The evolution of resistance to two-toxin pyramid transgenic crops

Bt corn hybrids expressing mCry3A and eCry3.1Ab Proteins protect corn roots against western corn rootworm injury

BACKGROUND

The western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), is one of the most serious pests of corn (Zea mays L.) In 2017 and 2018, studies were conducted in fields with and without known unexpected root injury to Cry3Bb1, to determine root protection by Bt corn hybrids expressing both mCry3A and eCry3.1Ab insecticidal crystal proteins, and hybrids expressing either mCry3A or eCry3.1Ab only against the WCR root injury. Node injury was evaluated using the Iowa State University 0-3 node-injury scale (NIS), and the consistency of root protection was also determined.

RESULTS

In 2017, with medium to high larval feeding pressure, the Bt corn hybrids expressing both mCry3A and eCry3.1Ab in the breeding stack, molecular stack, and Bt corn hybrid expressing eCry3.1Ab only, sustained low node injury compared with Bt corn hybrid expressing mCry3A only, and the non-Bt corn. In 2018, with low larval feeding pressure in most of the locations, node injury was not different for the Bt and Non-Bt corn hybrids. Across all locations in both years, the Bt corn hybrids expressing both mCry3A and eCry3.1Ab provided better and consistent node injury protection.

CONCLUSION

Bt corn hybrids expressing both mCry3A and eCry3.1Ab proteins provided better root protection and consistency than the Bt corn hybrid expressing mCry3A only, and non-Bt. Therefore, stacking of Bt traits will be the best option for managing insect resistance. © 2023 Society of Chemical Industry.

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.

Bacillus thuringiensis chimeric proteins Cry1A.2 and Cry1B.2 to control soybean lepidopteran pests: New domain combinations enhance insecticidal spectrum of activity and novel receptor contributions

Two new chimeric Bacillus thuringiensis (Bt) proteins, Cry1A.2 and Cry1B.2, were constructed using specific domains, which provide insecticidal activity against key lepidopteran soybean pests while minimizing receptor overlaps between themselves, current, and soon to be commercialized plant incorporated protectants (PIP's) in soybean. Results from insect diet bioassays demonstrate that the recombinant Cry1A.2 and Cry1B.2 are toxic to soybean looper (SBL) Chrysodeixis includens Walker, velvetbean caterpillar (VBC) Anticarsia gemmatalis Hubner, southern armyworm (SAW) Spodoptera eridania, and black armyworm (BLAW) Spodoptera cosmioides with LC50 values < 3,448 ng/cm2. Cry1B.2 is of moderate activity with significant mortality and stunting at > 3,448 ng/cm2, while Cry1A.2 lacks toxicity against old-world bollworm (OWB) Helicoverpa armigera. Results from disabled insecticidal protein (DIP) bioassays suggest that receptor utilization of Cry1A.2 and Cry1B.2 proteins are distinct from each other and from current, and yet to be commercially available, Bt proteins in soy such as Cry1Ac, Cry1A.105, Cry1F.842, Cry2Ab2 and Vip3A. However, as Cry1A.2 contains a domain common to at least one commercial soybean Bt protein, resistance to this common domain in a current commercial soybean Bt protein could possibly confer at least partial cross resistance to Cry1A2. Therefore, Cry1A.2 and Cry1B.2 should provide two new tools for controlling many of the major soybean insect pests described above.

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.

Evaluation of the safety and nutritional equivalency of maize grain with genetically modified event DP-Ø23211-2

Feeding studies were conducted with rats and broiler chickens to assess the safety and nutrition of maize grain containing event DP-Ø23211-2 (DP23211), a newly developed trait-pyramid product for corn rootworm management. Diets containing 50% ground maize grain from DP23211, non-transgenic control, or non-transgenic reference hybrids (P0928, P0993, and P1105) were fed to Crl:CD®(SD) rats for 90 days. Ross 708 broilers were fed phase diets containing up to 67% maize grain from each source for 42 days. Body weight, gain, and feed conversion were determined for comparisons between animals fed DP23211 and control diets in each study. Additional measures included clinical and neurobehavioral evaluations, ophthalmology, clinical pathology, organ weights, and gross and microscopic pathology for rats, and carcass parts and select organ yields for broilers. Reference groups were included to determine if any observed significant differences between DP23211 and control groups were likely due to natural variation. No diet-related effects on mortality or evaluation measures were observed between animal fed diets produced with DP23211 maize grain and animal fed diets produced with control maize grain. These studies show that maize grain containing event DP-Ø23211-2 is as safe and nutritious as non-transgenic maize grains when fed in nutritionally adequate diets. The results are consistent with previously published studies, providing further demonstration of the absence of hazards from edible-fraction consumption of genetically modified plants.

Models of Diabrotica Populations: Demography, Population Genetics, Geographic Spread, and Management

Simple Summary

Two beetles that are serious pests of maize, Diabrotica virgifera virgifera and Diabrotica barberi, have caused problems for farmers in the USA and Europe for many years. Because both species have developed resistance to several management tactics, including insecticides and crop rotation, mathematical modeling has been used to evaluate their life cycles for weaknesses and new tactics for value. This review highlights lessons learned from the past 35 years. Some models have focused on the probability of the beetles spreading across regions. Other models have been developed to estimate the risk of the evolution of resistance. These models are thoroughly reviewed with respect to the biological attributes incorporated in these models and the impact of those attributes on the evolution of resistance.

Abstract

Both Diabrotica virgifera virgifera LeConte and D. barberi Smith and Lawrence are among the most damaging insects impacting corn in North America. D. virgifera virgifera has also invaded Europe and has become an important pest in that region. Computer models have become an important tool for understanding the impact and spread of these important pests. Over the past 30 years, over 40 models have been published related to these pests. The focus of these models range from occupancy models (particularly for Europe), impact of climate change, range expansion, economics of pest management, phenology, to the evolution of resistance to toxins and crop rotation. All of these models share characteristics. We elaborate on the methods in which modelers have incorporated the biology of these pests, including density-dependence, movement, fecundity and overwintering mortality. We discuss the utility of both spatially-explicit, complex models and spatially-implicit, generational models and where each might be appropriate. We review resistance models that either explain past evolution to crop rotation, insecticides or insecticidal traits or attempt to predict the consequences of resistance management strategies.

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.

Combining refuges with transgenic insect releases for the management of an insect pest with non-recessive resistance to Bt crops in agricultural landscapes

Reinforcing the high-dose/refuge strategy with releases of transgenic insects has been suggested as a method for simultaneously managing agricultural pest populations and resistance to transgenic crops. Theoretical and empirical studies have shown that these approaches can work when deployed against closed populations and the assumptions of the HDR strategy are met. However, field-evolved resistance is often linked to non-recessive resistance or refuge non-compliance, and pest management regimes are likely to take place at the landscape-level. It is therefore important to understand how effective such strategies are when resistance is non-recessive, and how they could be employed in agricultural landscapes. We developed a spatially-explicit model to investigate the efficacy of strategies combining refuges with transgenic insect releases to manage a pest with non-recessive resistance in agricultural landscapes. We compared two release strategies, area-wide releases and localised releases targeted at population hotspots, and analysed the effects of refuge and release parameters on population and resistance dynamics. Area-wide releases reliably achieved landscape-level pest eradication. Localised releases also eradicated the pest when low release thresholds were combined with high release ratios, and maintained the pest at low densities when insufficient to achieve extinction. Reinforcing refuges with localised releases also greatly enhanced the probability of resistance extinction. However, when resistance remained in the population, localised releases prevented resistance from reaching fixation rather than greatly delaying or reversing resistance evolution. Our work indicates that combining refuges with simple release policies is effective for landscape-level pest suppression when the HDR assumptions are violated, but more nuanced release strategies may be required to enhance the benefits to resistance management.

Baseline Plant-to-Plant Larval Movement of Spodoptera eridania in Bt and Non-Bt Soybean and Its Possible Impacts on IRM

The widespread use of transgenic technologies has led to the emergence of insect populations resistant to Bt toxins. Some lepidopteran pest species also appear to naturally have some level of tolerance to certain proteins, such as some species of Spodoptera to Cry1Ac. One of the main strategies to manage resistance is the use of refuge areas, the success of which is in part dependent on larval movement of the target pest. Thus, in order to assess the viability of a refuge strategy addressing Spodoptera eridania Stoll (Lepidoptera: Noctuidae) in Bt soybean, it was evaluated the larval movement across plants in Bt and non-Bt soybean, as well as the larval development and mortality on Bt and non-Bt soybean cultivars. We concluded that apparent S. eridania incomplete resistance resulting from high larval mortality and low adaptability on Bt plants, high larval dispersal, nondirectional larval movement, and random larval spatial dispersion suggest that structured refuge is more suitable than mixed refuge for managing resistance in S. eridania populations.

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.

The Corn-Cotton Agroecosystem in the Mid-Southern United States: What Insecticidal Event Pyramids Should be Used in Each Crop to Extend Vip3A Durability

Recent studies suggest that resistance in Helicoverpa zea (Boddie) (Lepidoptera, Noctuidae) to Cry1A(b/c) and Cry2Ab2 toxins from the bacterium Bacillus thuringiensis Berliner (Bacillales: Bacillaceae) has increased and field efficacy is impacted in transgenic corn and cotton expressing these toxins. A third toxin, Vip3A, is available in pyramids expressing two or more Bt toxins in corn hybrids and cotton varieties, but uncertainty exists regarding deployment strategies. During a growing season, H. zea infests corn and cotton, and debate arises over use of Vip3A toxin in corn where H. zea is not an economic pest. We used a three-locus, spatially explicit simulation model to evaluate when using Vip3A in corn might hasten evolution of resistance to Vip3A, with implications in cotton where H. zea is a key pest. When using a conventional refuge in corn and initial resistance allele frequencies of Cry1A and Cry2A were 10%, transforming corn with Vip3A slowed resistance to these toxins and delayed resistance evolution to the three-toxin pyramid as a whole. When Cry resistance allele frequencies exceeded 30%, transforming corn with Vip3A hastened the evolution of resistance to the three-toxin pyramid in cotton. When using a seed blend refuge strategy, resistance was delayed longest when Vip3A was not incorporated into corn and used only in cotton. Simulations of conventional refuges were generally more durable than seed blends, even when 75% of the required refuge was not planted. Extended durability of conventional refuges compared to other models of resistance evolution are discussed as well as causes for unusual survivorship in seed blends.

Seasonal Declines in Cry1Ac and Cry2Ab Concentration in Maturing Cotton Favor Faster Evolution of Resistance to Pyramided Bt Cotton in Helicoverpa zea (Lepidoptera: Noctuidae)

Under ideal conditions, widely adopted transgenic crop pyramids producing two or more distinct insecticidal proteins from Bacillus thuringiensis (Bt) that kill the same pest can substantially delay evolution of resistance by pests. However, deviations from ideal conditions diminish the advantages of such pyramids. Here, we tested the hypothesis that changes in maturing cotton producing Cry1Ac and Cry2Ab affect evolution of resistance in Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae), a pest with low inherent susceptibility to both toxins. In terminal leaves of field-grown Bt cotton, the concentration of both toxins was significantly higher for young, squaring plants than for old, fruiting plants. We used laboratory bioassays with plant material from field-grown cotton to test H. zea larvae from a strain selected for resistance to Cry1Ac in the laboratory, its more susceptible parent strain, and their F1 progeny. On young Bt cotton, no individuals survived to pupation. On old Bt cotton, survival to pupation was significantly higher for the lab-selected strain and the F1 progeny relative to the unselected parent strain, indicating dominant inheritance of resistance. Redundant killing, the extent to which insects resistant to one toxin are killed by another toxin in a pyramid, was complete on young Bt cotton, but not on old Bt cotton. No significant fitness costs associated with resistance were detected on young or old non-Bt cotton. Incorporation of empirical data into simulations indicates the observed increased selection for resistance on old Bt cotton could accelerate evolution of resistance to cotton producing Cry1Ac and Cry2Ab in H. zea.

Performance of Bt-susceptible and -heterozygous genotypes of Spodoptera frugiperda (J.E. Smith) possessing single- or dual-gene resistance alleles in sequential feedings of non-Bt and Cry1A.105/Cry2Ab2 maize leaf tissues

We simulated larval feeding behavior in seed blends of non-Bt and Bt maize to determine if seed blends create more favorable conditions for heterozygous-resistant insects over their Bt-susceptible counterparts. Survival, growth, development, and progeny production of four genotypes of the fall armyworm, Spodoptera frugiperda, Bt-susceptible (aabb), Cry1A.105 heterozygous resistant (Aabb), Cry2Ab2 heterozygous resistant (aaBb), and Cry1A.105/Cry2Ab2 heterozygous resistant (AaBb), were evaluated in eight feeding sequences (Seq 1-8) of non-Bt and MON89034 Bt maize leaf tissue expressing the Cry1A.105 and Cry2Ab2 proteins. We report variation in the performance of the four genotypes across the feeding sequences and biological parameters measured. Three heterozygous genotypes generally outperformed the susceptible genotype in larval survival, pupation rate, pupal weight, and progeny production. The performance was greater for Aabb over aaBb, AaBb over Aabb or aaBb, in two of the feeding sequences. The findings of this study could have important implications in assessing the risk of seed blends as refuge plantings for Bt crop resistance management where resistance in the target pest is not functionally recessive.

Effects of transgenic Bacillus thuringiensis cotton on insecticide use, heliothine counts, plant damage, and cotton yield: A meta-analysis, 1996-2015

The primary management tactic for lepidopteran pests of cotton in the United States of America (USA) is the use of transgenic cotton that produces Bacillus thuringiensis Berliner (Bt) toxins. The primary target pests of this technology are Helicoverpa zea (Boddie) and Heliothis virescens (F.) in the eastern and central Cotton Belt of the USA. Concerns over the evolution of resistance in H. zea to Bt toxins and scrutiny of the necessity of Bt crops has escalated. We reviewed published and unpublished data from field trials of Bt cotton in the eastern and central Cotton Belt of the USA through 2015 to evaluate the effectiveness of Bt cotton (Bollgard, Bollgard II, WideStrike, WideStrike 3, and TwinLink). Bt cotton reduced insecticide usage, reduced heliothine pest numbers and damage, and provided a yield benefit, but Bollgard II and WideStrike efficacy declined in the Midsouth over the period evaluated. In the Southeastern region, heliothine damage remained constant through 2015, but yield benefits declined from 2010 until 2015. Resistance of H. zea to several Bt toxins is the most plausible explanation for the observed changes in Bt cotton efficacy. The introduction of new Bt toxins such as found in Widestrike 3 and Twinlink may preserve the benefits of Bt crops. However, while both Widestrike 3 and Twinlink had less damage than Widestrike, damage levels of both were similar to Bollgard II.

Effects of seasonal changes in cotton plants on the evolution of resistance to pyramided cotton producing the Bt toxins Cry1Ac and Cry1F in Helicoverpa zea

BACKGROUND

In pests with inherently low susceptibility to Bacillus thuringiensis (Bt) toxins, seasonal declines in the concentration of Bt toxins in transgenic crops could accelerate evolution of resistance by increasing the dominance of resistance. Here, we evaluated Helicoverpa zea survival on young and old cotton plants that produced the Bt toxins Cry1Ac and Cry1F or did not produce Bt toxins.

RESULTS

Using a strain selected for resistance to Cry1Ac in the laboratory, its parent strain that was not selected in the laboratory, and their F₁ progeny, we showed that resistance to Cry1Ac + Cry1F cotton was partially dominant on young and old plants. On Cry1Ac + Cry1F cotton, redundant killing was incomplete on young plants but nearly complete on old plants. No significant fitness costs on non-Bt cotton occurred on young plants, but large recessive costs affected survival on old plants. Simulation models incorporating the empirical data showed that the seasonal changes in fitness could delay resistance to Cry1Ac + Cry1F cotton by inducing low equilibrium frequencies of resistance alleles when refuges are sufficiently large.

CONCLUSION

Our results suggest that including effects of seasonal changes in fitness of pests on Bt crops and refuge plants can enhance resistance risk assessment and resistance management. © 2017 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.

Optimal management strategy of insecticide resistance under various insect life histories: Heterogeneous timing of selection and interpatch dispersal

Although theoretical studies have shown that the mixture strategy, which uses multiple toxins simultaneously, can effectively delay the evolution of insecticide resistance, whether it is the optimal management strategy under different insect life histories and insecticide types remains unknown. To test the robustness of this management strategy over different life histories, we developed a series of simulation models that cover almost all the diploid insect types and have the same basic structure describing pest population dynamics and resistance evolution with discrete time steps. For each of two insecticidal toxins, independent one‐locus two‐allele autosomal inheritance of resistance was assumed. The simulations demonstrated the optimality of the mixture strategy either when insecticide efficacy was incomplete or when some part of the population disperses between patches before mating. The rotation strategy, which uses one insecticide on one pest generation and a different one on the next, did not differ from sequential usage in the time to resistance, except when dominance was low. It was the optimal strategy when insecticide efficacy was high and premating selection and dispersal occur.

Is a larger refuge always better? Dispersal and dose in pesticide resistance evolution

The evolution of resistance against pesticides is an important problem of modern agriculture. The high-dose/refuge strategy, which divides the landscape into treated and nontreated (refuge) patches, has proven effective at delaying resistance evolution. However, theoretical understanding is still incomplete, especially for combinations of limited dispersal and partially recessive resistance. We reformulate a two-patch model based on the Comins model and derive a simple quadratic approximation to analyze the effects of limited dispersal, refuge size, and dominance for high efficacy treatments on the rate of evolution. When a small but substantial number of heterozygotes can survive in the treated patch, a larger refuge always reduces the rate of resistance evolution. However, when dominance is small enough, the evolutionary dynamics in the refuge population, which is indirectly driven by migrants from the treated patch, mainly describes the resistance evolution in the landscape. In this case, for small refuges, increasing the refuge size will increase the rate of resistance evolution. Our analysis distils major driving forces from the model, and can provide a framework for understanding directional selection in source-sink environments.

Spring phenology of cotton bollworm affects wheat yield

Climate change has changed numerous species phenologies. Understanding the asynchronous responses between pest insects and host plants to climate change is helpful in improving integrated pest management. It is necessary to use long-term data to analyze the effects of climate change on cotton bollworm and wheat anthesis. Data for cotton bollworm, wheat yield, and wheat anthesis collected since 1990 were analyzed using linear regression and partial least-squares regression, as well as the Mann-Kendall test. The results showed that warmer temperatures in the spring advanced the phenologies of cotton bollworm and wheat anthesis, but the phenology changes in overwintering cotton bollworm were faster than those in wheat anthesis, and the eclosion period of overwintering was prolonged, resulting in an increase in overwintering adult abundance. This might lead to more first-generation larvae and subsequent wheat damage. An early or late first-appearance date significantly affected the eclosion days. The abrupt changes of phenologies in cotton bollworm, wheat anthesis, and climate were asynchronous, but the abrupt phenology changes occurred after or around the climate abrupt change, especially after or around the abrupt changes of temperature in March and April. The expansion of asynchronous responses in the change rate of wheat anthesis and overwintering cotton bollworm would likely decrease wheat yield due to climate warming in the future. Accumulated temperature was the major affecting factor on the first eclosion date (t1), adult abundance, and eclosion days. Temperatures in March and April and precipitation in the winter mainly affected the prepeak date (t2), peak date (t3), and postpeak date (t4), respectively, and these factors indirectly affected wheat yield. Thus, the change in the spring phenology of the cotton bollworm and wheat anthesis, and hence wheat yield, was affected by climate warming.

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.

Sixteen Years of Bt Maize in the EU Hotspot: Why Has Resistance Not Evolved?

The majority of Bt maize production in the European Union (EU) is concentrated in northeast Spain, which is Europe's only hotspot where resistance might evolve, and the main target pest, Sesamia nonagrioides, has been exposed to Cry1Ab maize continuously since 1998. The cropping system in northeast Spain has some similar characteristics to those that probably led to rapid resistance failures in two other target noctuid maize pests. These include repeated cultivation of Bt maize in the same fields, low use of refuges, recurring exposure of larvae to non-high dose concentrations of Cry1Ab toxin during the first years of cultivation, low migratory potential, and production concentrated in an irrigated region with few alternative hosts. Available data reveal no evidence of resistance in S. nonagrioides after 16 years of use. We explore the possible reasons for this resistance management success using evolutionary models to consider factors expected to accelerate resistance, and those expected to delay resistance. Low initial adoption rates and the EU policy decision to replace Event 176 with MON 810 Bt maize were key to delaying resistance evolution. Model results suggest that if refuge compliance continues at the present 90%, Bt maize might be used sustainably in northeast Spain for at least 20 more years before resistance might occur. However, obtaining good estimates of the present R allele frequency and level of local assortative mating are crucial to reduce uncertainty about the future success of resistance management.

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.

On-Plant Larval Movement and Feeding Behavior of Fall Armyworm (Lepidoptera: Noctuidae) on Reproductive Corn Stages

Spodoptera frugiperda J.E. Smith (fall armyworm) is considered one of the most destructive pests of corn throughout the Americas. Although this pest has been extensively studied, little is known about its larval movement and feeding behavior on reproductive compared to vegetative corn stages. Thus, we conducted studies with two corn stages (R1 and R3) and four corn plant zones (tassel, above ear, ear zone, and below ear) in the field at Concord, NE (USA), and in the field and greenhouse at Botucatu, SP (Brazil), to investigate on-plant larval movement. The effects of different corn tissues (opened tassel, closed tassel, silk, kernel, and leaf), two feeding sequence scenarios (closed tassel-leaf-silk-kernel and leaf-silk-kernel), and artificial diet (positive control) on larval survival and development were also evaluated in the laboratory. Ear zone has a strong effect on feeding choice and survival of fall armyworm larvae regardless of reproductive corn stage. Feeding site choice is made by first-instar. Corn leaves of reproductive plants were not suitable for early instar development, but silk and kernel tissues had a positive effect on survival and development of fall armyworm larvae on reproductive stage corn.

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.

Resistance Risk Assessment of Spodoptera frugiperda (Lepidoptera: Noctuidae) and Diatraea saccharalis (Lepidoptera: Crambidae) to Vip3Aa20 Insecticidal Protein Expressed in Corn

Transgenic Agrisure Viptera 3 corn that expresses Cry1Ab, Vip3Aa20, and EPSPS proteins and Agrisure Viptera expressing Vip3Aa20 are used for control of Spodoptera frugiperda (J.E. Smith) and Diatraea saccharalis (F.) in Brazil. To support a resistance management program, resistance risk assessment studies were conducted to characterize the dose expression of Vip3Aa20 protein and level of control against these species. The Vip3Aa20 expression in Agrisure Viptera 3 and Agrisure Viptera decreased from V6 to V10 stage of growth. However, Vip3Aa20 expression in Agrisure Viptera 3 at V6 and V10 stages was 13- and 16-fold greater than Cry1Ab, respectively. The Vip3Aa20 expression in lyophilized tissue of Agrisure Viptera 3 and Agrisure Viptera diluted 25-fold in an artificial diet caused complete larval mortality of S. frugiperda and D. saccharalis. In contrast, lyophilized tissue of Bt11 at the same dilution does not provide complete mortality of these species. Agrisure Viptera 3 and Agrisure Viptera also caused a high level of mortality against S. frugiperda and D. saccharalis. Moreover, 100% mortality was observed for S. frugiperda larvae (neonates through fifth-instar larvae) when fed in corn with the Vip trait technology. Viptera corn achieves a high level of control against S. frugiperda and D. saccharalis providing a high dose, which is an important determination to support the refuge strategy for an effective resistance management program.

Effect of Seed Blends and Soil-Insecticide on Western and Northern Corn Rootworm Emergence from mCry3A+eCry3.1Ab Bt Maize

Seed blends containing various ratios of transgenic Bt maize (Zea mays L.) expressing the mCry3A+eCry3.1Ab proteins and non-Bt maize (near-isoline maize) were deployed alone and in combination with a soil applied pyrethroid insecticide (Force CS) to evaluate the emergence of the western corn rootworm, Diabrotica virgifera virgifera LeConte, in a total of nine field environments across the Midwestern United States in 2010 and 2011. Northern corn rootworm, Diabrotica barberi Smith & Lawrence emergence was also evaluated in four of these environments. Both western and northern corn rootworm beetle emergence from all Bt treatments was significantly reduced when compared with beetle emergence from near-isoline treatments. Averaged across all environments, western corn rootworm beetle emergence from 95:5, 90:10, and 80:20 seed blend ratios of mCry3A+eCry3.1Ab: near-isoline were 2.6-, 4.2-, and 6.7-fold greater than that from the 100:0 ratio treatment. Northern corn rootworm emergence from the same seed blend treatments resulted in 2.8-, 3.2-, and 4.2-fold more beetles than from the 100:0 treatment. The addition of Force CS (tefluthrin) significantly reduced western corn rootworm beetle emergence for each of the three treatments to which it was applied. Force CS also significantly delayed the number of days to 50% beetle emergence in western corn rootworms. Time to 50% beetle emergence in the 100% mCry3A+eCry3.1Ab treatment with Force CS was delayed 13.7 d when compared with western corn rootworm beetle emergence on near-isoline corn. These data are discussed in terms of rootworm resistance management.

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.

Effects of Pyramided Bt Corn and Blended Refuges on Western Corn Rootworm and Northern Corn Rootworm (Coleoptera: Chrysomelidae)

The western corn rootworm, Diabrotica virgifera virgifera LeConte, and the northern corn rootworm, Diabrotica barberi Smith & Lawrence (Coleoptera: Chrysomelidae), are major pests of corn (Zea mays L). Several transgenic corn events producing insecticidal toxins derived from the bacterium Bacillus thuringiensis (Bt) kill corn rootworm larvae and reduce injury to corn roots. However, planting of Bt corn imposes selection on rootworm populations to evolve Bt resistance. The refuge strategy and pyramiding of multiple Bt toxins can delay resistance to Bt crops. In this study, we assessed the impact of four treatments--1) non-Bt corn, 2) Cry3Bb1 corn, 3) corn pyramided with Cry3Bb1 and Cry34/35Ab1, and 4) pyramided corn with a blended refuge--on survival, time of adult emergence, and size of western and northern corn rootworm. All treatments with Bt corn led to significant reductions in the number of adults that emerged per plot. However, at one location, we identified Cry3Bb1-resistant western corn rootworm. In some cases Bt treatments reduced size of adults and delayed time of adult emergence, with effects most pronounced for pyramided corn. For both species, the number of adults that emerged from pyramided corn with a blended refuge was significantly lower than expected, based solely on emergence from pure stands of pyramided corn and non-Bt corn. The results of this study indicate that pyramided corn with a blended refuge substantially reduces survival of both western and northern corn rootworm, and as such, should be a useful tool within the context of a broader integrated pest management strategy.

Survival and Development of Spodoptera frugiperda and Chrysodeixis includens (Lepidoptera: Noctuidae) on Bt Cotton and Implications for Resistance Management Strategies in Brazil

In Brazil, Spodoptera frugiperda (J. E. Smith) and Chrysodeixis includens (Walker) are important cotton pests and target of control of Bollgard II (Cry1Ac/Cry2Ab2) and WideStrike (Cry1Ac/Cry1F) cotton technologies. To subsidize an insect resistance management program, we conducted laboratory studies to evaluate the toxicity of these Bt cotton plants throughout larval development of S. frugiperda and C. includens. In bioassays with leaf disc, the efficacy of both Bt cotton plants against neonates was >80% for S. frugiperda and 100% for C. includens. However, S. frugiperda larvae that survived on Bt cotton had >76% of growth inhibition and stunting. In bioassays with S. frugiperda and C. includens larvae fed on non-Bt near-isoline during different time period (from 3 to 18 d) and then transferred to Bollgard II or WideStrike leaves showed that larval susceptibility decreased as larval age increased. For Bollgard II cotton, in all S. frugiperda instars, there were larvae that reached the pupal and adult stages. In contrast, on WideStrike cotton, a few larvae in fifth and sixth instar completed the biological cycle. For C. includens, some larvae in sixth instar originated adults in both Bt cotton plants. In conclusion, Bollgard II and WideStrike cotton technologies showed high efficacy against neonates of S. frugiperda and C. includens. However, the mortality of these species decreases as larval age increase, allowing insect survival in a possible seed mixture environment and favoring the resistance evolution.

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.

Strong oviposition preference for Bt over non-Bt maize in Spodoptera frugiperda and its implications for the evolution of resistance

BACKGROUND

Transgenic crops expressing Bt toxins have substantial benefits for growers in terms of reduced synthetic insecticide inputs, area-wide pest management and yield. This valuable technology depends upon delaying the evolution of resistance. The 'high dose/refuge strategy', in which a refuge of non-Bt plants is planted in close proximity to the Bt crop, is the foundation of most existing resistance management. Most theoretical analyses of the high dose/refuge strategy assume random oviposition across refugia and Bt crops.

RESULTS

In this study we examined oviposition and survival of Spodoptera frugiperda across conventional and Bt maize and explored the impact of oviposition behavior on the evolution of resistance in simulation models. Over six growing seasons oviposition rates per plant were higher in Bt crops than in refugia. The Cry1F Bt maize variety retained largely undamaged leaves, and oviposition preference was correlated with the level of feeding damage in the refuge. In simulation models, damage-avoiding oviposition accelerated the evolution of resistance and either led to requirements for larger refugia or undermined resistance management altogether. Since larval densities affected oviposition preferences, pest population dynamics affected resistance evolution: larger refugia were weakly beneficial for resistance management if they increased pest population sizes and the concomitant degree of leaf damage.

CONCLUSIONS

Damaged host plants have reduced attractiveness to many insect pests, and crops expressing Bt toxins are generally less damaged than conventional counterparts. Resistance management strategies should take account of this behavior, as it has the potential to undermine the effectiveness of existing practice, especially in the tropics where many pests are polyvoltinous. Efforts to bring down total pest population sizes and/or increase the attractiveness of damaged conventional plants will have substantial benefits for slowing the evolution of resistance.

Potential shortfall of pyramided transgenic cotton for insect resistance management

To delay evolution of pest resistance to transgenic crops producing insecticidal proteins from Bacillus thuringiensis (Bt), the "pyramid" strategy uses plants that produce two or more toxins that kill the same pest. In the United States, this strategy has been adopted widely, with two-toxin Bt cotton replacing one-toxin Bt cotton. Although two-toxin plants are likely to be more durable than one-toxin plants, the extent of this advantage depends on several conditions. One key assumption favoring success of two-toxin plants is that they kill insects selected for resistance to one toxin, which is called "redundant killing." Here we tested this assumption for a major pest, Helicoverpa zea, on transgenic cotton producing Bt toxins Cry1Ac and Cry2Ab. Selection with Cry1Ac increased survival on two-toxin cotton, which contradicts the assumption. The concentration of Cry1Ac and Cry2Ab declined during the growing season, which would tend to exacerbate this problem. Furthermore, analysis of results from 21 selection experiments with eight species of lepidopteran pests indicates that some cross-resistance typically occurs between Cry1A and Cry2A toxins. Incorporation of empirical data into simulation models shows that the observed deviations from ideal conditions could greatly reduce the benefits of the pyramid strategy for pests like H. zea, which have inherently low susceptibility to Bt toxins and have been exposed extensively to one of the toxins in the pyramid before two-toxin plants are adopted. For such pests, the pyramid strategy could be improved by incorporating empirical data on deviations from ideal assumptions about redundant killing and cross-resistance.

Transgenic approaches to western corn rootworm control

The western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae) is a significant corn pest throughout the United States corn belt. Rootworm larvae feed on corn roots causing yield losses and control expenditures that are estimated to exceed US$1 billion annually. Traditional management practices to control rootworms such as chemical insecticides or crop rotation have suffered reduced effectiveness due to the development of physiological and behavioral resistance. Transgenic maize expressing insecticidal proteins are very successful in protecting against rootworm damage and preserving corn yield potential. However, the high rate of grower adoption and early reliance on hybrids expressing a single mode of action and low-dose traits threatens the durability of commercialized transgenic rootworm technology for rootworm control. A summary of current transgenic approaches for rootworm control and the corresponding insect resistance management practices is included. An overview of potential new modes of action based on insecticidal proteins, and especially RNAi targeting mRNA coding for essential insect proteins is provided.

Bt pollen dispersal and Bt kernel mosaics: integrity of non-Bt refugia for lepidopteran resistance management in maize

Field trials were conducted at Rosemount, MN in 2009 and 2010, to measure pollen movement from Bt corn to adjacent blocks of non-Bt refuge corn. As the use of Bt corn hybrids continues to increase in the United States, and new insect resistance management (IRM) plans are implemented, it is necessary to measure the efficacy of these IRM plans. In Minnesota, the primary lepidopteran pests of corn include the European corn borer, Ostrinia nubilalis (Hübner) and corn earworm, Helicoverpa zea (Boddie). The primary IRM plan in transgenic corn is the use of hybrids expressing a high dose of insecticidal proteins and an insect refuge containing hybrids not expressing insecticidal proteins that produce susceptible insects. Wind-assisted pollen movement in corn occurs readily, and is the primary method of pollination for corn. The combination of pollen movement and viability determines the potential for cross pollination of refuge corn. In 2009 and 2010, cross pollination occurred with the highest frequency on the north and east sides of Bt corn fields, but was found at some level in all directions. Highest levels of cross pollination (75%) were found within the first four rows (3 m) of non-Bt corn adjacent to Bt corn, and in general decreasing levels of cross pollination were found the further the non-Bt corn was planted from the Bt corn. A mosaic of Bt cross-pollinated kernels was found throughout the ear, but in both years the ear tip had the highest percentage of cross-pollinated kernels; this pattern may be linked to the synchrony of pollen shed and silking between Bt and non-Bt corn hybrids. The dominant wind direction in both years was from WNW. However, in both years, there were also prevailing winds from SSW and WSW. Further studies are needed to quantify Bt levels in cross-pollinated kernels, measure the Bt dose of such kernels and associated lepidopteran pest survival, and measure the impact of Bt pollen on lepidopteran pests, particularly when considering the seed mixture refuge configuration.

Application of pyramided traits against Lepidoptera in insect resistance management for Bt crops

Since initial launch of insect protected transgenic crops, the most effective strategy to manage the potential for target pests to evolve resistance has been the use of a single mode of action with "high dose" and structured refuge. However, the effectiveness of this strategy is limited if mortality of certain pests does not reach "high dose" criteria, inconsistent implementation of refuges and non-rare resistance alleles. More recently, several pyramided trait products, which include multiple modes of action against key target pests, have been developed. These products offer the potential for dramatically improved resistance management with smaller refuges and less dependence on high mortality of susceptible and heterozygous insects and rare resistance alleles. We show that products such as SmartStax and PowerCore offer compelling resistance management benefits compared with single mode of action products and allow for the option of products containing refuge seed mixtures rather than structured refuges to effectively delay resistance. We conclude that all stakeholders, including technology developers, growers, crop advisors, extensions services and regulatory authorities should continue to encourage the development, deployment and adoption of pyramided trait products for improved pest management and improved resistance management.

Multimodal protein constructs for herbivore insect control

Transgenic plants expressing combinations of microbial or plant pesticidal proteins represent a promising tool for the efficient, durable control of herbivorous insects. In this review we describe current strategies devised for the heterologous co-expression of pesticidal proteins in planta, some of which have already shown usefulness in plant protection. Emphasis is placed on protein engineering strategies involving the insertion of single DNA constructs within the host plant genome. Multimodal fusion proteins integrating complementary pesticidal functions along a unique polypeptide are first considered, taking into account the structural constraints associated with protein or protein domain grafting to biologically active proteins. Strategies that allow for the co- or post-translational release of two or more pesticidal proteins are then considered, including polyprotein precursors releasing free proteins upon proteolytic cleavage, and multicistronic transcripts for the parallel translation of single protein-encoding mRNA sequences.