRM strategies are discussed.

Susceptibility of Different Instars of Striacosta albicosta (Lepidoptera: Noctuidae) to Vip3A, a Bacillus thuringiensis (Bacillaceae: Bacillales) Protein

Striacosta albicosta (Smith) (Lepidoptera: Noctuidae) is an important pest of corn, Zea mays L. in the Great Lakes region, which can be controlled by transgenic corn expressing Vip3A protein from Bacillus thuringiensis. To inform insect resistance management, the susceptibility, survival, and development of first, third, and fifth instar S. albicosta to Vip3A was determined using protein-overlay and corn tissue bioassays. Tissue bioassays were also used to determine the quantity of corn tissues with and without Vip3A-expression consumed by various instars. In diet bioassays, third and fifth instars were significantly less susceptible to Vip3A compared with first instars; however, no significant difference was observed in susceptibility of older instars. In tissue bioassays, survival was lowest for larvae fed Vip3A-expressing tissues, ranging from 0 to 21%, however, developmental measures of larvae fed Vip3A-expressing tissues did not differ from those fed artificial diet or tissues of other Bt events. Consumption of Vip3A × Cry1Ab tissues did not differ from that of Cry1Ab for each instar. Estimated Vip3A exposure of first instars ranged from 3 to 57 times higher than the concentration required for 99% mortality (LC99) based on the product of the reported Vip3A expression in transgenic corn tissues and the consumption observed in tissue bioassays; however, the estimated exposure of third and fifth instars to Vip3A was lower than their respective LC99. These findings suggest that first instar S. albicosta maybe exposed to a high dose of Vip3A under field conditions; however, Vip3A-expression in corn may not be high dose against older instars, increasing the risk of resistance development.

Effects of seed mixture sowing with transgenic Bt rice and its parental line on the population dynamics of target stemborers and leafrollers, and non-target planthoppers

The widespread planting of insect-resistant crops has caused a dramatic shift in agricultural landscapes, thus raising concerns about the potential impacts on both target and non-target pests. In this study, we examined the potential effects of intra-specific seed mixture sowing with transgenic Bt rice (Bt) and its parental non-transgenic line (Nt) (100% Bt rice [Bt₁₀₀ ], 5% Nt+95% Bt [Nt₀₅ Bt₉₅ ], 10% Nt+90% Bt [Nt₁₀ Bt₉₀ ], 20% Nt+80% Bt [Nt₂₀ Bt₈₀ ], 40% Nt+60% Bt [Nt₄₀ Bt₆₀ ] and 100% Nt rice [Nt₁₀₀ ]) on target and non-target pests in a 2-year field trial in southern China. The occurrence of target pests, Sesamia inferens, Chilo suppressalis and Cnaphalocrocis medinalis, decreased with the increased ratio of Bt rice, and the mixture ratios with more than 90% Bt rice (Bt₁₀₀ and Nt₀₅ Bt₉₅ ) significantly increased the pest suppression efficiency, with the lowest occurrences of non-target planthoppers, Nilaparvata lugens and Sogatella furcifera in Nt₁₀₀ and Nt₀₅ Bt₉₅ . Furthermore, there were no significant differences in 1000-grain dry weight and grain dry weight per 100 plants between Bt₁₀₀ and Nt₀₅ Bt₉₅ . Seed mixture sowing of Bt rice with ≤10% (especially 5%) of its parent line was sufficient to overcome potential compliance issues that exist with the use of block or structured refuge to provide most effective control of both target and non-target pests without compromising the grain yield. It is also expected that the strategy of seed mixture sowing with transgenic Bt rice and the non-transgenic parental line would provide rice yield stability while decreasing the insecticide use frequency in rice production.

Assessing the Efficacy of Bacillus thuringiensis (Bt) Pyramided Proteins Cry1F, Cry1A.105, Cry2Ab2, and Vip3Aa20 Expressed in Bt Maize Against Lepidopteran Pests in Brazil

Field studies across four states in maize-producing areas of Brazil were conducted to characterize the efficacy of a new pyramided Bacillus thuringiensis (Bt) Berliner technology in maize, Zea mays L., and compare it to existing single and pyramided commercial Bt technologies, to control Helicoverpa zea Boddie (Lepidoptera: Noctuidae), Elasmopalpus lignosellus Zeller (Lepidoptera: Pyralidae), Agrotis ipsilon (Hufnagel) (Lepidoptera: Noctuidae), and Diatraea saccharalis F. (Lepidoptera: Crambidae). Bt maize expressing Vip3Aa20 protein and pyramided Bt maize expressing proteins Cry1F + Cry1A.105 + Cry2Ab2 + Vip3Aa20 provided excellent protection against kernel feeding by H. zea compared to Bt technologies expressing only Cry1F or Cry1A.105 + Cry2Ab2. Bt maize expressing Cry1F, Cry1A.105 + Cry2Ab2, Cry1F + Cry1A.105 + Cry2Ab2, and Cry1F + Cry1A.105 + Cry2Ab2 + Vip3Aa20 resulted in less than 5% of plants injured by E. lignosellus, significantly less than Bt maize expressing only Vip3Aa20 and a non-Bt maize iso-hybrid with and without a thiamethoxam seed treatment. The highest protection against plant cutting injury caused by A. ipsilon was observed in the pyramid Bt maize technology expressing Cry1F + Cry1A.105 + Cry2Ab2 + Vip3Aa20. However, it did not differ statistically from the Bt maize expressing Vip3Aa20, Cry1F, or Cry1F + Cry1A.105 + Cry2Ab2. All Bt maize hybrids evaluated in our study were highly effective in reducing tunneling injury caused by D. saccharalis. These results show that a new maize technology expressing pyramided Bt proteins Cry1F + Cry1A.105 + Cry2Ab2 + Vip3Aa20 offers a higher level of protection from feeding by the above lepidopteran pest complex compared to maize with a single Bt protein or double pyramided Bt proteins.

Effects of seed mixture sowing with resistant and susceptible rice on population dynamics of target planthoppers and non-target stemborers and leaffolders

BACKGROUND

The widespread planting of insect-resistant crops has caused a dramatic shift in agricultural landscapes, thus raising concerns about the potential impact on both target and non-target pests worldwide. In this study, we examined the potential effects of six seed mixture ratios of insect-resistance dominance [100% (R100), 95% (S05R95), 90% (S10R90), 80% (S20R80), 60% (S40R60), and 0% (S100)] on target and non-target pests in a 2-year field trial in southern China.

RESULTS

The occurrence of the target pests Nilaparvata lugens and Sogatella furcifera decreased with an increase in the ratio of resistant rice, and mixture ratios with ≥90% resistant rice significantly increased the pest suppression efficiency, with the lowest occurrences of the non-target pests Sesamia inferens, Chilo suppressalis and Cnaphalocrocis medinalis for S100 and S10R90 seed mixture ratios. Furthermore, there were no significant differences in the 1000-grain dry weight and grain yield between R100 and other treatments with ≥80% resistant seeds in the mixture (S20R80, S10R90 and S05R95).

CONCLUSION

S10R90 produced a good yield and provided the most effective control of both target and non-target pests, with the potential to significantly reduce the application of chemical pesticides for integrated pest management in paddy fields. It is further presumed that the strategy of seed mixture with resistant and susceptible rice would be advantageous for rice yield stability. © 2018 Society of Chemical Industry.

Measuring rootworm refuge function: Diabrotica virgifera virgifera emergence and mating in seed blend and strip refuges for Bacillus thuringiensis (Bt) maize

BACKGROUND

Current insect resistance management plans rely on refuges of plants without Bacillus thuringiensis (Bt) toxins to provide a gene pool of unexposed insects. Insects from refuges must mate with insects from Bt maize to slow resistance evolution. We used stable isotope labeling to observe Diabrotica virgifera virgifera emergence, dispersal, physical characteristics, and mating in Bt and refuge maize planted in different refuge configurations. Our objective was to assess how refuge type facilitates mating between insects from Bt and refuge plants.

RESULTS

Mating between D. v. virgifera beetles from different plant types was more likely in seed blends compared with strip refuges. Adult D. v. virgifera from refuge plants emerged before those from Bt plants. In strip refuges, D. v. virgifera from refuge plants did not disperse far from refuge boundaries. Larval host plant type did not affect adult size. Larger males and females were more likely to mate. Low proportions of D. v. virgifera from refuge plants were found in 5% seed blend refuges.

CONCLUSION

Seed blend refuges can help to facilitate gene flow between D. v. virgifera beetles from Bt and refuge maize, but current approaches do not meaningfully contribute to delaying resistance because numbers of refuge beetles produced are insufficient. © 2018 Society of Chemical Industry.

Density Dependence and Growth Rate: Evolutionary Effects on Resistance Development to Bt (Bacillus thuringiensis)

It has long been recognized that pest population dynamics can affect the durability of a pesticide, but dose remains the primary component of insect resistance management (IRM). For transgenic pesticidal traits such as Bt (Bacillus thuringiensis Berliner (Bacillales: Bacillaceae)), dose (measured as the mortality of susceptibles caused by a toxin) is a relatively fixed characteristic and often falls below the standard definition of high dose. Hence, it is important to understand how pest population dynamics modify durability and what targets they present for IRM. We used a deterministic model of a generic arthropod pest to examine how timing and strength of density dependence interacted with population growth rate and Bt mortality to affect time to resistance. As in previous studies, durability typically reached a minimum at intermediate doses. However, high population growth rates could eliminate benefits of high dose. The timing of density dependence had a more subtle effect. If density dependence operated simultaneously with Bt mortality, durability was insensitive to its strengths. However, if density dependence was driven by postselection densities, decreasing its strength could increase durability. The strength of density dependence could affect durability of both single traits and pyramids, but its influence depended on the timing of density dependence and size of the refuge. Our findings suggest the utility of a broader definition of high dose, one that incorporates population-dynamic context. That maximum growth rates and timing and strength of interactions causing density dependent mortality can all affect durability, also highlights the need for ecologically integrated approaches to IRM research.

Blended Refuge and Insect Resistance Management for Insecticidal Corn

In this review, we evaluate the intentional mixing or blending of insecticidal seed with refuge seed for managing resistance by insects to insecticidal corn (Zea mays). We first describe the pest biology and farming practices that will contribute to weighing trade-offs between using block refuges and blended refuges. Case studies are presented to demonstrate how the trade-offs will differ in different systems. We compare biological aspects of several abstract models to guide the reader through the history of modeling, which has played a key role in the promotion or denigration of blending in various scientific debates about insect resistance management for insecticidal crops. We conclude that the use of blended refuge should be considered on a case-by-case basis after evaluation of insect biology, environment, and farmer behavior. For Diabrotica virgifera virgifera, Ostrinia nubilalis, and Helicoverpa zea in the United States, blended refuge provides similar, if not longer, delays in the evolution of resistance compared to separate block refuges.

Effects of temperature on the feeding behavior of Alabama argillacea (Hübner) (Lepidoptera: Noctuidae) on Bt and non-Bt cotton plants

The host acceptance behavior and environmental factors as temperature affect the feeding behavior of Lepidoptera pests. Thus, they must be considered in studies about the risk potential of resistance evolution. The current study sets the differences in the feeding behavior of neonate Alabama argillacea (Hübner) (Lepidoptera: Noctuidae) larvae exposed to Bt and non-Bt cotton plants, under different temperatures and time gap after hatching. Two cotton cultivars were used: the Bt (DP 404 BG - bollgard) and the non-transformed isoline, DP 4049. We found that the feeding behavior of neonate A. argillacea is significantly different between Bt and non-Bt cotton. Based on the number of larvae with vegetal tissue in their gut found on the plant and in the organza as well as on the amount of vegetal tissue ingested by the larvae. A. argillacea shows feeding preference for non-Bt cotton plants, in comparison to that on the Bt. However, factors such as temperature and exposure time may affect detection capacity and plant abandonment by the larvae and it results in lower ingestion of vegetal tissue. Such results are relevant to handle the resistance of Bt cotton cultivars to A. argillacea and they also enable determining how the cotton seeds mix will be a feasible handling option to hold back resistance evolution in A. argillacea populations on Bt cotton, when it is compared to other refuge strategies. The results can also be useful to determine which refuge distribution of plants is more effective for handling Bt cotton resistance to A. argillacea.

Intraguild interactions and behavior of Spodoptera frugiperda and Helicoverpa spp. on maize

BACKGROUND

Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) is one of the major pests of maize and is in the same feeding guild as the noctuid pests Helicoverpa zea (Boddie) and Helicoverpa armigera (Hübner), recently reported in South and North America. The intraguild interactions of these species were assessed in laboratory and field conditions by determining the survival of larvae in interaction scenarios with non-Bt maize silks and ears. Moreover, a video tracking system was utilized to evaluate behavioral parameters during larval interactions in scenarios with or without food.

RESULTS

In intraguild interactions, S. frugiperda had greater survival (55-100%) when competing with Helicoverpa spp. in scenarios where larvae were the same instar or when they were larger (fourth versus second) than their competitor. Frequency and time in food of S. frugiperda larvae were negatively influenced by interactions. Larvae of S. frugiperda moved shorter distances (less than 183.03 cm) compared with H. zea.

CONCLUSION

Overall, S. frugiperda had a competitive advantage over Helicoverpa spp. This study provides significant information regarding noctuid behavior and larval survival during intraguild interactions, which may impact pest prevalence and population dynamics, thereby affecting integrated pest management and insect resistance management of these species in maize. © 2017 Society of Chemical Industry.

Pollen contamination in seed mixture increases the dominance of resistance to Bt maize in Spodoptera frugiperda (Lepidoptera: Noctuidae)

BACKGROUND

Seed mixture, also called 'RIB', has been used to provide refuge populations for delaying insect resistance. Pollen contamination in RIB could result in refuge kernels of non-Bt maize expressing variable Bt proteins. Data are lacking regarding the impact of pollen contamination on evolution of resistance for ear-feeding insects. Here, we used Spodoptera frugiperda and Cry1F-maize as a model to examine if pollen contamination in RIB increases the dominance of insect resistance.

RESULTS

Pollen contamination caused >66% refuge kernels in 5:95% (non-Bt:Bt) and 20:80% RIBs to express Cry1F protein. Survival at adult stage on pure non-Bt ears was similar (54.4-63.3%) among Cry1F-susceptible (SS), Cry1F-resistant (RR) and Cry1F-heterozygous (RS) S. frugiperda. On Bt ears, survival was similar between SS and RS (0.0-1.7%), but it was significantly less than that of RR (59.2%). On the two RIB refuge ears, survival at adult stage for RS (42.3% in 5:95% RIB; 50.0% in 20:80% RIB) was significantly higher than for SS (8.7% in 5:95% RIB; 10.0% in 20:80% RIB).

CONCLUSION

The results suggest that pollen contamination in RIB could increase the dominance of resistance for ear-feeding insects by significantly reducing susceptible refuge populations and supporting higher survival of heterozygotes relative to homozygous susceptible insects. © 2017 Society of Chemical Industry.

Evidence for Field-Evolved Resistance of Striacosta albicosta (Lepidoptera: Noctuidae) to Cry1F Bacillus thuringiensis Protein and Transgenic Corn Hybrids in Ontario, Canada

Western bean cutworm, Striacosta albicosta (Smith) (Lepidoptera: Noctuidae), is a pest of corn (Zea mays L.) that has recently expanded its range into Ontario, Canada. Control of S. albicosta damage to corn hybrids containing event TC1507-expressing Cry1F Bacillus thuringiensis protein alone or pyramided with event MON 89034 expressing Cry1A.105 and Cry2Ab2 Bt proteins was tested in 2011-2015 in Ontario in small- and large-scale field plots with natural infestation. In 2011, significantly lower incidence and severity of kernel damage was sustained by Cry1F × Cry1A.105 + Cry2Ab2 corn compared with a non-Bt near-isogenic hybrid. However, from 2012 to 2015, there was no difference in incidence or severity of damage comparing non-Bt hybrids with Cry1F hybrids alone or pyramided with Cry1A.105 and Cry2Ab2 planted as a pure stand or with an integrated refuge (95% Bt: 5% non-Bt seeds). In 2015, neonate larvae derived from Ontario field-collections were tested in concentration-response diet-overlay bioassays with lyophilized Cry1F protein at concentrations up to 75 µg cm-2. The concentrations at which mortality of 50% (LC50) of the collections occurred ranged from approximately 10 µg cm-2 (F0) to >28 µg cm-2 (F1) in a 7-d bioassay, indicating relative insensitivity to Cry1F. Results from field experiments, laboratory bioassays, and the history of exposure to Cry1F in corn show that S. albicosta in Ontario are not controlled by Cry1F-expressing corn hybrids and provide evidence for the conclusion that the evolution of resistance to Cry1F has occurred.

Role of dispersal in resistance evolution and spread

Gene flow via immigration affects rate of evolution of resistance to a pest management tactic, while emigration from a resistant population can spread resistance alleles spatially. Whether resistance detected across the landscape reflects ongoing de novo evolution in different hotspots or spread from a single focal population can determine the most effective mitigation strategy. Pest dispersal dynamics determine the spatio-temporal scale at which mitigation tactics must be applied to contain or reverse resistance in an area. Independent evolution of resistance in different populations appears common but not universal. Conversely, spatial spread appears to be almost inevitable. However, rate and scale of spread depends largely on dispersal dynamics and interplay with factors such as fitness costs, spatially variable selection pressure and whether resistance alleles are spreading through an established population or being carried by populations colonizing new territory.

Hybridizing transgenic Bt cotton with non-Bt cotton counters resistance in pink bollworm

Extensive cultivation of crops genetically engineered to produce insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) has suppressed some major pests, reduced insecticide sprays, enhanced pest control by natural enemies, and increased grower profits. However, these benefits are being eroded by evolution of resistance in pests. We report a strategy for combating resistance by crossing transgenic Bt plants with conventional non-Bt plants and then crossing the resulting first-generation (F₁) hybrid progeny and sowing the second-generation (F₂) seeds. This strategy yields a random mixture within fields of three-quarters of plants that produce Bt toxin and one-quarter that does not. We hypothesized that the non-Bt plants in this mixture promote survival of susceptible insects, thereby delaying evolution of resistance. To test this hypothesis, we compared predictions from computer modeling with data monitoring pink bollworm (Pectinophora gossypiella) resistance to Bt toxin Cry1Ac produced by transgenic cotton in an 11-y study at 17 field sites in six provinces of China. The frequency of resistant individuals in the field increased before this strategy was widely deployed and then declined after its widespread adoption boosted the percentage of non-Bt cotton plants in the region. The correspondence between the predicted and observed outcomes implies that this strategy countered evolution of resistance. Despite the increased percentage of non-Bt cotton, suppression of pink bollworm was sustained. Unlike other resistance management tactics that require regulatory intervention, growers adopted this strategy voluntarily, apparently because of advantages that may include better performance as well as lower costs for seeds and insecticides.

Movement and survival of Busseola fusca (Lepidoptera: Noctuidae) larvae within maize plantings with different ratios of non-Bt and Bt seed

BACKGROUND

Products of plant biotechnology, for example genetically modified Bt maize, provide useful tools for pest management. The benefits provided by insect-resistant plants are, however, threatened by the evolution of resistance by target pest species. The high-dose/refuge insect resistance management strategy (IRM) as well as seed mixtures are globally used as IRM strategies. Busseola fusca (Lepidoptera: Noctuidae), the target stem borer of Bt maize in Africa, evolved resistance to Bt maize expressing Cry1Ab protein in South Africa. Owing to high larval mobility and subsequent sublethal exposure of larvae moving between non-Bt and Bt plants, more rapid resistance evolution has been proposed as a possibility with deployment of seed mixture strategies.

RESULTS

Laboratory and field studies were conducted to study B. fusca larval mobility. In the laboratory, different scenarios of B. fusca larval movement between single-gene (Cry1Ab) and stacked-trait (Cry1A.105 and Cry2Ab2) Bt maize were studied. Data on larval survival and mass over time indicated that Cry proteins do not kill larvae above certain developmental stages. A 2 year field study with the single gene and the stacked event was conducted using seed mixtures containing 5, 10, 15 and 20% non-Bt seed as well as a control treatment (non-Bt seed only).

CONCLUSION

Larval movement continued for 5 weeks and resulted in a significant incidence of Bt and non-Bt damaged plants, indicating that the movement behaviour of B. fusca is of such a nature that seed mixtures as an IRM strategy may not be effective to delay resistance evolution. © 2016 Society of Chemical Industry.

Intraguild Competition of Three Noctuid Maize Pests

The western bean cutworm Striacosta albicosta (Smith), the fall armyworm Spodoptera frugiperda (J. E. Smith), and the corn earworm Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) are among the major lepidopteran pests of maize in the United States, belonging to the same guild and injuring the reproductive tissues of this crop. Here, intraguild competition of these lepidopterans on non-Bt maize was evaluated through survival analysis of each species under laboratory and field conditions. Competition scenarios were carried out in arenas containing maize silk or ear tissue, using larvae on different stadium of development. Fitness cost competition studies were conducted to examine the influence of intraguild competition and cannibalism and predation rates on larval development. The survival of S. albicosta competing with the other species was significantly lower than in intraspecific competition, even when the larvae were more developed than the competitor. For S. frugiperda, survival remained high in the different competition scenarios, except when competing in a smaller stadium with H. zea Larvae of H. zea had a high rate of cannibalism, higher survival when competing against S. albicosta than S. frugiperda, and reduced survival when the H. zea larvae were at the same development stadium or smaller than the competitors. Based on fitness cost results, the absence of a competitor for the feeding source may confer an advantage to the larval development of S. frugiperda and H. zea Our data suggest that S. frugiperda has a competitive advantage against the other species, while S. albicosta has the disadvantage in the intraguild competition on non-Bt maize.

Impact of Lepidoptera (Crambidae, Noctuidae, and Pyralidae) Pests on Corn Containing Pyramided Bt Traits and a Blended Refuge in the Southern United States

Blended refuge for transgenic plants expressing Bacillus thuringiensis (Bt) toxins has been approved in the northern United States as a resistance management strategy alternative to a structured refuge. A three-year study (2012-2014) was conducted with 54 trials across nine states in the southern United States to evaluate plant injury from lepidopteran pests of corn and yield in a corn hybrid expressing Cry1F × Cry1Ab × Vip3Aa20 (Pioneer Brand Optimum Leptra) planted as a pure stand and in refuge blends of 5, 10, and 20% in both early and late plantings. Injury by corn earworm, Helicoverpa zea Boddie (Lepidoptera: Noctuidae), and fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), was generally proportional to the percentage of non-Bt corn within each refuge blend. Across locations, ear injury in plots with 100% Cry1F × Cry1Ab × Vip3Aa20 (Optimum Leptra) corn ranged from no injury to a maximum of 0.42 cm(2) per ear in Mississippi in 2013. Leaf injury ratings in 100% non-Bt plots in early and late planted trials in 2014 were 86- and 70-fold greater than in 100% Cry1F × Cry1Ab × Vip3Aa20 (Optimum Leptra) plots. Plants in plots with blended refuges had significantly greater leaf injury in 2012 (5, 10, and 20% refuge blends), in the early-planted corn in 2013 (10 and 20% only), and in both early- and late-planted corn in 2014 (20% only) as compared with leaf injury in a pure stand of Cry1F × Cry1Ab × Vip3Aa20 (Optimum Leptra) seen during these years. Corn ears in plots with blended refuges also had significantly greater area of kernels injured in 2012 (5, 10, and 20%), in early- and late-planted corn in 2013 (5, 10, and 20%), and in early (10 and 20% only)- and late-planted corn (5, 10, and 20%) in 2014 as compared with ear injury in a pure stand of Cry1F × Cry1Ab × Vip3Aa20 (Optimum Leptra) seen during these years. Infestations of southwestern corn borer, Diatraea grandiosella Dyar (Lepidoptera: Crambidae), were also significantly reduced by Cry1F × Cry1Ab × Vip3Aa20 (Optimum Leptra). Despite these differences in injury, yield averaged across locations varied among refuge blends only in the late-planted trials in 2013, with greater yields in the 0% refuge blend than in the 20% blend; however, when examining yield separately by location, only two of nine locations had higher yields in the 100% Bt plots than in any of the blended refuge plots. As a complement to studying the contribution of blended refuge to delaying resistance, quantifying injury and yield in a range of refuge blends is a necessary step to provide management information on the range of lepidopteran pests that occur in the southern United States.

Plant-to-Plant Movement of Striacosta albicosta (Lepidoptera: Noctuidae) and Spodoptera frugiperda (Lepidoptera: Noctuidae) in Maize ( Zea mays )

Integrated pest management (IPM) and insect resistance management (IRM) in various cropping systems demand a comprehensive understanding of insect behavior. Among the needed information is basic charaterizations of larval movement and dispersion of some insect-pests, such as the noctuids Striacosta albicosta (Smith) and Spodoptera frugiperda (J.E. Smith). We investigated the plant-to-plant movement of western bean cutworm and fall armyworm larvae in field of maize. Experiments on S. albicosta were conducted between 2008 and 2010. A main study with this pest was performed in 2012 in a randomized complete block design (RCBD) with nine replications. An S. frugiperda study was performed in 2013 in an RCBD with eight replications. The plant-to-plant movement and larval survival were measured in plots with maize nontoxic to the insects. The larval survival of S. albicosta presented high variety throughout the years. Although S. frugiperda survival was relatively low during 2013, it did not compromise the larval assessment. Larvae of both species dispersed governed by nondirectional sensory information, and presented aggregated and symmetrical distribution; however, fall armyworm remained nearer the release point. These results may help the IPM components, such as scouting and economic threshold, as well as the implementation of refuge and seed mixture strategies for IRM.

Impact of violated high-dose refuge assumptions on evolution of Bt resistance

Transgenic crops expressing Bacillus thuringiensis (Bt) toxins have been widely and successfully deployed for the control of target pests, while allowing a substantial reduction in insecticide use. The evolution of resistance (a heritable decrease in susceptibility to Bt toxins) can pose a threat to sustained control of target pests, but a high‐dose refuge (HDR) management strategy has been key to delaying countervailing evolution of Bt resistance. The HDR strategy relies on the mating frequency between susceptible and resistant individuals, so either partial dominance of resistant alleles or nonrandom mating in the pest population itself could elevate the pace of resistance evolution. Using classic Wright‐Fisher genetic models, we investigated the impact of deviations from standard refuge model assumptions on resistance evolution in the pest populations. We show that when Bt selection is strong, even deviations from random mating and/or strictly recessive resistance that are below the threshold of detection can yield dramatic increases in the pace of resistance evolution. Resistance evolution is hastened whenever the order of magnitude of model violations exceeds the initial frequency of resistant alleles. We also show that the existence of a fitness cost for resistant individuals on the refuge crop cannot easily overcome the effect of violated HDR assumptions. We propose a parametrically explicit framework that enables both comparison of various field situations and model inference. Using this model, we propose novel empiric estimators of the pace of resistance evolution (and time to loss of control), whose simple calculation relies on the observed change in resistance allele frequency.

Early Detection and Mitigation of Resistance to Bt Maize by Western Corn Rootworm (Coleoptera: Chrysomelidae)

Transgenic Bt maize that produces less than a high-dose has been widely adopted and presents considerable insect resistance management (IRM) challenges. Western corn rootworm, Diabrotica virgifera virgifera LeConte, has rapidly evolved resistance to Bt maize in the field, leading to local loss of efficacy for some corn rootworm Bt maize events. Documenting and responding to this resistance has been complicated by a lack of rapid diagnostic bioassays and by regulatory triggers that hinder timely and effective management responses. These failures are of great concern to the scientific and agricultural community. Specific challenges posed by western corn rootworm resistance to Bt maize, and more general concerns around Bt crops that produce less than a high-dose of Bt toxin, have caused uncertainty around current IRM protocols. More than 15 years of experience with IRM has shown that high-dose and refuge-based IRM is not applicable to Bt crops that produce less than a high-dose. Adaptive IRM approaches and pro-active, integrated IRM-pest management strategies are needed and should be in place before release of new technologies that produce less than a high-dose. We suggest changes in IRM strategies to preserve the utility of corn rootworm Bt maize by 1) targeting local resistance management earlier in the sequence of responses to resistance and 2) developing area-wide criteria to address widespread economic losses. We also favor consideration of policies and programs to counteract economic forces that are contributing to rapid resistance evolution.

Bt Maize Seed Mixtures for Helicoverpa zea (Lepidoptera: Noctuidae): Larval Movement, Development, and Survival on Non-transgenic Maize

In 2012 and 2013, field trials were conducted near Rosemount, MN, to assess the movement and development of Helicoverpa zea (Boddie) larvae on non-Bt refuge corn plants within a seed mixture of non-Bt and Bt corn. The Bt corn hybrid expressed three Bt toxins-Cry1Ab, Cry1F, and Vip3A. As the use of seed mixtures for insect resistance management (IRM) continues to be implemented, it is necessary to further characterize how this IRM approach impacts resistance development in ear-feeding Lepidopteran pests. The potential for Bt pollen movement and cross pollination of the non-Bt ears in a seed mixture may lead to Bt toxin exposure to larvae developing on those refuge ears. Larval movement and development by H. zea, feeding on non-Bt refuge plants adjacent to either transgenic Bt or non-Bt plants, were measured to investigate the potential for unintended Bt exposure. Non-Bt plants were infested with H. zea eggs and subplots were destructively sampled twice per week within each treatment to assess larval development, location, and kernel injury. Results indicate that H. zea larval movement between plants is relatively low, ranging from 2-16% of larvae, and occurs mainly after reaching the second instar. Refuge plants in seed mixtures did not produce equivalent numbers of H. zea larvae, kernel injury, and larval development differed as compared with a pure stand of non-Bt plants. This suggests that there may be costs to larvae developing on refuge plants within seed mixtures and additional studies are warranted to define potential impacts.

Analysis of resistance to Cry1Ac in field-collected pink bollworm, Pectinophora gossypiella (Lepidoptera:Gelechiidae), populations

High survivorship of pink bollworrm, Pectinophora gossypiella in bolls of Bollgard® cotton hybrids and resistance to Cry1Ac protein, expressed in Bollgard cotton were reported in field-populations collected from the state of Gujarat (western India) in 2010. We have found Cry1Ac-resistance in pink bollworm populations sourced from Bollgard and non-Bt cotton fields in the adjoining states of Maharashtra and Madhya Pradesh in Central India. Further, we observed reduced binding of labeled Cry1Ac protein to receptors localized on the brush-border membrane of pink bollworm larval strains with high tolerance to Cry1Ac. These strains were sourced from Bollgard and conventional cotton fields. A pooled Cry1Ac-resistant strain, further selected on Cry1Ac diet also showed significantly reduced binding to Cry1Ac protein. The reduced binding of Cry1Ac to receptors could be an underlying mechanism for the observed resistance in pink bollworm populations feeding on Bollgard hybrids.

Inhibition of Helicoverpa zea (Lepidoptera: Noctuidae) Growth by Transgenic Corn Expressing Bt Toxins and Development of Resistance to Cry1Ab

Transgenic corn, Zea mays L., that expresses the Bacillus thuringiensis (Bt) toxin Cry1Ab is only moderately toxic to Helicoverpa zea (Boddie) and has been planted commercially since 1996. Growth and development of H. zea was monitored to determine potential changes in susceptibility to this toxin over time. Small plots of corn hybrids expressing Cry1F, Cry1F × Cry1Ab, Cry1Ab × Cry3Bb1, Cry1A.105 × Cry2Ab2 × Cry3Bb1, Cry1A.105 × Cry2Ab2, and Vip3Aa20 × Cry1Ab × mCry3A were planted in both 2012 and 2013 inNorth and South Carolina with paired non-Bt hybrids from the same genetic background. H. zea larvae were sampled on three time periods from ears and the following factors were measured: kernel area injured (cm(2)) by H. zea larvae, larval number per ear, larval weight, larval length, and larval head width. Pupae were sampled on a single time period and the following factors recorded: number per ear, weight, time to eclosion, and the number that eclosed. There was no reduction in larval weight, number of insect entering the pupal stadium, pupal weight, time to eclosion, and number of pupae able to successfully eclose to adulthood in the hybrid expressing Cry1Ab compared with a non-Bt paired hybrid. As Cry1Ab affected these in 1996, H. zea may be developing resistance to Cry1Ab in corn, although these results are not comprehensive, given the limited sampling period, size, and geography. We also found that the negative impacts on larval growth and development were greater in corn hybrids with pyramided traits compared with single traits.

A seed mixture increases dominance of resistance to Bt cotton in Helicoverpa zea

Widely grown transgenic crops producing insecticidal proteins from Bacillus thuringiensis (Bt) can benefit agriculture, but adaptation by pests threatens their continued success. Refuges of host plants that do not make Bt toxins can promote survival of susceptible insects and delay evolution of resistance, particularly if resistance is inherited as a recessive trait. However, data have been lacking to compare the dominance of resistance when Bt and non-Bt seeds are planted in random mixtures versus separate blocks. Here we report results from greenhouse experiments with transgenic cotton producing Bt toxin Cry1Ac and the bollworm, Helicoverpa zea, showing that the dominance of resistance was significantly higher in a seed mixture relative to a block of Bt cotton. The proportion of larvae on non-Bt cotton plants in the seed mixture was also significantly higher than expected under the null hypothesis of random distribution. In simulations based on observed survival, resistance evolved 2- to 4.5-fold faster in the seed mixture relative to separate blocks of Bt and non-Bt cotton. These findings support previous modelling results indicating that block refuges may be more effective than seed mixtures for delaying resistance in pests with mobile larvae and inherently low susceptibility to the toxins in Bt crops.

A simultaneous extraction method for organophosphate, pyrethroid, and neonicotinoid insecticides in aqueous samples

A method was developed for the extraction and analysis of 2 organophosphate, 8 pyrethroid, and 5 neonicotinoid insecticides from the same water sample. A salted liquid-liquid extraction (LLE) was optimized with a solid-phase extraction (SPE) step that separated the organophosphates (OPs) and pyrethroids from the neonicotinoids. Factors that were optimized included volume of solvent and amount of salt used in the LLE, homogenization time for the LLE, and type and volume of eluting solvent used for the SPE. The OPs and pyrethroids were quantified using gas chromatography-mass spectrometry, and the neonicotinoids were quantified using liquid chromatography-diode array detector. Results showed that the optimized method was accurate, precise, reproducible, and robust; recoveries in river water spiked with 100 ng L(-1) of each of the insecticides were all between 86 and 114 % with RSDs between 2 and 8 %. The method was also sensitive with method detection limits ranging from 0.1 to 27.2 ng L(-1) depending on compounds and matrices. The optimized method was thus appropriate for the simultaneous extraction of 15 widely applied insecticides from three different classes and was shown to provide valuable information on their environmental fate from field-collected aqueous samples.

Lepidoptera (Crambidae, Noctuidae, and Pyralidae) Injury to Corn Containing Single and Pyramided Bt Traits, and Blended or Block Refuge, in the Southern United States

Fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae); corn earworm, Helicoverpa zea Boddie (Lepidoptera: Noctuidae); southwestern corn borer, Diatraea grandiosella Dyar (Lepidoptera: Crambidae); sugarcane borer, Diatraea saccharalis F. (Lepidoptera: Crambidae); and lesser cornstalk borer, Elasmopalpus lignosellus Zeller (Lepidoptera: Pyralidae), are lepidopteran pests of corn, Zea mays L., in the southern United States. Blended refuge for transgenic plants expressing the insecticidal protein derivative from Bacillus thuringiensis (Bt) has recently been approved as an alternative resistance management strategy in the northern United States. We conducted a two-year study with 39 experiments across 12 states in the southern United States to evaluate plant injury from these five species of Lepidoptera to corn expressing Cry1F and Cry1Ab, as both single and pyramided traits, a pyramid of Cry1Ab×Vip3Aa20, and a pyramid of Cry1F×Cry1Ab plus non-Bt in a blended refuge. Leaf injury and kernel damage from corn earworm and fall armyworm, and stalking tunneling by southwestern corn borer, were similar in Cry1F×Cry1Ab plants compared with the Cry1F×Cry1Ab plus non-Bt blended refuge averaged across five-plant clusters. When measured on an individual plant basis, leaf injury, kernel damage, stalk tunneling (southwestern corn borer), and dead or injured plants (lesser cornstalk borer) were greater in the blended non-Bt refuge plants compared to Cry1F×Cry1Ab plants in the non-Bt and pyramided Cry1F×Cry1Ab blended refuge treatment. When non-Bt blended refuge plants were compared to a structured refuge of non-Bt plants, no significant difference was detected in leaf injury, kernel damage, or stalk tunneling (southwestern corn borer). Plant stands in the non-Bt and pyramided Cry1F×Cry1Ab blended refuge treatment had more stalk tunneling from sugarcane borer and plant death from lesser cornstalk borer compared to a pyramided Cry1F×Cry1Ab structured refuge treatment. Hybrid plants containing Cry1F×Cry1Ab within the pyramided Cry1F×Cry1Ab blended refuge treatment had significantly less kernel damage than non-Bt structured refuge treatments. Both single and pyramided Bt traits were effective against southwestern corn borer, sugarcane borer, and lesser cornstalk borer.

Emergence and Abundance of Western Corn Rootworm (Coleoptera: Chrysomelidae) in Bt Cornfields With Structured and Seed Blend Refuges

To slow evolution of western corn rootworm (Diabrotica virgifera virgifera LeConte) resistance to Bt (Bacillus thuringiensis Berliner) corn hybrids, non-Bt "refuges" must be planted within or adjacent to Bt cornfields, allowing susceptible insects to develop without exposure to Bt toxins. Bt-susceptible adults from refuges are expected to find and mate with resistant adults that have emerged from Bt corn, reducing the likelihood that Bt-resistant offspring are produced. The spatial and temporal distribution of adults in four refuge treatments (20, 5, and 0% structured refuges and 5% seed blend) and adjacent soybean fields was compared from 2010 to 2012. Adult emergence (adults/trap/day) from refuge corn in structured refuge treatments was greater than that from Bt corn, except during the post-pollination period of corn phenology when emergence from refuge and Bt plants was often the same. Abundance of free-moving adults was greatest in and near refuge rows in structured refuge treatments during vegetative and pollination periods. By post-pollination, adult abundance became evenly distributed. In contrast, adult abundance in 5% seed blends and 0% refuges was evenly distributed, or nearly so, across plots throughout the season. The persistent concentration of adults in refuge rows suggests that structured refuge configurations may not facilitate the expected mixing of adults from refuge and Bt corn. Seed blends produce uniform distributions of adults across the field that may facilitate mating between Bt and refuge adults and ultimately delay the evolution of Bt resistance.

A challenge for the seed mixture refuge strategy in Bt maize: impact of cross-pollination on an ear-feeding pest, corn earworm

To counter the threat of insect resistance, Bacillus thuringiensis (Bt) maize growers in the U.S. are required to plant structured non-Bt maize refuges. Concerns with refuge compliance led to the introduction of seed mixtures, also called RIB (refuge-in-the-bag), as an alternative approach for implementing refuge for Bt maize products in the U.S. Maize Belt. A major concern in RIB is cross-pollination of maize hybrids that can cause Bt proteins to be present in refuge maize kernels and negatively affect refuge insects. Here we show that a mixed planting of 5% nonBt and 95% Bt maize containing the SmartStax traits expressing Cry1A.105, Cry2Ab2 and Cry1F did not provide an effective refuge for an important above-ground ear-feeding pest, the corn earworm, Helicoverpa zea (Boddie). Cross-pollination in RIB caused a majority (>90%) of refuge kernels to express ≥ one Bt protein. The contamination of Bt proteins in the refuge ears reduced neonate-to-adult survivorship of H. zea to only 4.6%, a reduction of 88.1% relative to larvae feeding on ears of pure non-Bt maize plantings. In addition, the limited survivors on refuge ears had lower pupal mass and took longer to develop to adults.

Feeding and dispersal behavior of the cotton leafworm, Alabama argillacea (Hübner) (Lepidoptera: Noctuidae), on Bt and non-Bt cotton: implications for evolution and resistance management

The host acceptance of neonate Alabama argillacea (Hübner) (Lepidoptera: Noctuidae) larvae to Bt cotton plants exerts a strong influence on the potential risk that this pest will develop resistance to Bt cotton. This will also determine the efficiency of management strategies to prevent its resistance such as the “refuge-in-the-bag” strategy. In this study, we assessed the acceptance of neonate A. argillacea larvae to Bt and non-Bt cotton plants at different temperatures during the first 24 h after hatching. Two cotton cultivars were used in the study, one a Bt DP 404 BG (Bollgard) cultivar, and the other, an untransformed isoline, DP 4049 cultivar. There was a greater acceptance by live neonate A. argillacea larvae for the non-Bt cotton plants compared with the Bt cotton plants, especially in the time interval between 18 and 24 h. The percentages of neonate A. argillacea larvae found on Bt or non-Bt plants were lower when exposed to temperatures of 31 and 34°C. The low acceptance of A. argillacea larvae for Bt cotton plants at high temperatures stimulated the dispersion of A. argillacea larvae. Our results support the hypothesis that the dispersion and/or feeding behavior of neonate A. argillacea larvae is different between Bt and non-Bt cotton. The presence of the Cry1Ac toxin in Bt cotton plants, and its probable detection by the A. argillacea larvae tasting or eating it, increases the probability of dispersion from the plant where the larvae began. These findings may help to understand how the A. argillacea larvae detect the Cry1Ac toxin in Bt cotton and how the toxin affects the dispersion behavior of the larvae over time. Therefore, our results are extremely important for the management of resistance in populations of A. argillacea on Bt cotton.

Current European corn borer, Ostrinia nubilalis, injury levels in the northeastern United States and the value of Bt field corn

BACKGROUND

Recent evidence indicates that some populations of European corn borer (ECB), Ostrinia nubilalis (Hübner), have declined to historic lows owing to widespread adoption of Bt corn hybrids. To understand current ECB populations in Pennsylvania field corn, the authors assessed larval damage in Bt and non-Bt corn hybrids at 29 sites over 3 years. The influence of Bt adoption rates, land cover types and moth activity on levels of ECB damage was also considered.

RESULTS

Bt hybrids reduced ECB damage when compared with non-Bt, but these differences inconsistently translated to higher yields and, because of higher seed costs, rarely improved profits. No relationships were detected between land use or Bt adoption and ECB damage rates, but positive relationships were found between plant damage and captures of Z-race ECB moths in pheromone traps in the PestWatch network.

CONCLUSIONS

ECB damage levels were generally low and appear to be declining across Pennsylvania. In many locations, farmers may gain greater profits by planting competitive non-Bt hybrids; however, Bt hybrids remain valuable control options, particularly in the parts of Pennsylvania where ECB populations persist. Moth captures from PestWatch appear to provide insight into where Bt hybrids are most valuable.

Modeling a western corn rootworm, Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae), maturation delay and resistance evolution in Bt corn

BACKGROUND

Emergence delay and female-skewed sex ratios among adults of Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae) from Bt corn have been reported in field studies. The authors used a simulation model to study the effect of a maturation delay and a female-skewed sex ratio for D. v. virgifera emerging from Bt corn on the evolution of Bt resistance.

RESULTS

The effect of skewed toxin mortality in one sex on evolution of Bt resistance was insignificant. An emergence delay among resistant beetles from Bt corn slowed resistance evolution. A shift in the time of emergence for homozygous susceptible beetles from Bt corn did not have a significant effect on the evolution of Bt resistance in D. v. virgifera.

CONCLUSION

This simulation study suggested that skewed toxin mortality in one sex and an emergence delay for beetles in Bt corn are not major concerns for managing resistance by D. v. virgifera to single-toxin or pyramided Bt corn.

Occurrence and larval movement of Diatraea saccharalis (Lepidoptera: Crambidae) in seed mixes of non-Bt and Bt pyramid corn

BACKGROUND

Larval movement of target pest populations among Bt and non-Bt plants is a major concern in the use of a seed mixture refuge strategy for Bt resistance management. In this study, occurrence and larval movement of the sugarcane borer, Diatraea saccharalis (F.), were evaluated in four planting patterns of non-Bt and Bt plants containing Genuity® SmartStax(TM) traits in 2009-2011. The four planting patterns were: (1) a pure stand of 27 Bt plants; (2) one non-Bt plant in the center, surrounded by 26 Bt plants; (3) a pure stand of 27 non-Bt plants; (4) one Bt plant in the center, surrounded by 26 non-Bt plants. Studies were conducted under four conditions: (1) open field with natural infestation; (2) greenhouse with artificial infestations; open field with artificial infestations (3) on the center plants only and (4) on every plant. The major objective of this study was to determine whether refuge plants in a seed mixture strategy could provide a comparable refuge population of D. saccharalis to a 'structured refuge' planting.

RESULTS

Larvae of D. saccharalis showed the ability to move from infested plants to at least four plants away, as well as to adjacent rows, but the majority remained within the infested row. However, the number of larvae found on the non-Bt plants in the mixture plantings was not significantly reduced compared with the pure stand of non-Bt corn.

CONCLUSION

The results of this study show that refuge plants in a seed mixture may be able to provide a comparable refuge population of D. saccharalis to a structured refuge planting.