non-Bt Refuge Research Articles

Effect of Blended Bt Corn Refuge on Corn Earworm (Lepidoptera: Noctuidae) Infestation and Grain Yield

Blended refuges for corn-expressing toxins from Bacillus thuringiensis (Bt) Berliner controls have been approved in the United States as an alternative resistance management approach to structured refuge. This study examined the impact of blended refuges up to 30% non-Bt seed on the corn earworm, Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae), larval numbers, and kernel injury, and its effect on corn grain yield and test weights. The percentage of infested ears, larval numbers, and kernel injury of the 20% blend were not significantly different from the non-Bt and nonblended Bt for the Cry1A.105 + Cry2Ab + Cry1Fa2 treatment, but infested ears, larval numbers, and kernel injury of the nonblended Cry1A.105 + Cry2Ab treatment was lower than the comparable non-Bt hybrids, with the 20% blend being intermediate. The nonblended Cry1A.105 + Cry2Ab + Vip3Aa20 had virtually no larvae in ears and no kernel injury. Ear infestation, larval numbers, and kernel injury of the non-Bt blends with Vip3Aa20 were proportional to the percentage of non-Bt seed in the blend, and all infested ears were from the non-Bt refuge plants. Grain yield and test weight were not significantly different among nonblended or blended treatments of any Bt product tested. Results indicate losses of grain yield and test weight by corn earworm in seed blends up to 30% non-Bt seed are unlikely with infestation levels observed in this study.

Yield analysis and corn earworm feeding in Bt and non-Bt corn hybrids across diverse locations.

Corn, Zea mays L. (Poales: Poaceae), growers in the US Cotton Belt are required to plant 20% of total corn acres to non-Bt hybrids for resistance management (non-Bt refuge). Most growers do not meet this requirement, in part, because they perceive non-Bt hybrids to yield less than Bt hybrids. We planted multiple non-Bt and Bt hybrids from a single company in small-plot replicated trials at a single location from 2019 to 2023, as well as in small-plot replicated trials at multiple locations during 2022 and 2023. In the single location, we measured kernel injury from corn earworm, Helicoverpa zea Boddie (Lepidoptera: Noctuidae), and we recorded yield at all locations. In the single location trial, yields only separated among hybrids in 3 out of 5 years. In the multiple location trial, yields were variable between both years. We found that Bt hybrids tended to yield higher than non-Bt hybrids overall, but this was influenced by the inclusion of non-Bt hybrids that had a lower overall genetic yield potential in the environments we tested them in. In both tests, when hybrids were analyzed during each year, both Bt and non-Bt hybrids were among the statistically highest yielders. Our study demonstrates the importance of comparing multiple Bt and non-Bt hybrids to draw yield comparisons. This highlights the need for corn seed company breeders to put effort into improving yield for non-Bt hybrids. Hopefully this effort will translate into increased planting of non-Bt refuge corn for growers in the US Cotton Belt.

Enhancing non-Bt corn refuge based on corn grower and seed dealer surveys

Abstract To delay Bacillus thuringiensis (Bt) resistance, southern US corn growers are required to plant 20% of their total corn acres to non-Bt hybrids. However, most do not, possibly due to negative perceptions or the lack of availability of non-Bt hybrids. We surveyed 174 corn growers and 32 corn seed dealers in eastern North Carolina, an area that historically has planted little non-Bt corn. Our questions focused on planting behavior and hybrid choice, barriers to planting non-Bt corn, and preferences and challenges for non-Bt corn sales. Only 32% of growers surveyed planted the mandated non-Bt refuge acres. Growers from counties with smaller crop operations and in counties where growers had less graduate education planted less non-Bt seed. Grower perceptions of yield differences between Bt and non-Bt hybrids were ambiguous, but only a few had limited availability or knowledge of non-Bt seed options. Seed dealers were familiar with non-Bt corn hybrids in their portfolio, but not as familiar with non-Bt refuge requirements. Furthermore, dealers indicated obstacles in providing diverse herbicide packages and seed treatments for non-Bt hybrids. We highlight the need for enhanced communication and education initiatives. Strengthening educational outreach to clarify refuge requirements and addressing perceived yield disparities between Bt and non-Bt hybrids could also increase planting of non-Bt refuge. Finally, incentivizing seed dealers to offer and promote non-Bt seed options, coupled with diverse herbicide packages, might increase refuge planting. Our surveys underscore the complexity surrounding grower behaviors, seed dealer dynamics, and the importance of educational efforts and incentives to non-Bt refuge.

Grower perceptions and adoption of IPM and non-Bt refuge in field corn: a survey in North and South Carolina

Abstract Production of field corn in the southeastern United States is often impacted by a number of insect pests. However, surveys to assess stakeholder perception of the relative importance of insect pests and adopted management decisions for these pests have not been extensively reported in the region. A survey was conducted in North and South Carolina to address deficiencies in our knowledge about how corn producers perceive insect pests, implement management and insect resistance strategies, and understand and implement integrated pest management (IPM). The survey indicated that stink bugs (Hemiptera: Pentatomidae) and corn earworm, Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae), were the most commonly managed insect pests in field corn. For transgenic corn expressing insecticidal toxins from Bacillus thuringiensis (Bt) (Berliner), the rate of resistance management (i.e., planting of non-Bt corn refuge) implemented (59%) by growers was higher than previously reported. The majority of grower respondents identified ear development as the most critical corn growth stage for managing stink bugs (52%), with the most commonly reported management strategy being applications of insecticide at the economic threshold. Growers also reported understanding the concepts of IPM and implementing it in field corn. Assessing the adoption of preventative approaches, such as Bt corn and insecticide seed treatments, and strategies to manage insect resistance, in the case of Bt corn in particular, is critical for tailoring research and Extension efforts to protect the durability of these management tactics.

Refuges of conventional host plants counter dominant resistance of cotton bollworm to transgenic Bt cotton

Transgenic crops have revolutionized insect pest control, but evolution of resistance by pests threatens their continued success. The primary strategy for combating pest resistance to crops producing insecticidal proteins from Bacillus thuringiensis (Bt) uses refuges of non-Bt host plants to allow survival of susceptible insects. The prevailing paradigm is that refuges delay resistance that is rare and recessively inherited. However, we discovered refuges countered resistance to Bt cotton that was neither rare nor recessive. In a 15-year field study of the cotton bollworm, the frequency of a mutation conferring dominant resistance to Bt cotton surged 100-fold from 2006 to 2016 yet did not rise from 2016 to 2020. Computer simulations indicate the increased refuge percentage from 2016 to 2020 is sufficient to explain the observed halt in the evolution of resistance. The results also demonstrate the efficacy of a Bt crop can be sustained by non-Bt refuges of other crops.

Local and landscape factors determining the abundance and richness of insect pests and arthropod predators in Bt and non-Bt soybean

Agricultural landscapes of the Pampas and Campos biome have changed considerably due to the expansion of annual crops over non-crop areas, soybean seeding and the adoption of lepidopteran-resistant varieties (Bt technology). These changes have a great impact on predatory and phytophagous arthropods, which have been scarcely studied in Uruguay. The following aspects were evaluated: (a) the effect of vegetation cover composition (landscape factor), Bt technology (varieties expressing Cry1Ac protein) and field margins (local factors), on the abundance of the main pests and predators of soybean; b) the role of refuge areas (non-Bt soybean) in determining the abundance and richness of arthropod predators in Bt soybean crops, and c) the effect of Cry1Ac protein on the biology, reproduction and feeding behavior of Piezodorus guildinii (Hemiptera: Pentatomidae). In the northern Uruguayan littoral, during two years, predators and pests were sampled in 90 commercial non-Bt and Bt soybean fields and refuge areas. Biological and reproductive parameters of P. guildinii fed with Bt soybean pods and their possible effect on feeding behavior were evaluated using the electrical penetration graph technique. The main results obtained were: a) i. Bt soybean presented lower abundance of defoliating lepidoptera, without affecting predator abundance; ii. Soybean predators’ abundance was negatively related to the proportion of this crop in the landscape, but positively related to natural grassland and other crops; iii. Predators’ abundance between margins and within soybean fields was positively associated; b) Bt predators’ abundance and richness was positively associated with their values in refuge areas, evidencing for the first time their role in the conservation of beneficial fauna; c) biological and reproductive parameters and feeding behavior of P. guildinii were not affected by the ingestion of Cry1Ac protein expressed in Bt soybean pods. These results contribute to the possibilities of ecological intensification designs of national and regional production systems through pest management approached from a landscape perspective.

Varied frequencies of resistance alleles to Cry1Ab and Cry1Ac among Brazilian populations of the sugarcane borer, Diatraea saccharalis (F.).

Brazil is the largest grower of the world's 26 million ha of sugarcane, Saccharum officinarum. Pest damage mainly by the sugarcane borer, Diatraea saccharalis (F.), is a great challenge to the sugarcane industry. To control D.saccharalis, Brazil launched the world's first commercial use of Bt sugarcane in 2017. As part of the resistance management programs for Bt sugarcane planting, 535 F2 isoline families of D.saccharalis collected from three major sugarcane planting states (Goiás, Minas Gerais and São Paulo) in Brazil during 2019-2020 were screened for resistance to two Bt sugarcane varieties: CTC20BT expressing Cry1Ab and CTC9001BT expressing Cry1Ac. Here we report the results of the first study related to Bt resistance in a sugarcane cropping system. Larval survivorships of these families in an F2 screen on CTC20BT were highly correlated with their survival on CTC9001BT, whereas the Cry1Ac tissues exhibited greater insecticidal activities than Cry1Ab. Resistance allele frequencies (RAFs) for populations from Goiás and Minas Gerais were relatively low at 0.0034 for Cry1Ab and 0.0045 to Cry1Ac. By contrast, RAFs for São Paulo populations were considerably greater (0.0393 to Cry1Ab, 0.0245 to Cry1Ac). RAFs to Cry1Ab and Cry1Ac varied among Brazilian D.saccharalis populations. Prior selection resulting from an intensive use of single-gene Bt maize under low compliance of refuge planting could be a main factor contributing to the high RAF in São Paulo. The results suggest that mitigation measures including sufficient non-Bt maize refuge planting, effective resistance monitoring, and use of pyramided Bt sugarcane traits should be implemented promptly to prevent further increase in the RAF to ensure the sustainable use of Bt sugarcane in Brazil. To control Diatraea saccharalis, Brazil launched the world's first commercial use of Bt sugarcane in 2017. As part of the resistance management programs for Bt sugarcane planting in Brazil, 535 F2 isoline families of D.saccharalis collected from three major sugarcane planting states (Goiás, Minas Gerais and São Paulo) in Brazil during 2019-2020 were screened for resistance to Cry1Ab and Cry1Ac sugarcane plants Resistance allele frequencies (RAFs) for the populations from Goiás and Minas Gerais were relatively low at 0.0034 for Cry1Ab and 0.0045 to Cry1Ac. By contrast, RAFs for the São Paulo populations were considerably greater (0.0393 to Cry1Ab, 0.0245 to Cry1Ac). Prior selection resulting from an intensive use of single-gene Bt maize under low compliance of non-Bt maize refuge planting could be a main factor contributing to the high RAF in São Paulo. The results suggest that effective mitigation measures including sufficient non-Bt maize refuge planting, effective resistance monitoring and use of pyramided Bt sugarcane traits should be implemented promptly to prevent further increase in the RAF to ensure the sustainable use of Bt sugarcane in Brazil. © 2022 Society of Chemical Industry.

Soybeans as a non-Bt refuge for Helicoverpa zea in maize-cotton agroecosystems

Geospatial models are crucial for identifying likely ‘hot-spots’ of Bt resistance evolution in Helicoverpa zea (Lepidoptera: Noctuidae), thereby improving regional insecticide resistance management (IRM) strategies and planted refuge compliance. To characterize H. zea distributions in relation to land use, we used historical trapping data collected from 2008 to 2019 in North Carolina to model the spatial and temporal abundance of H. zea populations across Bt-dominated landscapes. Helicoverpa zea abundance was standardized across site-year observations, and candidate landscape composition and configuration predictors of H. zea abundance were obtained. Spatiotemporal Bayesian hierarchical models were developed to make posterior predictions of H. zea abundance from environmental covariates, and results were used to generate interpolation prediction maps to visualize H. zea abundance across the sampled region. Our results suggest inverse distance weighted (IDW) soybeans is the most important predictor of H. zea abundance through time in row crop agroecosystems in North Carolina. Soybeans in North Carolina and southeastern U.S. likely serves as a critical non-Bt refuge for delaying H. zea resistance to Bt toxins in landscapes dominated by Bt maize and cotton. Moreover, soybean abundance can be used to predict the spatial abundance of H. zea in this region. Results can be applied to understand population dynamics of H. zea in landscapes dominated by genetically engineered (GE) crops expressing Bt toxins and will enable the development of sound insect resistance management strategies of H. zea populations to GE toxins targeting noctuid pests of maize and cotton. This work will also drive future geospatial studies investigating environmental predictors of resistance evolution in arthropod pests to GE technologies in crop production systems.

SWOT Analysis of Refuge-in-Bag for Bt-cotton In India

Growing a refuge crop is an integral part of transgenic crops with gene(s) from Bacillus thuringiensis (Bt) and a key requirement to delay evolution of Bt resistance in target pests. Poor refuge planting with Bt-cotton in India, for many years, is a major reason for the outbreak of Bt resistance in pink bollworm and consequent erosion in the efficacy of Bt-cotton technology. As a remedial measure, the Ministry of Agriculture, Government of India has directed complete shift to the marketing of Bt-cotton seeds blended with non-Bt refuge cotton seeds (refuge-in-bag; RIB) from this (2020) season onwards. The strengths, weaknesses, opportunities and threats of RIB Bt-cotton combination in the context of known behaviour of cotton lepidopteran pests and prior exposure to the same Bt toxins since 2002, is discussed. Bt-cotton with RIB, if implemented with strong stewardship, will have far-reaching benefits and is expected to delay Bt resistance development in target lepidopteran cotton pests. However, the relevance of RIB for Bt-resistant pink bollworm could be in question.

Multiple origins of a single point mutation in the cotton bollworm tetraspanin gene confers dominant resistance to Bt cotton.

Transgenic crops producing insecticidal proteins derived from Bacillus thuringiensis (Bt) are used globally to kill key insect pests and provide numerous benefits, including improved pest management, increased profits, reduced insecticide use, and increased biological control. Unfortunately, such benefits are rapidly being lost by the evolution of Bt resistance by pests. The main strategy to delay resistance relies on the use of non-Bt refuge plants to produce sufficient susceptible insects that mate with rare resistant insects emerging from Bt crops, essentially diluting and/or removing resistance alleles from pest populations. A key assumption for the success of this refuge strategy is that inheritance of resistance is recessive. In China, dominant resistance to Cry1Ac Bt cotton by the cotton bollworm Helicoverpa armigera is increasing and is associated with a mutation in the tetraspanin HaTSPAN1 gene, conferring more than 125-fold resistance. Here, we used amplicon sequencing to test the hypotheses that the HaTSPAN1 mutation either arose from a single event and spread or that the mutation evolved independently several times throughout northern China. From three laboratory strains and 28 field populations sampled from northern China, we identified six resistant and 50 susceptible haplotypes. Phylogenetic analysis indicates that the HaTSPAN1 mutation arose from at least four independent origins and spread to their current distributions. The results provide valuable information about the evolutionary origins of dominant resistance to Cry1Ac Bt cotton in northern China and offer rationale for the rapid increase in field-evolved resistance in these areas, where the implementation of additional practical resistance management is needed. © 2020 Society of Chemical Industry.

Applying a Selection Experiment to Test for Fitness Costs of Bt Resistance in Western Corn Rootworm (Coleoptera: Chrysomelidae) and the Effect of Density on Fitness Costs.

Western corn rootworm, Diabrotica virgifera virgifera LeConte, is a serious pest of corn and is often managed with transgenic corn producing insecticidal toxins from the bacterium Bacillus thuringiensis (Bt). This pest has developed field-evolved resistance to all commercially available Bt traits, beginning with Cry3Bb1 in 2009. Fitness costs may accompany Bt resistance, where individuals with alleles for Bt resistance have reduced fitness on non-Bt corn compared to Bt-susceptible individuals. In conjunction with non-Bt refuges, fitness costs can delay the evolution of Bt resistance. Importantly, ecological factors may affect the presence and magnitude of fitness costs. For western corn rootworm, available data suggest that fitness costs of Bt resistance may be present in some cases. Using two Cry3Bb1-resistant western corn rootworm strains (Hopkinton and Cresco), a fitness-cost experiment was performed by rearing rootworm in the absence of Bt for six generations to test for fitness costs of Cry3Bb1 resistance and the effect of larval rearing density on fitness costs. Fitness costs were detected for both strains; however, strains were still resistant to Cry3Bb1 corn at the end of the experiment. Cresco experienced a greater loss of resistance at low versus high density, but no effect of density was detected in Hopkinton. Our study shows that fitness costs can accompany Bt resistance in western corn rootworm and may be more pronounced under low larval density. Even though fitness costs were present, it appears that rootworm populations may remain resistant to Cry3Bb1 corn for years after resistance has evolved.

Larval movement and survival of Helicoverpa zea (Boddie) in seed blends of non-Bt and Bt maize containing Agrisure Viptera® trait: Implications for resistance management

A seed blend refuge has been used to host susceptible insects for Bt maize insect resistance management (IRM) in the U.S. Maize Belt. One major concern in the use of seed blends is that larval movement among non-Bt and Bt plants could cause additional mortalities of susceptible insects. To address this concern, three caged-field trials were conducted to evaluate larval movement and survival of the corn earworm, Helicoverpa zea (Boddie), in eight seed blends of Cry1Ab/Vip3A maize with 0–30% non-Bt refuges. Each plot consisted of five rows and 21 plants in each row. Each plot was infested on the ear silks of the central plant at R1 stage with 35 F1 neonates produced from field-collected H. zea. After 9–13 d, no live larvae or kernel damages were observed on all Bt plants across all planting patterns and trials. In pure non-Bt plantings, 64.8% of the total larvae recovered moved away from the infested ears and survived on other plants, but most of the recovered larvae (86.8%) were located within a distance of three plants from the originally infested plants, and all live larvae were found on the originally infested or its adjacent rows. Larval survival and distribution were similar among seed blends with non-Bt plants in the centers of the plots, in which larvae predominantly located on the central plants (85.7% of the total recovered) with 100% being found on the central rows. Overall, larval recovery from central non-Bt plants in seed blends was 27.5% less than that from the pure non-Bt plantings. Kernel damage levels followed similar patterns as the number of larvae recovered in the plants, and larvae recovered developed similarly across treatments. Results of the study should have value in understanding the larval movement behavior of H. zea, assessing resistance evolution risk, and refining Bt maize IRM modeling for managing this pest. Additional studies are needed to determine the extent to which the reduction of refuge populations in seed blends influences resistance development of the pest to the Bt maize.

The oviposition behavior of fall armyworm moths is unlikely to compromise the refuge strategy in genetically modified Bt crops

Non-random oviposition preference by target pest species on Bt and non-Bt refuge plants may increase the proportion of the population under selection pressure, reducing the value of the refuge strategy for resistance management. Here, we tested the oviposition preference for Bt over non-Bt plants and the damage-avoiding oviposition behavior of moths of the fall armyworm (FAW), Spodoptera frugiperda, a worldwide pest species with three reported cases of field-evolved resistance to Bt maize. In the greenhouse, choice assays indicated no oviposition preference for Cry1F Bt maize over its non-Bt isoline whatever intact or injured by FAW larvae. In a field experiment in a region of intensive agriculture in Brazil, the number of egg masses and FAW larvae was recorded on seven Bt maize varieties and two non-Bt maize varieties during vegetative and reproductive growth stages. There was only a weak oviposition preference by FAW moths for less damaged plants in the V10 stage. When using Cry1A.105 + Cry2Ab Bt maize and 50% refuge in a field experiment in another region in Southeastern Brazil, the number of egg masses on Bt and non-Bt plants also indicated no oviposition preference by the moths. Consistently, these results indicate that FAW moths do not distinguish Bt and non-Bt plants for oviposition and that conspecific larval damage has minor interference in the oviposition choice. Therefore, there seems to be random egg-laying preference of FAW moths in Bt maize fields, an oviposition behavior typical of generalist moth species and unlikely to be associated with the rapid selection of Bt-toxin-resistant FAWs.

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

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

Global Patterns of Resistance to Bt Crops Highlighting Pink Bollworm in the United States, China, and India.

Crops genetically engineered to produce insecticidal proteins from Bacillus thuringiensis (Bt) have advanced pest control, but their benefits have been reduced by evolution of resistance in pests. The global monitoring data reviewed here reveal 19 cases of practical resistance to Bt crops, which is field-evolved resistance that reduces Bt crop efficacy and has practical consequences for pest control. Each case represents the responses of one pest species in one country to one Bt toxin. The results with pink bollworm (Pectinophora gossypiella) and Bt cotton differ strikingly among the world's three leading cotton-producing nations. In the southwestern United States, farmers delayed resistance by planting non-Bt cotton refuges from 1996 to 2005, then cooperated in a program that used Bt cotton, mass releases of sterile moths, and other tactics to eradicate this pest from the region. In China, farmers reversed low levels of pink bollworm resistance to Bt cotton by planting second-generation hybrid seeds from crosses between Bt and non-Bt cotton. This approach yields a refuge of 25% non-Bt cotton plants randomly interspersed within fields of Bt cotton. Farmers adopted this tactic voluntarily and unknowingly, not to manage resistance, but apparently because of its perceived short-term agronomic and economic benefits. In India, where non-Bt cotton refuges have been scarce and pink bollworm resistance to pyramided Bt cotton producing Cry1Ac and Cry2Ab toxins is widespread, integrated pest management emphasizing shortening of the cotton season, destruction of crop residues, and other tactics is now essential.

Bt Resistance Implications for Helicoverpa zea (Lepidoptera: Noctuidae) Insecticide Resistance Management in the United States

Both maize and cotton genetically engineered to express Bt toxins are widely planted and important pest management tools in the United States. Recently, Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) has developed resistance to two toxin Bt maize and cotton (Cry1A and Cry2A). Hence, growers are transitioning to three toxin Bt cotton and maize that express both Cry toxins and the Vip3Aa toxin. H. zea susceptibility to Vip3Aa is threatened by 1) a lack of availability of non-Bt refuge crop hosts, including a 1-5% annual decline in the number of non-Bt maize hybrids being marketed; 2) the ineffectiveness of three toxin cultivars to function as pyramids in some regions, with resistance to two out of three toxins in the pyramid; and 3) the lack of a high dose Vip3Aa event in cotton and maize. We propose that data should be collected on current Cry-resistant H. zea in the field to inform future Bt resistance models and that the deployment of Bt toxins and non-Bt refuge crops should be adjusted to favor susceptibility of H. zea to Bt toxins such as Vip3Aa. Finally, maize growers should be incentivized to plant non-Bt structured refuge and have access to hybrids with high-yielding genetic potential at a reasonable price.

Estimating the landscape distribution of eggs by Helicoverpa spp., with implications for Bt resistance management

Transgenic crops expressing insecticidal toxins of Bacillus thuringiensis (Bt) have been deployed in agricultural landscapes around the globe. While the key strategy to delay resistance is the mandatory planting of a non-Bt refuge crop that is preferred by the target pest, the efficacy of this resistance management strategy across different landscape contexts over time is rarely considered. Here, we develop an individual-based model to simulate the spatio-temporal distribution of a highly mobile, polyphagous, global pest, Helicoverpa spp, across agricultural landscapes dominated by transgenic cotton. The simulation model allows us to explore refuge ‘electivity’, the relative utilization of refuge habitat by female Helicoverpa, in relation to Bt cotton habitat. Refuge electivity is an emergent function of egg distributions resulting from individual moth behavior, within multiple landscapes during different seasons and crop phenology.The individual-based model is validated against independent data collected from the field. Our findings suggest that refuge electivity is sensitive to the spatial and temporal context of the attractiveness of host crops in the landscape and the preferences of the moths. The attractiveness of mandated refuges, such as pigeon pea relative to Bt cotton, influences how effective they are in the landscape. Dynamics between other host crops, such as sorghum, also play an important role that varies over time and space. We use the model to identify scenarios where refuge strategies are likely to be most effective in terms of boosting susceptible populations and increasing landscape movement (genetic mixing). This dynamic approach has potential to inform better refuge design for Bt resistance management across a wide range of landscape contexts. For example, these findings justify the removal of sorghum as an option for mandated refuge in the Risk Management Plan (RMP) for Bt cotton in Australia.

Spatio-Temporal Variation in Landscape Composition May Speed Resistance Evolution of Pests to Bt Crops.

Transgenic crops that express insecticide genes from Bacillus thuringiensis (Bt) are used worldwide against moth and beetle pests. Because these engineered plants can kill over 95% of susceptible larvae, they can rapidly select for resistance. Here, we use a model for a pyramid two-toxin Bt crop to explore the consequences of spatio-temporal variation in the area of Bt crop and non-Bt refuge habitat. We show that variability over time in the proportion of suitable non-Bt breeding habitat, Q, or in the total area of Bt and suitable non-Bt habitat, K, can increase the overall rate of resistance evolution by causing short-term surges of intense selection. These surges can be exacerbated when temporal variation in Q and/or K cause high larval densities in refuges that increase density-dependent mortality; this will give resistant larvae in Bt fields a relative advantage over susceptible larvae that largely depend on refuges. We address the effects of spatio-temporal variation in a management setting for two bollworm pests of cotton, Helicoverpa armigera and H. punctigera, and field data on landscape crop distributions from Australia. Even a small proportion of Bt fields available to egg-laying females when refuges are sparse may result in high exposure to Bt for just a single generation per year and cause a surge in selection. Therefore, rapid resistance evolution can occur when Bt crops are rare rather than common in the landscape. These results highlight the need to understand spatio-temporal fluctuations in the landscape composition of Bt crops and non-Bt habitats in order to design effective resistance management strategies.

Factors Associated With Willingness to Plant Non-Bt Maize Refuge and Suggestions for Increasing Refuge Compliance

Southern U.S. growers who plant Bt maize (Zea mays L.) must plant a separate non-Bt refuge to maintain Bt susceptibility to pests. North Carolina maize growers were surveyed for intention to plant non-Bt maize refuge, understanding of the importance of refuge, and for their perceptions on actions that would encourage refuge plantings. Identical surveys were conducted and compared before and after a short verbal presentation during 2014 on the logic and necessity of planting refuge. An identical survey was given during 2016, but without a verbal presentation and growers were not asked about actions to encourage planting of refuge. Survey responses from 2014 and 2016 were compared and 2016 survey responses were investigated for possible correlations to environmental factors, such as planted crop area and demographic factors by county. Only 38.3 to 44.3% growers indicated they were planning to plant refuge, and 22 to 29.4% were uncertain about this. Additionally, the verbal county meeting presentation did not increase grower intention to plant more refuge. Although this medium increased understanding directly following the presentation, understanding did not change two years later. Total cropland and farm size were most consistently correlated with intention to plant refuge and understanding of the importance of planting refuge. Future efforts to increase compliance and maintain Bt susceptibility in southern U.S. states like North Carolina should focus on reaching smaller-sized growers. Furthermore, the seed industry could also focus on improved breeding, seed availability, and marketing efforts toward companion non-Bt refuge hybrids, as this was popular with growers.

IPM Use With the Deployment of a Non-High Dose Bt Pyramid and Mitigation of Resistance for Western Corn Rootworm (Diabrotica virgifera virgifera).

Recent detection of western corn rootworm resistance to Bt (Bacillus thuringiensis) corn prompted recommendations for the use of integrated pest management (IPM) with planting refuges to prolong the durability of Bt technologies. We conducted a simulation experiment exploring the effectiveness of various IPM tools at extending durability of pyramided Bt traits. Results indicate that some IPM practices have greater merits than others. Crop rotation was the most effective strategy, followed by increasing the non-Bt refuge size from 5 to 20%. Soil-applied insecticide use for Bt corn did not increase the durability compared with planting Bt with refuges alone, and both projected lower durabilities. When IPM participation with randomly selected management tools was increased at the time of Bt commercialization, durability of pyramided traits increased as well. When non-corn rootworm expressing corn was incorporated as an IPM option, the durability further increased. For corn rootworm, a local resistance phenomenon appeared immediately surrounding the resistant field (hotspot) and spread throughout the local neighborhood in six generations in absence of mitigation. Hotspot mitigation with random selection of strategies was ineffective at slowing resistance, unless crop rotation occurred immediately; regional mitigation was superior to random mitigation in the hotspot and reduced observed resistance allele frequencies in the neighborhood. As resistance alleles of mobile pests can escape hotspots, the scope of mitigation should extend beyond resistant sites. In the case of widespread resistance, regional mitigation was less effective at prolonging the life of the pyramid than IPM with Bt deployment at the time of commercialization.