Type of fitness cost influences the rate of evolution of resistance to transgenic Bt crops

Biotechnological advances in the production of unusual fatty acids in transgenic plants and recombinant microorganisms

Certain plants and microorganisms can produce high amounts of unusual fatty acids (UFAs) such as hydroxy, conjugated, cyclic, and very long-chain polyunsaturated fatty acids, which have distinct physicochemical properties and significant applications in the food, feed, and oleochemical industries. Since many natural sources of UFAs are not ideal for large-scale agricultural production or fermentation, it is attractive to produce them through synthetic biology. Although several UFAs have been commercially or pre-commercially produced in transgenic plants and microorganisms, their contents in transgenic hosts are generally much lower than in natural sources. Moreover, reproducing this success for a wider spectrum of UFAs has remained challenging. This review discusses recent advancements in our understanding of the biosynthesis, accumulation, and heterologous production of UFAs, and addresses the challenges and potential strategies for achieving high UFA content in engineered plants and microorganisms.

Discovery of insecticide resistance in field-collected populations of the aphid pest, Acyrthosiphon kondoi Shinji

BACKGROUND

The bluegreen aphid (Acyrthosiphon kondoi) is a worldwide pest of alfalfa, pulses, and other legume crops. An overreliance on insecticides to control A. kondoi has potentially placed populations under selection pressure favouring resistant phenotypes, but to date, there have been no documented cases of insecticide resistance. Recently, Australian growers began reporting that conventional insecticides were failing to adequately control A. kondoi populations, prompting this laboratory-based investigation into whether these populations have evolved resistance.

RESULTS

We discovered four A. kondoi populations with moderate resistance (10-40-fold) to three different insecticide groups: organophosphates, carbamates and pyrethroids. However, A. kondoi populations showed no resistance to the butenolide, flupyradifurone. We were unable to identify general metabolic mechanisms using synergist assays (cytochromes P450, glutathione S-transferases, or esterases), indicating that further detailed molecular investigations to characterise the putative resistance mechanism are needed.

CONCLUSION

Insecticide-resistant A. kondoi present an emerging challenge to Australian agriculture. Growers require new tools and updated strategies, including access to newer chemistries, to alleviate their reliance on the few insecticides currently registered against A. kondoi. The implications of insecticide resistant A. kondoi for future management, the potential mechanisms of resistance, and future research priorities are discussed. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Modeling the Resistance Evolution to Insecticides Driven by Lepidopteran Species Competition in Cotton, Soybean, and Corn Crops

Simple Summary

Lepidopteran species commonly interact in the same niches in multiple crops. Interspecific competition has been neglected as a pressure selection agent in insecticide resistance studies. Our results showed that competition may act as an agent to speed up the evolution of diamide resistance in Spodoptera frugiperda and Helicoverpa armigera.

Abstract

Intra- and interspecific competition is considered a fundamental phenomenon in ecology. It acts as one of the most powerful selective forces that drives ecological diversity, the spatiotemporal distribution of organisms, fitness, and evolutionary aspects. Spodoptera frugiperda and Helicoverpa armigera are devastating pests and can co-occur in systems consisting of multiple agricultural crops and compete for food resources. Insecticide resistance in populations of these species has been a major threat to the sustainability of agroecosystems. No study to date has shown the effect of intra- and interspecific competition as a selective pressure agent on the evolution of insecticide resistance in lepidopteran pests in an experimental and theoretical way. Our study developed a parameterized computational model with experimental results for S. frugiperda and H. armigera competition. We simulated the behavior of heterozygous individuals with a competition capacity 100% equal to homozygous individuals resistant (100 RR) or susceptible to insecticides (00 RR), and intermediate between them (50 RR). Competition involving strains of these insect species can accelerate the evolution of their resistance to insecticides in agricultural crops. We found that competitive processes can result in a high probability of competitive exclusion for individuals with the susceptibility allele of these lepidopteran species. The results of this study are of paramount importance for understanding the impact of ecological factor competition on the evolution of insecticide resistance in lepidopteran pests, which until now has been neglected in these types of evolutionary dynamics studies.

Susceptibility of the Cowpea Aphid (Hemiptera: Aphididae) to Widely Used Insecticides in Australia

Globally, 27 aphid species have evolved resistance to almost 100 insecticide active ingredients. A proactive approach to resistance management in pest aphids is needed; this should include risk analysis, followed by regular baseline susceptibility assays for species deemed at high risk of evolving resistance. The cowpea aphid (Aphis craccivora Koch) has evolved insecticide resistance to multiple insecticides outside Australia and was recently identified as a high-risk species in Australia. In this study, we generated toxicity data against four insecticides (representing four unique chemical Mode of Action groups) for populations of A. craccivora collected across Australia. Alpha-cypermethrin was the most toxic chemical to A. craccivora in leaf-dip laboratory bioassays with an average LC50 value across nine populations of 0.008 mg a.i./L, which was significantly lower than dimethoate (1.17 mg a.i./L) and pirimicarb (0.89 mg a.i./L). Small, but significant, differences in sensitivity were detected in some populations against pirimicarb and dimethoate, whereas responses to alpha-cypermethrin and imidacloprid were not significantly different across all aphid populations examined in this study. For all insecticides, the field rate controlled 100% of individuals tested. The data generated will be important for future monitoring of insecticide responses of A. craccivora. Proactive management, including increased reliance on non-chemical pest management approaches and routine insecticide baseline sensitivity studies, is recommended for A. craccivora.

An Interactive Teaching Tool Describing Resistance Evolution and Basic Economics of Insecticide-Based Pest Management

Simple Summary

To attract bright, creative, and curious students to the academic fields of applied entomology and sustainable food production, instructors of undergraduate and graduate student courses should discuss experiences with lectures and lab sessions and share effective interactive teaching tools. This communication describes how a simple population model in an Excel spreadsheet can be used in teaching both insecticide resistance evolution and basic economics of insecticide-based pest management. A tutorial video and the model as an Excel spreadsheet are freely available. Through hands-on experience with this and similar interactive teaching tools, students will acquire fundamental knowledge about basic structures population models and they will acquire experience with quantitative data interpretation. Teachers can use this tool and accompanying tutorials to demonstrate how models can be used to describe and visualize complex interactions between insect genetics and crop management. Furthermore, data from published studies can be analyzed and discussed using this interactive teaching tool.

Abstract

Effective teaching of complex concepts relies heavily on the ability to establish relevance of topics and to engage students in a constructive dialogue. To connect students with abstract concepts and basic theory, instructors foster and facilitate an engaging teaching environment. Population modeling is a cornerstone in applied entomology. However, it is also a topic and skill set that requires both basic mathematical and biological knowledge, and it may be perceived by students as being abstract and exceedingly theoretical. As a way to introduce entomology students at both that undergraduate and graduate levels to hands-on experience with population modeling, a well-established and widely used deterministic genetic population model is presented as an interactive teaching tool. Moreover, the general model describes three genotypes (SS = homozygous susceptible, SR = heterozygous, and RR = homozygous resistant) during 30 discrete and univoltine generations under a shared population density dependence (carrying capacity). Based on user inputs for each genotype (survival, fitness cost, reproductive rate, emigration, and immigration) and an initial resistance allele frequency, model outputs related to resistance evolution are produced. User inputs related to insecticide-based pest management (pest density action threshold, crop damage rate, insecticide treatment costs, and profit potential) can also be introduced to examine and interpret the basic economic effects of different insect pest management scenarios. The proposed model of resistance evolution and basic economics of pest management relies on a large number of important simplifications, so it may only have limited ability to predict the outcomes of real-world (commercial) scenarios. However, as a teaching tool and to introduce students to a well-known and widely used genetic population model structure, the interactive teaching tool is believed to have considerable utility and relevance.

Demographic Performance of Helicoverpa zea Populations on Dual and Triple-Gene Bt Cotton

Insecticidal toxins from Bacillus thuringiensis (Bt) are valuable tools for pest management worldwide, contributing to the management of human disease insect vectors and phytophagous insect pests of agriculture and forestry. Here, we report the effects of dual and triple Bt toxins expressed in transgenic cotton cultivars on the fitness and demographic performance of Helicoverpa zea (Boddie)—a noctuid pest, known as cotton bollworm and corn earworm. Life-history traits were determined for individuals of three field populations from a region where H. zea overwintering is likely. Triple-gene Bt cotton cultivars that express Cry and Vip3Aa toxins killed 100% of the larvae in all populations tested. In contrast, dual-gene Bt cotton that express Cry1Ac+Cry1F and Cry1Ac+Cry2Ab allowed population growth with the intrinsic rate of population growth (r_m) 38% lower than on non-Bt cotton. The insects feeding on Bt cotton plants that express Cry1Ac+Cry2Ab, Cry1Ac+Cry1F, or Cry1Ab+Cry2Ae exhibited reduced larval weight, survival rate, and increased development time. Additionally, fitness parameters varied significantly among the insect populations, even on non-Bt cotton plants, likely because of their different genetic background and/or previous Bt toxin exposure. This is the first report of the comparative fitness of H. zea field populations on dual-gene Bt cotton after the recent reports of field resistance to certain Bt toxins. These results document the population growth rates of H. zea from an agricultural landscape with 100% Bt cotton cultivars. Our results will contribute to the development and validation of resistance management recommendations.

Escalating insecticide resistance in Australian grain pests: contributing factors, industry trends and management opportunities

Insecticide resistance is an ever-increasing problem that threatens food production globally. Within Australia, the grain industry has a renewed focus on resistance due to diminishing chemical options available to farmers and the increasing prevalence and severity of resistance encountered in the field. Chemicals are too often used as the major tool for arthropod pest management, ignoring the potent evolutionary forces from chemical selection pressures that lead to resistance. A complex array of factors (biological, social, economic, political, climatic) have contributed to current trends in insecticide usage and resistance in the Australian grain industry. We review the status of insecticide resistance and provide a context for how resistance is currently managed. We discuss emerging technologies and research that could be applied to improve resistance management. This includes generating baseline sensitivity data for insecticides before they are launched, developing genetic diagnostics for the full complement of known resistances, expanding resistance monitoring programs, and utilizing new technologies. Additional benefits are likely to be achieved through a combination of industry awareness and engagement, risk modeling, adoption of integrated pest management tactics, greater collaboration between industry stakeholders, and policy changes around chemical use and record keeping. The Australian grain context provides lessons for other agricultural industries. © 2018 Society of Chemical Industry.

Insect pest control, approximate dynamic programming, and the management of the evolution of resistance

Ecological decision problems, such as those encountered in agriculture, often require managing conflicts between short-term costs and long-term benefits. Dynamic programming is an ideal method for optimally solving such problems but agricultural problems are often subject to additional complexities that produce state spaces intractable to exact solutions. In contrast, look-ahead policies, a class of approximate dynamic programming (ADP) algorithm, may attempt to solve problems of arbitrary magnitude. However, these algorithms focus on a temporally truncated caricature of the full decision problem over a defined planning horizon and as such are not guaranteed to suggest optimal actions. Thus, look-ahead policies may offer promising means of addressing detail-rich ecological decision problems but may not be capable of fully utilizing the information available to them, especially in scenarios where the best short- and long-term solutions may differ. We constructed and applied look-ahead policies to the management of a hypothetical, stage-structured, continually reproducing, agricultural insect pest. The management objective was to minimize the combined costs of management actions and crop damage over a 16-week growing season. The manager could elect to utilize insecticidal sprays or one of six release ratios of male-selecting transgenic insects where the release ratio determines the number of transgenic insects to be released for each wild-type male insect in the population. Complicating matters was the expression of insecticide resistance at non-trivial frequencies in the pest population. We assessed the extent to which look-ahead policies were able to recognize the potential threat of insecticide resistance and successfully integrate insecticides and transgenic releases to capitalize upon their respective benefits. Look-ahead policies were competent at anticipating and responding to ecological and economic information. Policies with longer planning horizons made fewer, better-timed insecticidal sprays and made more frequent transgenic releases, which consequently facilitated lower resistance allele frequencies. However, look-ahead policies were ultimately inefficient resistance managers, and directly responded to resistance only when it was dominant and prevalent. Effective long-term agricultural management requires the capacity to anticipate and respond to the evolution of resistance. Look-ahead policies can accommodate all the information pertinent to making the best long-term decision but may lack the perspective to actually do so.

Genetics-based methods for agricultural insect pest management

The sterile insect technique is an area-wide pest control method that reduces agricultural pest populations by releasing mass-reared sterile insects, which then compete for mates with wild insects. Contemporary genetics-based technologies use insects that are homozygous for a repressible dominant lethal genetic construct rather than being sterilized by irradiation.Engineered strains of agricultural pest species, including moths such as the diamondback moth Plutella xylostella and fruit flies such as the Mediterranean fruit fly Ceratitis capitata, have been developed with lethality that only operates on females.Transgenic crops expressing insecticidal toxins are widely used; the economic benefits of these crops would be lost if toxin resistance spread through the pest population. The primary resistance management method is a high-dose/refuge strategy, requiring toxin-free crops as refuges near the insecticidal crops, as well as toxin doses sufficiently high to kill wild-type insects and insects heterozygous for a resistance allele.Mass-release of toxin-sensitive engineered males (carrying female-lethal genes), as well as suppressing populations, could substantially delay or reverse the spread of resistance. These transgenic insect technologies could form an effective resistance management strategy.We outline some policy considerations for taking genetic insect control systems through to field implementation.

Beyond dose: Pulsed antibiotic treatment schedules can maintain individual benefit while reducing resistance

The emergence of treatment-resistant microbes is a key challenge for disease treatment and a leading threat to human health and wellbeing. New drugs are always in development, but microbes regularly and rapidly acquire resistance. We must consider if altering how we administer drugs at the individual level could slow development of resistance. Here we use mathematical models to show that exposing microbes to drug pulses could greatly reduce resistance without increasing individual pathogen load. Our results stem from two key factors: the presence of antibiotics creates a selection pressure for antibiotic resistant microbes, and large populations of bacteria are more likely to harbor drug resistance than small populations. Drug pulsing targets these factors simultaneously. Short duration pulses minimize the time during which there is selection for resistance, and high drug concentrations minimize pathogen abundance. Our work provides a theoretical basis for the design of in vitro and in vivo experiments to test how drug pulsing might reduce the impact of drug resistant infections.

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.

Species Diversity Hinders Adaptation to Toxicants

Environmental toxicants such as pesticides exert strong selection pressure on many species. While the resulting development of pesticide resistance in agricultural pest species is well-known, reports on the extent of adaptation in exposed nontarget species are contradictory. These contradictory reports highlight a continuing challenge in understanding the relevant ecological mechanisms that facilitate or hinder adaptation to toxicants in the field. Here we show that species diversity hinders the adaptation to toxicants. In agricultural streams with low diversity, we observed an up to 8-fold increase in insecticide tolerance in a total of 17 macroinvertebrate species that was not observed in more diverse communities under similar contamination. High species diversity occurred independently from adjacent nonpolluted refuge areas. Therefore, the low level of adaptation in diverse streams cannot be explained by an increased recolonization of sensitive individuals from refuge areas. Instead, high intraspecific competition may facilitate the selection for increased tolerance in low-diverse communities. In diverse communities, by contrast, species interactions may reduce intraspecific competition and, thus, the potential for developing toxicant resistance. We suggest that this mechanism may be the general case in adaptation to environmental stressors. Additionally, we conclude that the current framework for risk assessment of pesticides is not protective against selection for tolerant organisms and the associated risk of genetic erosion.

Type of fitness cost influences the rate of evolution of resistance to transgenic Bt crops

The evolution of resistance to pesticides by insect pests is a significant challenge for sustainable agriculture. For transgenic crops expressing Bacillus thuringiensis (Bt), crystalline (Cry) toxins resistance evolution may be delayed by the high‐dose/refuge strategy in which a non‐toxic refuge is planted to promote the survival of susceptible insects. The high‐dose/refuge strategy may interact with fitness costs associated with resistance alleles to further delay resistance. However, while a diverse range of fitness costs are reported in the field, they are typically represented as a fixed reduction in survival or viability which is insensitive to ecological conditions such as competition. Furthermore, the potential dynamic consequences of restricting susceptible insects to a refuge which represents only a fraction of the available space have rarely been considered.

We present a generalized discrete time model which utilizes dynamic programming methods to derive the optimal management decisions for the control of a theoretical insect pest population exposed to Bt crops. We consider three genotypes (susceptible homozygotes, resistant homozygotes and heterozygotes) and implement fitness costs of resistance to Bt toxins as either a decrease in the relative competitive ability of resistant insects or as a penalty on fecundity. Model analysis is repeated and contrasted for two types of density dependence: uniform density dependence which operates equally across the landscape and heterogeneous density dependence where the intensity of competition scales inversely with patch size and is determined separately for the refuge and Bt crop.

When the planting of Bt is decided optimally, fitness costs to fecundity allow for the planting of larger areas of Bt crops than equivalent fitness costs that reduce the competitive ability of resistant insects.

Heterogeneous competition only influenced model predictions when the proportional area of Bt planted in each season was decided optimally and resistance was not recessive.

Synthesis and applications. The high‐dose/refuge strategy alone is insufficient to preserve susceptibility to transgenic Bacillus thuringiensis (Bt) crops in the long term when constraints upon the evolution of resistance are not insurmountable. Fitness costs may enhance the delaying effect of the refuge, but the extent to which they do so depends upon how the cost is realized biologically. Fitness costs which apply independently of other variables may be more beneficial to resistance management than costs which are only visible to selection under a limited range of ecological conditions.

The high‐dose/refuge strategy alone is insufficient to preserve susceptibility to transgenic Bacillus thuringiensis (Bt) crops in the long term when constraints upon the evolution of resistance are not insurmountable. Fitness costs may enhance the delaying effect of the refuge, but the extent to which they do so depends upon how the cost is realized biologically. Fitness costs which apply independently of other variables may be more beneficial to resistance management than costs which are only visible to selection under a limited range of ecological conditions.