Insect Resistance Management Strategies Research Articles

Effects of combining soil-applied insecticide and Bt corn for integrated pest management and resistance management of western corn rootworm (Coleoptera: Chrysomelidae).

The western corn rootworm, (Diabrotica virgifera virgifera LeConte, Coleoptera: Chrysomelidae), is a serious pest of corn (Zea mays Linnaeus, Cyperales: Poaceae) in the midwestern United States. Management practices for corn rootworm larvae include crop rotation, transgenic corn producing insecticidal toxins from the bacterium Bacillus thuringiensis Berliner (Bacillales: Bacillaceae) (Bt), and soil-applied insecticides. The extent to which combining soil-applied insecticide with Bt corn would be beneficial from the perspective of insect resistance management (IRM) or integrated pest management (IPM) remains uncertain. We conducted a 3-yr field study to characterize the implications of combining a soil-applied insecticide and Bt corn for IRM and IPM of western corn rootworm. Experimental treatments were Bt corn, a soil-applied insecticide, the combination of these factors, and an experimental control in which both factors were absent. Data were collected on root injury to corn by rootworm, survival to adulthood, adult size, and emergence time for western corn rootworm. We found that mortality caused by the soil-applied insecticide was insufficient to delay resistance to Bt corn. While combining Bt corn and a soil-applied insecticide may provide a short-term economic benefit, additional research is needed to determine appropriate economic thresholds for combining these tactics. Additionally, combining a soil-applied insecticide and Bt corn would not be sustainable over multiple growing seasons because of its potential to rapidly select for Bt resistance. In general, a more sustainable IRM strategy for rootworm management would include using crop rotation and alternating between non-Bt corn with soil-applied insecticide and Bt corn without soil-applied insecticide.

Continental-scale migration patterns and origin of Helicoverpa zea (Lepidoptera: Noctuidae) based on a biogeochemical marker.

Insect migrations have ecological and economic impacts, particularly in agriculture. However, there is limited knowledge about the migratory movements of pests at the continental scale, which is an important factor influencing the spread of resistance genes. Understanding the migratory patterns of economic pests, like Helicoverpa zea (Boddie), is essential for improving Integrated Pest Management (IPM) and Insect Resistance Management (IRM) strategies. In this study, we used stable hydrogen isotopic ratios in wing tissue as a biogeochemical marker to examine migratory patterns and estimate the native origins of H. zea individuals collected across a wide latitudinal gradient in North America. Samples collected at higher latitudes (Ontario, Canada and Minnesota, USA) exhibited a greater proportion (60%-96%) of nonlocal individuals, with an increased probability of origin from the southeastern United States. Populations from mid-latitudes (Florida, North Carolina, and South Carolina) showed a blend of local and nonlocal (40%-60%) individuals. Finally, 15% of the southernmost population individuals (Puerto Rico) were classified as migratory, with some having a probability of origin at higher latitudes (>30°). Overall, our results provide evidence of a northward spring/summer migration of H. zea in North America and underscore the significance of the southeastern United States as a hub for genetic flow. In addition, based on stable hydrogen isotopic ratios, there is strong evidence of reverse (southward) migration of H. zea from the continental United States to Puerto Rico. Our study highlights the implications for IPM and IRM programs and the need for management strategies that account for both northward and southward migration patterns.

Impact of Seed Blend and Structured Maize Refuge on Helicoverpa zea (Lepidoptera: Noctuidae) Potential Phenological Resistance Development Parameters in Pupae and Adults.

Helicoverpa zea, an economic pest in the southeastern U.S., has evolved practical resistance to Bt Cry toxins in maize and cotton. Insect resistance management (IRM) programs have historically required planting of structured non-Bt maize, but because of its low adoption, the use of seed blends has been considered. To generate knowledge on target pest biology and ecology to help improve IRM strategies, nine field trials were conducted in 2019 and 2020 in Florida, Georgia, North Carolina, and South Carolina to evaluate the impact of Bt (Cry1Ab + Cry1F or Cry1Ab + Cry1F + Vip3A) and non-Bt maize plants in blended and structured refuge treatments on H. zea pupal survival, weight, soil pupation depth, adult flight parameters, and adult time to eclosion. From a very large sample size and geography, we found a significant difference in pupal mortality and weight among treatments in seed blends with Vip3A, implying that cross-pollination occurred between Bt and non-Bt maize ears. There was no treatment effect for pupation depth, adult flight distance, and eclosion time. Results of this study demonstrate the potential impact of different refuge strategies on phenological development and survival of an important pest species of regulatory concern. This article is protected by copyright. All rights reserved.

How can policymakers and researchers develop effective insect resistance management guidelines? A quantitative and qualitative study of Brazilian farmers' perspectives and attitudes

Societal Impact StatementToday, over 80% of the US and Brazil row crop acreage has plants expressing insecticidal proteins to prevent the damage caused by caterpillars. These plants (crops expressing Bacillus thuringiensis, Bt, toxins) have brought several benefits to farmers, the environment, and society. However, these can be eroded when insects develop resistance to these toxins. Researchers and regulatory agencies have developed tactics that should be followed by farmers to avoid resistance but with limited efficacy. Our research provides recommendations for researchers and policymakers that are based on farmers' perspectives, thereby offering changes for current guidelines to successfully manage insect resistance and protect Bt crops' efficacy.Summary Genetically engineered crops expressing insecticidal proteins produced by Bacillus thuringiensis (Bt) have brought numerous benefits; however, pest resistance evolution poses a threat to the sustainability of this technology. Insect resistance management (IRM) for Bt crops has been defined as a wicked problem as it involves sociobiological complexities. A main challenge in IRM is the adoption of non‐Bt refuge, which is one out of the few strategies amenable to human intervention. This study investigated farmers' perspectives on information sources and IRM practices in Brazil using quantitative and qualitative data collection. A total of 145 farmers responded to online Qualtrics surveys, and 13 farmers participated in person to open‐ended interviews. This study demonstrates that farmers rely on strong social networks for information exchange and that sources with expertise based on local field experience are the most reliable channels of communication. We identified new challenges for refuge adoption such as the need to spray insecticides for pests not targeted by Bt and the intangible aspect of resistance evolution. Based on results of sources of information and perspectives on IRM practices, we discuss strategies that may be successful in delaying insecticide resistance evolution based on local contexts. This is the first study to investigate Brazilian farmers' perceptions on information sources and IRM strategies using qualitative data. Our results provide important elements to orient research development and decision‐making in biotechnology policies for the agricultural sector in Brazil and other similar contexts.

Monitoring, cross-resistance, inheritance, and fitness costs of brown planthoppers, Nilaparvata lugens, resistance to pymetrozine in China.

The brown planthopper, Nilaparvata lugens, is considered the most destructive pest of rice in many Asian countries including China. Use of pymetrozine in insect resistance management (IRM) has been one strategy to control this pest. In this study, we reported the status of pymetrozine resistance in Nilaparvata lugens (Stål) collected from China over the period 2017-2021 and selected a strain of N. lugens resistant to pymetrozine and evaluated the cross-resistance, inheritance and fitness costs of the resistance. Monitoring data (2017-2021) showed that field populations of N. lugens in China developed moderate- to high-level pymetrozine resistance during these 5 years. By continuous selection with pymetrozine in the lab, the pymetrozine selected N. lugens strain (Pym-R98 ) developed a 225.2-fold resistance compared to a susceptible strain. The Pym-R98 strain showed high cross-resistance to dinotefuran (66.6-fold) and low cross-resistance to nitenpyram (5.2-fold) and sulfoxaflor (5.8-fold). Inheritance pattern analysis of Pym-R93 revealed that resistance to pymetrozine was polygenic, autosomal and incompletely dominant. Fitness costs of pymetrozine resistance were present in Pym-R90 and WA2020 strains with a relative fitness of 0.72 and 0.60, respectively. The developmental duration of Pym-R90 and WA2020 was significantly longer and hatchability was significantly lower compared to pymetrozine-susceptible strain (Pym-S). N. lugens has developed high level of resistance to pymetrozine. Pymetrozine-resistance brown planthopper had cross-resistance with some of neonicotinoids such as dinotefuran, nitenpyram and sulfoxaflor. The autosomal, incompletely dominant and polygenic resistance to pymetrozine in N. lugens and the fitness costs associated with this resistance can be exploited in IRM strategies to preserve the lifetime of pymetrozine for control of N. lugens in China. © 2022 Society of Chemical Industry.

Insect resistance management facing African smallholder farmers under climate change.

Changes in climatic conditions affect pest populations and ultimately result in increased pest status and yield losses. While pesticide application is usually the first defensive tool used to control pest species that threaten crop production, genetically modified (GM) crops with insecticidal traits (Bt crops) are becoming more common. The indiscriminate and over use of insecticides, and absence of insect resistance management (IRM) strategies ultimately lead to evolution of resistance against these technologies. IRM faces significant challenges in the African context. In this paper we use examples of cotton, maize, cowpea and tomato pests to illustrate their potential to evolve resistance to insecticides and also highlight the importance of IRM strategies, both with regard to the use of pesticides and the cultivation of Bt cotton, Bt maize and Bt cowpea.

Comparative susceptibility of Anticarsia gemmatalis Hübner (Lepidoptera: Erebidae) and Chrysodeixis includens (Walker) (Lepidoptera: Noctuidae) to insecticides

ABSTRACT: Chrysodeixis includens (Walker) and Anticarsia gemmatalis Hübner are important soybean defoliators. The chemical control of A. gemmatalis has been easier and more effective than that of C. includens. It is hypothesized that C. includens is natural tolerant to insecticides. This study quantified and compare the susceptibility of C. includens and A. gemmatalis to the insecticides flubendiamide, methomyl, and spinetoram. A susceptible population of each species, maintained under laboratory conditions without insecticides selection pressure for more than 17 generations, was used. Ingestion bioassays using five to eight concentrations of each insecticide applied on the artificial diet surface were used to estimate the LC50 and LC99 (LC = Lethal Concentration). The tolerance ratio (TR) was calculated by dividing the LC50 or LC99 of the most tolerant species by the respective value of the most susceptible species. Chrysodeixis includens was more tolerant to all insecticides tested than A. gemmatalis, with TR50 values of 45.9-, 10.0- and 2.6-fold for methomyl, flubendiamide, and spinetoram. These findings indicated that the risk of evolution of resistance may be higher for C. includens than A. gemmatalis due to differential survival when exposed to the same dose of insecticide applied in soybean fields. Therefore, to improve the control of both species, integrated pest management (IPM) and insect resistance management (IRM) strategies should be considered to avoid control failures in field conditions.

Effective Use of Emamectin Benzoate for the Management of Spodoptera frugiperda (J. E. Smith) in Maize

Emamectin benzoate has been largely used for controling lepidopterans larvae in grains and fiber crops and, therefore, selection of resistant populations to this pesticide is a real risk if ignored the insect resistance management (IRM) strategies. We studied the susceptibility of Spodoptera frugiperda (Smith, 1797) populations in the state of Mato Grosso - Brazil to Emamectin benzoate as well as a field study about Mode of Action rotation including this compound. Low variations in susceptibility (resistance ratios, 1,01-7,31 fold) were detected among the evalueted S. frugiperda populations. The field study reinforced our monitorings outcome that Emamectin Benzoate controls S. frugiperda and its use in IRM in Mato Grosso is practicable. Susceptibility monitoring should be continuous in order to detect possible increases of tolerance rates not only for this pesticide but also for all compounds used for this pest management aiming to apply early mitigation actions.

Exposure to a sublethal dose of imidacloprid induces cellular and physiological changes in Periplaneta americana: Involvement of α2 nicotinic acetylcholine subunit in imidacloprid sensitivity

Neonicotinoids are the most important class of insecticides used as pest management tools during several decades. Exposition of insect to sublethal dose of insecticide induces physiological and cellular changes that could contribute to the adaptation of the insects in order to loss their sensitivity to insecticides. The aim of our study is to demonstrate that a subchronic exposure to a sublethal dose of a neonicotinoid imidacloprid is sufficient to induce molecular changes leading to a loss of imidacloprid sensitivity. We report that in the cockroach, Periplaneta americana, subchronic exposure to a sublethal dose of imidacloprid induced weak changes in detoxification enzyme activity and a significant decrease of the nicotinic acetylcholine α2 mRNA. This molecular effect is correlated to a decrease of imidacloprid sensitivity of cockroaches. Using RNA interference, we shown the key role of nicotinic acetylcholine α2 subunit in imidacloprid sensitivity. Thus, quantitative changes in insecticide targets lead to decreased sensitivity to insecticides. This parameter needs to be considered in order to develop sustainable insect resistance management strategies.

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.

Landscape-level variation in Bt crops predict Helicoverpa zea (Lepidoptera: Noctuidae) resistance in cotton agroecosystems.

Helicoverpa zea (Boddie) damage to Bt cotton and maize has increased as a result of widespread Bt resistance across the USA Cotton Belt. Our objective was to link Bt crop production patterns to cotton damage through a series of spatial and temporal surveys of commercial fields to understand how Bt crop production relates to greater than expected H.zea damage to Bt cotton. To do this, we assembled longitudinal cotton damage data that spanned the Bt adoption period, collected cotton damage data since Bt resistance has been detected, and estimated local population susceptibility using replicated on-farm studies that included all Bt pyramids marketed in cotton. Significant year effects of H.zea damage frequency in commercial cotton were observed throughout the Bt adoption period, with a recent damage increase after 2012. Landscape-level Bt crop production intensity over time was positively associated with the risk of H.zea damage in two- and three-toxin pyramided Bt cotton. Helicoverpa zea damage also varied across Bt toxin types in spatially replicated on-farm studies. Landscape-level predictors of H.zea damage in Bt cotton can be used to identify heightened Bt resistance risk areas and serves as a model to understand factors that drive pest resistance evolution to Bt toxins in the southeastern United States. These results provide a framework for more effective insect resistance management strategies to be used in combination with conventional pest management practices that improve Bt trait durability while minimizing the environmental footprint of row crop agriculture. © 2021 Society of Chemical Industry. This article has been contributed to by US Government employees and their work is in the public domain in the USA.

Distribution and diversity of alternate hosts of Maruca vitrata\xa0Fabricius in three West African countries

The evolution of resistance to the Bacillus thuringiensis (Bt) toxins by insect pests is a major threat to Bt technology. However, the rate of resistance can be slowed with appropriate integrated insect resistance management (IRM) strategies. Surveys were conducted to identify alternate host species for Maruca vitrata (commonly called the legume pod borer or Maruca) that could serve as refuges for Pod-Borer Resistant (PBR) cowpea in three West African countries (Ghana, Nigeria, and Burkina Faso). Survey sites included 25 in northern Ghana, 44 in northern Nigeria, and 52 in north-central and southwestern Burkina Faso. Alternate hosts of Maruca identified from plant species belonging to the Fabaceae family that showed signs of Maruca damage on cowpea tissues were collected and dissected. Larvae that were found during these dissections were reared to adult moths in the laboratory then identified to species. The alternate host plants including species of Crotolaria, Sesbania, Tephrosia, and Vigna were the most frequently encountered among sites and locations. Flowering and podding of these plants overlapped with flowering and podding of the nearby (~200 m) cowpea crop. Abundance of these wild hosts and overlapping flowering patterns with the cowpea crop in most locations have the potential to sustain ample numbers of Bt susceptible Maruca that will mate with possible resistant Maruca and deter resistance development. Further quantitative studies, however, are required from each location to determine if actual Maruca production from alternate hosts is sufficient for a PBR IRM strategy. If verified, this approach would be compatible with the high dose/refuge IRM strategy that includes alternate hosts and non-Bt crops as refuges.

Cotton Fleahopper1 Biology and Ecology Relevant to Development of Insect Resistance Management Strategies

Since the success of the U.S. Boll Weevil Eradication Program and widespread adoption of genetically modified crops, the cotton fleahopper, Pseudatomoscelis seriatus (Reuter), has reemerged as a significant cotton pest. Current management strategies for the pest are based primarily on foliar applications of insecticides. A line of cotton that produces modified Bt Cry51Aa2 protein was developed for Lygus spp., but the protein has also shown activity against the cotton fleahopper. Consequently, before this new Bt line can be released commercially, refuge requirements and other Insect Resistance Management (IRM) strategies mandated by the Environmental Protection Agency need to be established to prevent or delay development of resistance by cotton fleahopper to the new toxin. In response, we provide and discuss relevant information on the biology and ecology of the cotton fleahopper, including life history, host preference, dispersal, and population genetic structure, that may be useful for developing IRM strategies for the pest.

The implications of homozygous vip3Aa20- and cry1Ab-maize on Spodoptera frugiperda control

Historically, Bt maize hybrids have been produced with the Bt alleles in a hemizygous state. Homozygous Bt transgenes increase the expression of Bt proteins in plants; however, recent studies have only briefly explored the zygosity of Bt transgenes in maize hybrids and its effects on the control of lepidopteran pests. In this study, we investigated whether an additional allelic dose of the single Bt events Bt11 and MIR162, and a pyramided version (Bt11 + MIR162 + GA21) would impact Bt protein expression in leaves and increase the control of fall armyworm, Spodoptera frugiperda, in leaves and grains of near-isogenic maize hybrids. Our results revealed that homozygous Bt hybrids had higher Cry1Ab and Vip3Aa20 protein concentrations in leaves than their hemizygous versions. However, higher concentrations of Bt proteins in homozygous versions did not increase the control of S. frugiperda. Hybrids expressing Cry1Ab, hemizygous and homozygous for the transgene, had similar mortality of S. frugiperda fed on leaves and grains as the non-Bt near-isogenic version, likely due to resistance of the pest to Cry1Ab. High susceptibility to Vip3Aa20 caused all larvae to die on hemizygous and homozygous MIR162 leaves. Also, larvae that survived on Vip3Aa20 grains did not gain weight after four days of feeding, regardless of the Bt zygosity of Vip3Aa20-expressing grains. Additional implications of Bt zygosity in maize hybrids on insect resistance management strategies are discussed.

A Framework for Effective Bt Maize IRM Programs: Incorporation of Lessons Learned From Busseola fusca Resistance Development.

Bt maize is genetically engineered to express insecticidal proteins from the bacterium Bacillus thuringiensis. Bt maize is used extensively by South African farmers to reduce yield losses caused by lepidopteran larvae. Starting in the 2004/2005 season, severe Busseola fusca-associated damage to Cry1Ab-expressing Bt maize was noted by South African farmers. The unsatisfactory pest control was eventually attributed to the development of insect resistance to the Cry1Ab protein in the Bt maize hybrids. An assessment of the historical events surrounding the development of resistance by B. fusca showed that there was room for improvement both in the insect resistance management (IRM) strategy selected and the implementation of the strategy. With the recent arrival of fall armyworm (Spodoptera frugiperda) in Africa, it is important to have IRM programs that are appropriate for all of the pests that constitute the maize lepidopteran pest complex. After the identification of shortcomings in the IRM programs implemented in South Africa, a framework is proposed for effective Bt maize IRM programs. The IRM framework integrates pre-marketing research, post-marketing monitoring, and two-level remedial action plans (RAPs). The core of the framework is a regulator-approved IRM strategy that is based on comprehensive pre-marketing research and serves to guide stakeholders during the post-marketing phase. The framework will assist technology developers and regulators, especially those with nascent regulatory systems, to select and implement IRM strategies that facilitate sustainable pest management.

Inheritance of Bacillus thuringiensis Cry2Ab2 protein resistance in Helicoverpa zea (Lepidoptera: Noctuidae).

The corn earworm, Helicoverpa zea (Boddie), is a major target pest of pyramided Bt corn and cotton in the United States. Field-evolved practical resistance to Cry1 and Cry2 proteins in H. zea has been documented in multiple locations in the United States. Understanding the genetic basis of Bt resistance is essential in developing insect resistance management (IRM) strategies for the sustainable use of the Bt crop technology. In this study, we characterized the genetic bases of Cry2Ab2 resistance in H. zea using diet-overlay bioassays with two different forms of Cry2Ab2 protein. Laboratory bioassays using a Cry2Ab2-resistant (RR) strain, a susceptible (SS) strain, as well as cross and backcross strains, revealed that resistance to Cry2Ab2 was autosomally inherited and controlled by more than one locus. In diet bioassays, the dominance of Cry2Ab2 resistance in H. zea varied from incompletely recessive to incompletely dominant across all tested Cry2Ab2 concentrations of either Bt corn leaf powder or solubilized protein. On leaf tissue of TwinLink cotton (expressing Cry1Ab and Cry2Ae), Cry2Ab2 resistance in H. zea was completely dominant. These results have significant implications for understanding the widespread field-evolved resistance of H. zea against Cry1 and Cry2 proteins in Bt corn and cotton and should be useful in developing effective IRM strategies for H. zea. © 2020 Society of Chemical Industry.

Resistance of Spodoptera frugiperda (Lepidoptera: Noctuidae) to spinetoram: inheritance and cross-resistance to spinosad.

The use of spinosyn insecticides is one of the major control strategies of the fall armyworm, Spodoptera frugiperda (J. E. Smith) in Brazil. In this study, we selected a spinetoram-resistant strain from a field-population of S. frugiperda to characterize the inheritance of the resistance and cross-resistance relationship between spinosyn insecticides. The values of LC50 (95% CI) obtained from concentration-response bioassays were 0.63 (0.55-0.73) μg spinetoram mL-1 for the susceptible strain (SUS), and 1170.96 (1041.61-1323.89) μg spinetoram mL-1 for the strain resistant to spinetoram (SPT-R). These values resulted in a resistance ratio of 1844-fold. The SPT-R strain showed cross-resistance with spinosad (resistance ratio = 1196-fold). The reciprocal crosses showed LC50 values of 3.91 (2.97-5.84) and 5.37 (4.52-6.52) μg spinetoram mL-1 , suggesting that the resistance of S. frugiperda to spinetoram is autosomal and incompletely recessive. The backcrosses of the F1 progeny with the SPT-R strain suggest a resistance with polygenic effect. Estimates of the effective number of loci with equal contributions to the resistance effect were from 1.18 to 1.76, suggesting that resistance to spinetoram is associated with a few genes. The inheritance pattern of resistance of S. frugiperda to spinetoram was characterized as autosomal, incompletely recessive and polygenic. Cross-resistance between spinosyns was confirmed in S. frugiperda. The importance of this information for implementing insect resistance management strategies is discussed in this paper. © 2020 Society of Chemical Industry.

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

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

Bigger, Faster, Stronger: Implications of Inter-Species Interactions for IRM of Lepidopteran Pests of Bt Maize in Africa

Abstract A hypothetical scenario of mixed populations of Busseola fusca (Fuller) (Lepidoptera: Noctuidae), Chilo partellus (Swinhoe) (Lepidoptera: Crambidae), and Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) was used as a model to investigate the potential effects of mixed populations of lepidopteran pests, on the design and implementation of insect resistance management (IRM) strategies for Bt maize (L.) (Poaceae) on smallholder farms in Africa. To predict the structure of such mixed populations in different agroecological zones, the biological and behavioral characteristics that affect the competitiveness of these species were identified and analyzed. Additionally, the validity of the assumptions that underlie the high-dose/refuge strategy was compared among the three species. Differences between the species, and the influence thereof on the choice of IRM strategy for a specific environment, were explored through analysis of three hypothetical scenarios. We suggest that the use of separate refuges as a component of an IRM strategy against mixed pest populations in smallholder Bt maize fields may be unwise. A seed mixture approach, coupled with an effective integrated pest management (IPM) strategy, would be more practical and sensible since it could limit the opportunity for a single species to dominate the species complex. The dynamic interactions in a multi-species community and domination of the species complex by a single species may influence moth and larval response to maize plants, which could lead to an increased infestation of Bt plants, and subsequent increased selection pressure for resistance evolution. This article provides insights into the unique challenges that face the deployment 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.