Multigenerational migration of fall armyworm, a pest insect

Seasonal migration pattern of an important rice pest, Nilaparvata lugens (Hemiptera: Delphacidae), on Hainan Island, China

The brown planthopper, Nilaparvata lugens (Hemiptera: Delphacidae) (BPH), is a major, regionally migratory pest of rice in Asia. Despite intensive migratory studies, the seasonal migratory pattern of this species in the year-round breeding region (i.e., Hainan Island) remains largely unknown. To understand the migration dynamics, we conducted relevant research based on BPH light trap catches on Hainan in 2017-2022. Results showed that the occurrence dynamics of BPH in Hainan oscillated in different years and seasons. Overall, there are 4 migration peak periods on Hainan, with outbound peak periods in April-May and August-September and inbound peak periods in June and October. Trajectory and wind fields showed that in August-September, Hainan had complex meteorological conditions, and the outbound paths of BPH were variable, mainly landing in southeastern Guangxi and southwestern Guangdong. In contrast, April-May was influenced by southeasterly winds, with a wider outbound range and a further distance, landing along northeastern Vietnam from west to east toward Guangxi and Guangdong areas with time. Wind-rain fields, trajectory, and hydrogen isotope showed that in June, southwesterly winds transported BPH northward from the Indochina peninsula, where it was hindered by rainfall in the Leizhou Peninsula area and the migrants will be "rained out" into Hainan. Identically, in October, the interactions between northeasterly winds and rainfall belts on the southwest shore of Hainan increased the opportunities for BPH to migrate southward to reach Hainan. Our results suggest that Hainan is not only an important source of BPH migrating from East Asia but also plays the role of a bridge.

Phenological Mapping of Invasive Insects: Decision Support for Surveillance and Management

Readily accessible and easily understood forecasts of the phenology of invasive insects have the potential to support and improve strategic and tactical decisions for insect surveillance and management. However, most phenological modeling tools developed to date are site-based, meaning that they use data from a weather station to produce forecasts for that single site. Spatial forecasts of phenology, or phenological maps, are more useful for decision-making at area-wide scales, such as counties, states, or entire nations. In this review, we provide a brief history on the development of phenological mapping technologies with a focus on degree-day models and their use as decision support tools for invasive insect species. We compare three different types of phenological maps and provide examples using outputs of web-based platforms that are presently available for real-time mapping of invasive insects for the contiguous United States. Next, we summarize sources of climate data available for real-time mapping, applications of phenological maps, strategies for balancing model complexity and simplicity, data sources and methods for validating spatial phenology models, and potential sources of model error and uncertainty. Lastly, we make suggestions for future research that may improve the quality and utility of phenological maps for invasive insects.

Exposure to Cry1 Toxins Increases Long Flight Tendency in Susceptible but Not in Cry1F-Resistant Female Spodoptera frugiperda (Lepidoptera: Noctuidae)

The fall armyworm (JE Smith) (Spodoptera frugiperda) is a polyphagous pest targeted by selected Cry and Vip3A insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) that are produced in transgenic Bt corn and cotton. Available evidence suggests that sublethal larval exposure to Cry1Ac increases flight activity in adult Spodoptera spp. However, it is not known whether this effect is also observed in survivors from generally lethal exposure to Cry1Ac. Moreover, while multiple cases of field-evolved resistance to Bt proteins have been described in the native range of S. frugiperda, the effect of resistance on flight behavior has not been examined. Long-distance migratory flight capacity of S. frugiperda is of concern given its ongoing global spread and the possibility that migrants may be carrying resistance alleles against pesticides and Bt crops. In this study, we used rotational flight mills to test the effects of generally lethal exposure to Cry1Ac in susceptible and sublethal exposure in Cry1F-resistant S. frugiperda strains. The results detected altered pupal weight after larval feeding on diet containing Cry proteins, which only translated in significantly increased tendency for longer flights in female moths from the susceptible strain. This information has relevant implications when considering current models and assumptions for resistance management of Bt crops.

Adaptive migratory orientation of an invasive pest on a new continent

Many species of insects undertake long-range, seasonally reversed migrations, displaying sophisticated orientation behaviors to optimize their migratory trajectories. However, when invasive insects arrive in new biogeographical regions, it is unclear if migrants retain (or how quickly they regain) ancestral migratory traits, such as seasonally preferred flight headings. Here we present behavioral evidence that an invasive migratory pest, the fall armyworm moth (Spodoptera frugiperda), a native of the Americas, exhibited locally adaptive migratory orientation less than three years after arriving on a new continent. Specimens collected from China showed flight orientations directed north-northwest in spring and southwest in autumn, and this would promote seasonal forward and return migrations in East Asia. We also show that the driver of the seasonal switch in orientation direction is photoperiod. Our results thus provide a clear example of an invasive insect that has rapidly exhibited adaptive migratory behaviors, either inherited or newly evolved, in a completely alien environment.

Using genetic comparisons of populations from Arizona, Mexico, and Texas to investigate fall armyworm migration in the American southwest

Fall armyworm (FAW) is a global agricultural pest, causing substantial economic losses in corn and many other crops. Complicating efforts to control this pest is its capacity for long distance flights, which has been described in greatest detail for the central and eastern sections of the United States. FAW infestations are also routinely found in agricultural areas in southern Arizona, which lie beyond the western limits of the mapped migratory pathways. Climate suitability analysis found that the affected Arizona locations cannot support permanent FAW populations, indicating that these FAW most likely arise from annual migrations. A better understanding of this migration would provide insights into how large moth populations can move across desert habitats as well as the degree of gene flow occurring between FAW populations across the North American continent. In this study the Arizona populations were genetically characterized and compared to a selection of permanent and migratory FAW from multiple sites in the United States and Mexico. The results are consistent with migratory contributions from permanent populations in the states of Texas (United States) and Sinaloa (Mexico), while also providing evidence of significant barriers to gene flow between populations within Mexico. An unexpected finding was that two genetically distinct FAW subpopulations known as "host strains" have a differential distribution in the southwest that may indicate significant differences in their migration behavior in this region. These findings indicate that the combination of mitochondrial and Z-linked markers have advantages in comparing FAW populations that can complement and extend the findings from other methods.

Prediction of the Overseas Migration of the Fall Armyworm, Spodoptera frugiperda, to Japan

(1) Background: The fall armyworm, Spodoptera frugiperda, is an invasive migratory insect pest that first arrived in Japan in early July 2019. Since then, the species has immigrated to Japan mainly in the summer monsoon season and inflicted damage mainly on the maize used as animal feed in the western region, where major immigrations occur. In this study, to know the precise arrival timing and area of S. frugiperda for purposes of pest management, a prediction method for its overseas migration from neighboring source areas was developed. (2) Methods: The method uses the Weather Research and Forecast model to give numerical weather predictions and the GEARN-insect model to predict migration. Emigration source areas on the Chinese mainland and the island of Taiwan and the insect's take-off and flight behaviors were input to the GEARN-insect model to calculate the daily migration prediction figures. (3) Results: In a prediction evaluation using 2-year six-point trapping data in Japan, the prediction method achieved an average hitting ratio of 78%. (4) Conclusions: The method has sufficient prediction quality for operational use. The technique may be applicable to other migratory moths immigrating to Japan, such as the oriental armyworm, Mythimna separata.

Virulence of Beauveria sp. and Metarhizium sp. fungi towards fall armyworm (Spodoptera frugiperda)

The development of effective pest management strategies for Spodoptera frugiperda is a high priority for crop protection across its invasive ranges. Here, we examined six Beauveria and five Metarhizium fungal isolates against this pest. Two Beauveria isolates (B-0571, B-1311) induced high mortality toward 3rd and 6th instar caterpillars and adults. For B-0571 mortality was 82.81 ± 5.75%, 61.46 ± 6.83%, and 93.75 ± 3.61%, and 73.72 ± 2.51%, 71.88 ± 5.41%, and 97.92 ± 2.08% for B-1311, with deaths in caterpillars largely occurring under 24 h (3rd instar control 0.74 ± 0.33%, B-0571 73.96 ± 7.85% and B-1311 62.08 ± 3.67%; 6th instar control 0%, B-0571 66.67% ± 11.02% and B-1311 62.5% ± 9.55%). Infection from both Beauveria isolates fully prevented reproduction in surviving S. frugiperda females. In contrast, all five Metarhizium isolates tested and the remaining four Beauveria isolates exhibited lower virulence. The discovery of two highly virulent Beauveria fungal isolates to S. frugiperda opens avenues to develop novel biological control tools against this highly invasive pest.

Investigating the Migratory Behavior of Soybean Looper, a Major Pest of Soybean, through Comparisons with the Corn Pest Fall Armyworm Using Mitochondrial Haplotypes and a Sex-Linked Marker

The Noctuid moth soybean looper (SBL), Chrysodeixis includens (Walker), is an economically important pest of soybean (Glycine max (Linnaeus) Merrill). Because it is not known to survive freezing winters, permanent populations in the United States are believed to be limited to the southern regions of Texas and Florida, yet its geographical range of infestations annually extend to Canada. This indicates annual migrations of thousands of kilometers during the spring and summer growing season. This behavior is like that of the fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith), also a Noctuid that is a major global pest of corn. SBL and FAW are projected to have very similar distributions of permanent populations in North America based on climate suitability modeling and the overlap in the distribution of their preferred host plants (corn and soybean). It therefore seems likely that the two species will display similar migratory behavior in the United States. This was tested by identifying genetic markers in SBL analogous to those successfully used to delineate FAW migratory pathways and comparing the distribution patterns of the markers from the two species. Contrary to expectations, the results indicate substantial differences in migratory behavior that appear to be related to differences in the timing of corn and soybean plantings. These findings underscore the importance of agricultural practices in influencing pest migration patterns, in particular the timing of host availability relative to mean seasonal air transport patterns.

Screening for resistance alleles to Cry1 proteins through targeted sequencing in the native and invasive range of Spodoptera frugiperda (Lepidoptera: Noctuidae)

The fall armyworm, Spodoptera frugiperda (J. E. Smith), is a highly polyphagous pest native to the tropical Americas that has recently spread to become a global super-pest threatening food and fiber production. Transgenic crops producing insecticidal Cry and Vip3Aa proteins from Bacillus thuringiensis (Bt) are used for control of this pest in its native range. The evolution of practical resistance represents the greatest threat to sustainability of this technology and its potential efficacy in the S. frugiperda invasive range. Monitoring for resistance is vital to management approaches delaying S. frugiperda resistance to Bt crops. DNA-based resistance screening provides higher sensitivity and cost-effectiveness than currently used bioassay-based monitoring. So far, practical S. frugiperda resistance to Bt corn-producing Cry1F has been genetically linked to mutations in the SfABCC2 gene, providing a model to develop and test monitoring tools. In this study, we performed targeted SfABCC2 sequencing followed by Sanger sequencing to confirm the detection of known and candidate resistance alleles to Cry1F corn in field-collected S. frugiperda from continental USA, Puerto Rico, Africa (Ghana, Togo, and South Africa), and Southeast Asia (Myanmar). Results confirm that the distribution of a previously characterized resistance allele (SfABCC2mut) is limited to Puerto Rico and identify 2 new candidate SfABCC2 alleles for resistance to Cry1F, one of them potentially spreading along the S. frugiperda migratory route in North America. No candidate resistance alleles were found in samples from the invasive S. frugiperda range. These results provide support for the potential use of targeted sequencing in Bt resistance monitoring programs.

Revisiting fall armyworm population movement in the United States and Canada

Introduction

Biophysical approaches validated against haplotype and trap catch patterns have modeled the migratory trajectory of fall armyworms at a semi-continental scale, from their natal origins in Texas or Florida through much of the United States east of the Rocky Mountains. However, unexplained variation in the validation analysis was present, and misalignments between the simulated movement patterns of fall armyworm populations and the haplotype ratios at several locations, especially in the northeastern US and Canada, have been reported.

Methods

Using an expanded dataset extending into Canada, we assess the consistency of haplotype patterns that relate overwintered origins of fall armyworm populations to hypothesized dispersal trajectories in North America and compare the geographic distribution of these patterns with previous model projections.

Results and discussion

We confirm the general accuracy of previous modeling efforts, except for late in the season where our data suggests a higher proportion of Texas populations invading the northeast, extending into eastern Canada. We delineate geographic limits to the range of both overwintering populations and show that substantial intermixing of the Texas and Florida migrants routinely occurs north of South Carolina. We discuss annual variation to these migratory trajectories and test the hypothesis that the Appalachian Mountains influence geographic patterns of haplotypes. We discuss how these results may limit gene flow between the Texas and Florida natal populations and limit the hereditary consequences of interbreeding between these populations.

Spodoptera frugiperda: Ecology, Evolution, and Management Options of an Invasive Species

The fall armyworm (FAW), Spodoptera frugiperda (Lepidoptera, Noctuidae), is a well-known agricultural pest in its native range, North and South America, and has become a major invasive pest around the globe in the past decade. In this review, we provide an overview to update what is known about S. frugiperda in its native geographic ranges. This is followed by discussion of studies from the invaded areas to gain insights into S. frugiperda's ecology, specifically its reproductive biology, host plant use, status of insecticide resistance alleles, and biocontrol methods in native and invasive regions. We show that reference to host strains is uninformative in the invasive populations because multidirectional introduction events likely underpinned its recent rapid spread. Given that recent genomic analyses show that FAW is much more diverse than was previously assumed, and natural selection forces likely differ geographically, region-specific approaches will be needed to control this global pest.

Migration risk of fall armyworm (Spodoptera frugiperda) from North Africa to Southern Europe

With the development of globalization and agriculture trade, as well as its own strong migratory capacity, fall armyworm (FAW) (Spodoptera frugiperda) (J.E. Smith) has invaded more than 70 countries, posing a serious threat to the production of major crops in these areas. FAW has now also been detected in Egypt in North Africa, putting Europe, which is separated from it only by the Mediterranean Sea, at high risk of invasion. Therefore, this study integrated multiple factors of insect source, host plant, and environment to provide a risk analysis of the potential trajectories and time periods of migration of FAW into Europe in 2016~2022. First, the CLIMEX model was used to predict the annual and seasonal suitable distribution of FAW. The HYSPLIT numerical trajectory model was then used to simulate the possibility of the FAW invasion of Europe through wind-driven dispersal. The results showed that the risk of FAW invasion between years was highly consistent (P<0.001). Coastal areas were most suitable for the expansion of the FAW, and Spain and Italy had the highest risk of invasion, with 39.08% and 32.20% of effective landing points respectively. Dynamic migration prediction based on spatio-temporal data can enable early warning of FAW, which is important for joint multinational pest management and crop protection.

eCry1Gb.1Ig, A Novel Chimeric Cry Protein with High Efficacy against Multiple Fall Armyworm (Spodoptera frugiperda) Strains Resistant to Different GM Traits

Spodoptera frugiperda (fall armyworm, FAW) is one of the most devastating insect pests to corn and soybean production in the Americas and is rapidly expanding its range worldwide. It is known to be hard to control either by chemical insecticide applications or by GM. Although the use of GM traits can be an effective way to control this pest, it is very rare to find native insecticidal proteins that provide the necessary level of FAW control in crop fields where FAW pressure and damage are high. Insecticidal Cry proteins sourced from Bacillus thuringiensis have been heavily utilized in the development of crops with GM traits; however, it is increasingly difficult to identify Cry proteins with unique modes of action. Protein engineering via a phylogenetically guided Cry protein domain swapping approach enabled us to discover novel chimeric Cry proteins engineered from inactive parent sequences. Some of these chimeras show excellent efficacy against key biotypes of FAW from Brazil and North America. In this study, we characterized a Cry-based chimera eCry1Gb.1Ig that is a very potent FAW toxin. eCry1Gb.1Ig showed high efficacy against multiple FAW strains that are resistant to various traits, including Cry1Fa, Vip3Aa and Cry1A.105/Cry2Ab. These results clearly indicate that the FAW strains resistant to Cry1Fa, Vip3Aa or Cry1A.105/Cry2Ab demonstrate no cross-resistance to eCry1Gb.1Ig and strongly suggest that eCry1Gb.1Ig acts through a novel mode of action compared to the existing traits. In addition to its FAW activity, eCry1Gb.1Ig has also been shown to control Chrysodeixis includens (soybean looper, SBL) and Anticarsia gemmatalis (velvetbean caterpillar, VBC), which are significant pests of soybean. When eCry1Gb.1Ig was introduced into corn and soybean crops, transgenic events showed strong efficacy against FAW, SBL and VBC, but no adverse plant phenotypes. This suggests that the in planta expression of the eCry1Gb.1Ig protein does not compromise plant growth or reproduction and can protect plants from FAW-related damage. Therefore, this valuable discovery will provide a differentiating FAW control trait that will give growers another tool to help them reduce yield loss due to FAW.

Observations of genetic differentiation between the fall armyworm host strains

The threat of invasive species is increasing with the expansion of global trade and habitat disruption. A recent example is the establishment of fall armyworm (FAW), a noctuid moth native to the Americas, into most of the Eastern Hemisphere with projections of significant economic losses on a global scale. The species has traditionally been subdivided into two populations that differ in their propensity to use different plant hosts, a phenotype with clear relevance for identifying crops at risk. However, inconsistencies in the genetic and phenotypic descriptions of these “host strains” has led to controversy about their composition and even existence. In this study, the locus for the Triosephosphate isomerase gene (Tpi) is used both as a host strain marker and for phylogenetic analysis. Association of the host choice phenotype with the Tpi-derived phylogenetic tree uncovered genetic differentiation between populations that supports the existence of the host strains and provided evidence that they are subject to different selection pressures. This correspondence of differential host use with Tpi was demonstrated for populations from a broad geographical range and supports the involvement of one or more Z-chromosome functions controlling the phenotype. Comparisons of collections from multiple locations identified significant differences in the efficacy of different molecular markers that implicate regional variations in host strain behavior.

Large genomic deletion linked to field-evolved resistance to Cry1F corn in fall armyworm (Spodoptera frugiperda) from Florida

The fall armyworm (Spodoptera frugiperda) is a highly polyphagous lepidopteran pest of relevant food and fiber staple crops. In the Americas, transgenic corn and cotton producing insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) have controlled and reduced the damage caused by S. frugiperda. However, cases of field-evolved S. frugiperda resistance to Bt corn producing the Cry1F insecticidal protein have been documented in North and South America. When characterized, field resistance to Cry1F is linked to insertions and mutations resulting in a modified or truncated ABC transporter subfamily C2 (SfABCC2) protein that serves as Cry1F receptor in susceptible S. frugiperda. In this work, we present detection of a large genomic deletion (~ 8 kb) affecting the SfABCC2 and an ABC transporter gene subfamily 3 –like gene (SfABCC3) as linked to resistance to Cry1F corn in a S. frugiperda strain from Florida (FL39). Monitoring for this genomic deletion using a discriminatory PCR reaction in field-collected S. frugiperda moths detected individuals carrying this allele in Florida, but not in surrounding states. This is the first report of a large genomic deletion being involved in resistance to a Bt insecticidal protein.

Population Source of Third-Generation Oriental Armyworm in Jilin, China, Determined by Entomology Radar, Trajectory Analysis, and Mitochondrial COI Sequences

The armyworm, Mythimna separata (Walker) (Lepidoptera: Noctuidae), is an important polyphagous pest with a strong migratory ability. Recently, third-generation larvae have become an increasingly serious pest threat in Jilin Province of northeast China. To investigate the population source of this species, scanning entomological radar observations and insect mitochondrial cytochrome oxidase I (COI) genes were used in this study. Five main results were found: (1) The peak period in captured second-generation moths was from mid to late July. The temperature and wind speeds were optimum for the moths to have migrated. Strong southwesterly winds occurred during the peak migration period. (2) Radar observations indicated that most of the moths' migration took place at a height of 600 m, often in a dense layer which formed at heights of 350-800 m. (3) Analyses of adult ovarian development and larval haplotypes showed third-generation larvae were progeny of both locally produced progeny and immigrant moths. (4) Based on our back-tracking and haplotype analyses, immigration led to an outbreak originated in the same source area to the southwest. (5) Emigration of second-generation moths was confirmed by both radar observation and mtDNA analysis. Forward trajectories indicated that the moths were capable of immigrating far from their overwintering range. These results are useful for improving the forecasting systems of this insect pest species.

Universal Mitochondrial Multi-Locus Sequence Analysis (mtMLSA) to Characterise Populations of Unanticipated Plant Pest Biosecurity Detections

Simple Summary

Agricultural and environmental sustainability requires effective biosecurity responses that prevent the establishment or spread of exotic insect pests. Understanding where new detections may have come from or if recurrent detections are connected contributes to this. Suitable population genetic markers use relatively rapidly evolving gene regions which render the PCR method species-specific at best. Because resource limitations mean these are pre-emptively developed for the highest risk species, populations of other exotic pests are unable to be characterised at the time. Here we have developed a generic method that is useful across species within the same taxonomic Order, including where there is little or no prior knowledge of their gene sequences. Markers are formed by concomitant sequencing of four gene regions. Sequence concatenation was shown to retrieve higher resolution signatures than standard DNA barcoding. The method is encouragingly universal, as illustrated across species in ten fly and 11 moth superfamilies. Although as-yet untested in a biosecurity situation, this relatively low-tech, off-the-shelf method makes a proactive contribution to the toolbox of quarantine agencies at the time of detection without the need for impromptu species-specific research and development.

Abstract

Biosecurity responses to post-border exotic pest detections are more effective with knowledge of where the species may have originated from or if recurrent detections are connected. Population genetic markers for this are typically species-specific and not available in advance for any but the highest risk species, leaving other less anticipated species difficult to assess at the time. Here, new degenerate PCR primer sets are designed for within the Lepidoptera and Diptera for the 3′ COI, ND3, ND6, and 3′ plus 5′ 16S gene regions. These are shown to be universal at the ordinal level amongst species of 14 and 15 families across 10 and 11 dipteran and lepidopteran superfamilies, respectively. Sequencing the ND3 amplicons as an example of all the loci confirmed detection of population-level variation. This supported finding multiple population haplotypes from the publicly available sequences. Concatenation of the sequences also confirmed that higher population resolution is achieved than for the individual genes. Although as-yet untested in a biosecurity situation, this method is a relatively simple, off-the-shelf means to characterise populations. This makes a proactive contribution to the toolbox of quarantine agencies at the time of detection without the need for unprepared species-specific research and development.

Global population genomic signature of Spodoptera frugiperda (fall armyworm) supports complex introduction events across the Old World

Native to the Americas, the invasive Spodoptera frugiperda (fall armyworm; FAW) was reported in West Africa in 2016, followed by its chronological detection across the Old World and the hypothesis of an eastward Asia expansion. We explored population genomic signatures of American and Old World FAW and identified 12 maternal mitochondrial DNA genome lineages across the invasive range. 870 high-quality nuclear single nucleotide polymorphic DNA markers identified five distinct New World population clusters, broadly reflecting FAW native geographical ranges and the absence of host-plant preferences. We identified unique admixed Old World populations, and admixed and non-admixed Asian FAW individuals, all of which suggested multiple introductions underpinning the pest’s global spread. Directional gene flow from the East into eastern Africa was also detected, in contrast to the west-to-east spread hypothesis. Our study demonstrated the potential of population genomic approaches via international partnership to address global emerging pest threats and biosecurity challenges.

This population genomics study identifies the complex multiple introduction history of Spodoptera frugiperda (fall armyworm) from the Americas, into Africa and Asia. This provides new insight into the ‘east-to-west’ directionality of gene flow, and suggests ample genomic exchange at the nuclear level.

Insecticide resistance in Australian Spodoptera frugiperda (J.E. Smith) and development of testing procedures for resistance surveillance

Spodoptera frugiperda (J.E. Smith) is a highly invasive noctuid pest first reported in northern Australia during early 2020. To document current status of resistance in S. frugiperda in Australia, insecticide toxicity was tested in field populations collected during the first year of establishment, between March 2020 and March 2021. Dose-response was measured by larval bioassay in 11 populations of S. frugiperda and a susceptible laboratory strain of Helicoverpa armigera. Emamectin benzoate was the most efficacious insecticide (LC50 0.023μg/ml) followed by chlorantraniliprole (LC50 0.055μg/ml), spinetoram (LC50 0.098μg/ml), spinosad (LC50 0.526μg/ml), and methoxyfenozide (1.413μg/ml). Indoxacarb was the least toxic selective insecticide on S. frugiperda (LC50 3.789μg/ml). Emamectin benzoate, chlorantraniliprole and methoxyfenozide were 2- to 7-fold less toxic on S. frugiperda compared with H. armigera while spinosyns were equally toxic on both species. Indoxacarb was 28-fold less toxic on S. frugiperda compared with H. armigera. There was decreased sensitivity to Group 1 insecticides and synthetic pyrethroids in S. frugiperda compared with H. armigera: toxicity was reduced up to 11-fold for methomyl, 56 to 199-fold for cyhalothrin, and 44 to 132-fold for alpha cypermethrin. Synergism bioassays with metabolic inhibitors suggest involvement of mixed function oxidase in pyrethroid resistance. Recommended diagnostic doses for emamectin benzoate, chlorantraniliprole, spinetoram, spinosad, methoxyfenozide and indoxacarb are 0.19, 1.0, 0.75, 6, 12 and 48μg/μl, respectively.

Genetic studies of fall armyworm indicate a new introduction into Africa and identify limits to its migratory behavior

The fall armyworm, Spodoptera frugiperda (J.E. Smith) is native to the Americas and a major pest of corn and several other crops of economic importance. The species has characteristics that make it of particular concern as an invasive pest, including broad host range, long-distance migration behavior, and a propensity for field-evolved pesticide resistance. The discovery of fall armyworm in western Africa in 2016 was followed by what was apparently a remarkably rapid spread throughout sub-Saharan Africa by 2018, causing economic damage estimated in the tens of billions USD and threatening the food security of the continent. Understanding the history of the fall armyworm invasion of Africa and the genetic composition of the African populations is critical to assessing the risk posed to different crop types, the development of effective mitigation strategies, and to make Africa less vulnerable to future invasions of migratory moth pests. This paper tested and expanded on previous studies by combining data from 22 sub-Saharan nations during the period from 2016 to 2019. The results support initial descriptions of the fall armyworm invasion, including the near absence of the strain that prefers rice, millet, and pasture grasses, while providing additional evidence that the magnitude and extent of FAW natural migration on the continent is more limited than expected. The results also show that a second entry of fall armyworm likely occurred in western Africa from a source different than that of the original introduction. These findings indicate that western Africa continues to be at high risk of future introductions of FAW, which could complicate mitigation efforts.

Beneficial rhizobacteria inoculation on Ocimum basilicum reduces the growth performance and nutritional value of Spodoptera frugiperda

BACKGROUND

Plant growth-promoting rhizobacteria (PGPR) has a significant role in plant-insect interaction. However, the extent of their impact on insects is still not well understood. This investigation was designed to evaluate the role of inoculation with Bacillus amyloliquefaciens GB03 on sweet basil (Ocimum basilucum L.) in the development and nutritional parameters of Spodoptera frugiperda. In addition, the feeding preferences on inoculated and non-inoculated plants were assessed.

RESULTS

Spodoptera frugiperda larvae reared with inoculated sweet basil leaves had a strong negative effect on the development of the insect, resulting in lower larval and pupal weights, and a longer period for larval-adult development. Moreover, adult emergence was reduced, but the relative consumption rate (RCR) value was unaffected, thereby revealing no alteration of the palatability. Growth rate and nutritional indicators, such as the efficiency of conversion of ingested food (ECI) and the efficiency of conversion of digested food (ECD), were reduced in larvae reared from treated plants. In the choice test, larvae avoided feeding on inoculated leaves.

CONCLUSION

The higher occurrence of secondary metabolites in inoculated plants could have been the reason for the reduction of the plant nutritional rate and also for the food selection, since it has been previously reported that GB03 inoculated sweet basil increased the essential oil yield. Therefore, PGPR inoculation could be used as a growth promoter, making it a promising candidate for plant protection programs against insects in aromatic plant production. © 2021 Society of Chemical Industry.

Genetic Screening to Identify Candidate Resistance Alleles to Cry1F Corn in Fall Armyworm Using Targeted Sequencing

Simple Summary

Monitoring of resistance alleles is critical to the sustainability of transgenic crops producing insecticidal Cry proteins. Highly sensitive and cost-effective DNA-based methods are needed to improve current bioassay-based resistance screening. Our goal was to evaluate the use of targeted sequencing in detecting known and novel candidate resistance alleles to Cry proteins. As a model, we used field-collected fall armyworm (Spodoptera frugiperda) from Puerto Rico, the first location reporting continued practical field-evolved S. frugiperda resistance to corn producing the Cry1F insecticidal protein, and sequenced the SfABCC2 gene previously identified as critical to Cry1F toxicity. Targeted sequencing of SfABCC2 detected a previously reported Cry1F resistance allele and mutations originally identified in populations from Brazil. Importantly, targeted sequencing also identified nonsynonymous and frameshift mutations as novel candidate resistance alleles. These results advocate for the use of targeted sequencing in screening for resistance alleles to Cry proteins and support potential gene flow, including resistance alleles, between S. frugiperda from Brazil and the Caribbean.

Abstract

Evolution of practical resistance is the main threat to the sustainability of transgenic crops producing insecticidal proteins from Bacillus thuringiensis (Bt crops). Monitoring of resistance to Cry and Vip3A proteins produced by Bt crops is critical to mitigate the development of resistance. Currently, Cry/Vip3A resistance allele monitoring is based on bioassays with larvae from inbreeding field-collected moths. As an alternative, DNA-based monitoring tools should increase sensitivity and reduce overall costs compared to bioassay-based screening methods. Here, we evaluated targeted sequencing as a method allowing detection of known and novel candidate resistance alleles to Cry proteins. As a model, we sequenced a Cry1F receptor gene (SfABCC2) in fall armyworm (Spodoptera frugiperda) moths from Puerto Rico, a location reporting continued practical field resistance to Cry1F-producing corn. Targeted sequencing detected a previously reported Cry1F resistance allele (SfABCC2mut), in addition to a resistance allele originally described in S. frugiperda populations from Brazil. Moreover, targeted sequencing detected mutations in SfABCC2 as novel candidate resistance alleles. These results support further development of targeted sequencing for monitoring resistance to Bt crops and provide unexpected evidence for common resistance alleles in S. frugiperda from Brazil and Puerto Rico.

Ambient Humidity Affects Development, Survival, and Reproduction of the Invasive Fall Armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae), in China

Ambient humidity can directly affect the water balance in insects. The migratory fall armyworm, Spodoptera frugiperda Smith, has spread to more than 60 countries and regions in Africa, Asia, and Oceania that have a great difference in average ambient humidity. Understanding the effects of ambient humidity changes on its development, survival, and reproduction can help to predict its population dynamics in different habitats. Therefore, we evaluated the effects of atmospheric relative humidity (RH) on the development, survival, and reproduction and soil moisture on the pupation and emergence of fall armyworm. As a result, survival and pupal mass increased significantly with increasing RH. Among the five RHs tested, 80% RH was the most suitable for fall armyworm with the highest intrinsic rate of increase (r), finite rate of increase (λ), and net reproduction rate (R0). The population growth at the different RHs in decreasing order was 80 > 100 > 60 > 40 > 20%. A relative moisture (RM) of soil from 6.80 to 47.59% was suitable for fall armyworm pupation, survival, and eclosion, but fall armyworm could not pupate normally in soil with 88.39 and 95.19% RM. The survival and emergence rate of fall armyworm pupae were reduced by irrigation that increased the RM after the mature larvae entered the soil. These findings may be helpful for refining laboratory rearing protocols, population forecasting, and management of fall armyworm.

Trapping Spodoptera frugiperda (Lepidoptera: Noctuidae) Moths in Different Crop Habitats in Togo and Ghana

The economic impact of the invasion of Spodoptera frugiperda (J.E. Smith, Lepidoptera: Noctuidae) into Africa has so far been limited to maize agriculture but could potentially impact many other crops. Trapping based on pheromone lures provides a cost-effective method for detecting this important pest (commonly known as fall armyworm) and will be essential for large-scale monitoring of populations to determine its geographical distribution and migration behavior as the species equilibrates to its new environment. However, the effective use of pheromone trapping requires optimization for a given location. An earlier report demonstrated that two commercial lures (one 3-component and the other 4-component) that were effective for trapping S. frugiperda in maize fields in Togo, Africa. The current study extends these findings to agricultural areas that differ in plant host composition (maize, pasture grasses, rice, and sorghum) in multiple locations in Ghana and Togo. In two seasons, significantly higher numbers of moths were found in maize, and in one season, higher numbers were found in rice than in sorghum and pasture grass systems. The results confirm the effectiveness of pheromone trapping and identify pheromone lures and trapping methods best suited for the different agroecosystems common to West Africa and that are at risk of infestation by S. frugiperda.

Trajectory modeling revealed a southwest-northeast migration corridor for fall armyworm Spodoptera frugiperda (Lepidoptera: Noctuidae) emerging from the North China Plain

The fall armyworm Spodoptera frugiperda, an invasive insect pest native to the Americas, has established populations throughout eastern China. The North China Plain-a key corn-producing area in East China with a unique topography-was invaded by fall armyworm in 2019 and is seriously threatened by this migratory pest. However, the spatiotemporal extent of the migratory movements of fall armyworm from the North China Plain remains poorly understood. Using an air transport-based trajectory modeling approach that incorporates flight behavior, we simulated the potential nocturnal migration trajectories of fall armyworm from the North China Plain based on historical meteorological data from June to October of 2015-2019, and examined the night-time atmospheric conditions associated with their possible flights. The emigration patterns showed monthly variation in the main landing area and common migration direction. The displacement of newly emerged moths from the North China Plain was concentrated in the Northeast China Plain (including Liaoning, Jilin and Heilongjiang provinces) before late summer, after which they were most likely to undertake return flights to the south (especially into Hubei, Anhui and Hunan provinces). This southwest-northeast aerial migration corridor follows the topography of East China and is affected by the East Asian monsoon. These topographic-atmospheric conditions have resulted in the North China Plain becoming a key stopover for fall armyworm populations engaging in multigenerational long-distance migration across East China. These findings contribute to our knowledge of fall armyworm migration and will aid in the implementation of management and control strategies against this highly migratory agricultural pest.

Are extreme high temperatures at low or high latitudes more likely to inhibit the population growth of a globally distributed aphid?

Although climate warming can increase both mean temperature and its variability, it is often the effects of climate warming on short periods of extreme temperatures that are expected to have particularly large physiological and ecological consequences. Understanding the vulnerability of organisms at various latitudes to climate extremes is thus critical for understanding warming effects on regional biodiversity conservation and ecosystem management. While previous studies have shown that thermal responses depend on temperature regimes that organisms have previously experienced, this issue has not been considered much when comparing the effects of temperature extremes at different latitudes. To fill this gap, here we manipulated different combinations of amplitude and duration of daily high temperature extremes to simulate conditions at different latitudes. We tested the effects of those regimes on life-history traits and fitness of a globally-distributed aphid species, Rhopalosiphum padi. We compared our results with previous studies to better understand the extent to which these regimes affect conclusions based on comparisons under different mean temperatures. As a consequence of asymmetrical thermal performance curves, we hypothesized that the temperature regimes with higher daily maximum temperatures at higher latitudes would cause strong negative effects. Our results showed that these regimes with thermal extremes caused substantial decreases in life-history traits and fitness relative to the predictions from different mean temperatures. Specifically, the regime with higher daily maximum temperature reflecting a higher mid-latitude location had larger impacts on development, reproduction and population fitness than the regime representing a lower mid-latitude location. These findings have implications for understanding the vulnerability of organisms across latitudes to increasingly frequent extreme heat events under ongoing climate warming.

A Dynamic Energy Budget Approach for the Prediction of Development Times and Variability in Spodoptera frugiperda Rearing

A major challenge in insect rearing is the need to provide certain life cycle stages at a given time for the initiation of experimental trials. The timing of delivery, organism quality, and variability directly affect the outcome of such trials. Development times and intraspecific variability are directly linked to the availability of food and to the ambient temperature. Varying temperature regimes is an approach to adapt development times to fulfill experimental needs without impairment of larval quality. However, current practices of temperature setting may lead to increased variability in terms of development times and the frequency of particular life stages at a given point in time. In this study, we analyzed how resource availability and ambient temperature may affect the larval development of the economically important noctuid species Spodoptera frugiperda by means of dynamic energy budget modeling. More specifically, we analyzed how rearing practices such as raising of temperatures may affect the variability in larval development. Overall, the presented modeling approach provides a support system for decisions that must be made for the timely delivery of larvae and reduction of variability.

Whole genome comparisons reveal panmixia among fall armyworm (Spodoptera frugiperda) from diverse locations

Background

The fall armyworm (Spodoptera frugiperda (J.E. Smith)) is a highly polyphagous agricultural pest with long-distance migratory behavior threatening food security worldwide. This pest has a host range of > 80 plant species, but two host strains are recognized based on their association with corn (C-strain) or rice and smaller grasses (R-strain). The population genomics of the United States (USA) fall armyworm remains poorly characterized to date despite its agricultural threat.

Results

In this study, the population structure and genetic diversity in 55 S. frugiperda samples from Argentina, Brazil, Kenya, Puerto Rico and USA were surveyed to further our understanding of whole genome nuclear diversity. Comparisons at the genomic level suggest a panmictic S. frugiperda population, with only a minor reduction in gene flow between the two overwintering populations in the continental USA, also corresponding to distinct host strains at the mitochondrial level. Two maternal lines were detected from analysis of mitochondrial genomes. We found members from the Eastern Hemisphere interspersed within both continental USA overwintering subpopulations, suggesting multiple individuals were likely introduced to Africa.

Conclusions

Our research is the largest diverse collection of United States S. frugiperda whole genome sequences characterized to date, covering eight continental states and a USA territory (Puerto Rico). The genomic resources presented provide foundational information to understand gene flow at the whole genome level among S. frugiperda populations. Based on the genomic similarities found between host strains and laboratory vs. field samples, our findings validate the experimental use of laboratory strains and the host strain differentiation based on mitochondria and sex-linked genetic markers extends to minor genome wide differences with some exceptions showing mixture between host strains is likely occurring in field populations.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07492-7.

Sweet Corn Sentinel Monitoring for Lepidopteran Field-Evolved Resistance to Bt Toxins

As part of an insect resistance management plan to preserve Bt transgenic technology, annual monitoring of target pests is mandated to detect susceptibility changes to Bt toxins. Currently Helicoverpa zea (Boddie) monitoring involves investigating unexpected injury in Bt crop fields and collecting larvae from non-Bt host plants for laboratory diet bioassays to determine mortality responses to diagnostic concentrations of Bt toxins. To date, this monitoring approach has not detected any significant change from the known range of baseline susceptibility to Bt toxins, yet practical field-evolved resistance in H. zea populations and numerous occurrences of unexpected injury occur in Bt crops. In this study, we implemented a network of 73 sentinel sweet corn trials, spanning 16 U.S. states and 4 Canadian provinces, for monitoring changes in H. zea susceptibility to Cry and Vip3A toxins by measuring differences in ear damage and larval infestations between isogenic pairs of non-Bt and Bt hybrids over three years. This approach can monitor susceptibility changes and regional differences in other ear-feeding lepidopteran pests. Temporal changes in the field efficacy of each toxin were evidenced by comparing our current results with earlier published studies, including baseline data for each Bt trait when first commercialized. Changes in amount of ear damage showed significant increases in H. zea resistance to Cry toxins and possibly lower susceptibility to Vip3a. Our findings demonstrate that the sentinel plot approach as an in-field screen can effectively monitor phenotypic resistance and document field-evolved resistance in target pest populations, improving resistance monitoring for Bt crops.

Regional and seasonal activity predictions for fall armyworm in Australia

Since 2016, the fall armyworm (FAW), Spodoptera frugiperda, has undergone a significant range expansion from its native range in the Americas, to continental Africa, Asia, and in February 2020, mainland Australia. The large dispersal potential of FAW adults, wide host range of immature feeding stages, and unique environmental conditions in its invasive range creates large uncertainties in the expected impact on Australian plant production industries. Here, using a spatial model of population growth and spread potential informed by existing biological and climatic data, we simulate seasonal population activity potential of FAW, with a focus on Australia's grain production regions. Our results show that, in Australia, the large spread potential of FAW will allow it to exploit temporarily favourable conditions for population growth across highly variable climatic conditions. It is estimated that FAW populations would be present in a wide range of grain growing regions at certain times of year, but importantly, the expected seasonal activity will vary markedly between regions and years depending on climatic conditions. The window of activity for FAW will be longer for growing regions further north, with some regions possessing conditions conducive to year-round population survival. Seasonal migrations from this permanent range into southern regions, where large areas of annual grain crops are grown annually, are predicted to commence from October, i.e. spring, with populations subsequently building up into summer. The early stage of the FAW incursion into Australia means our predictions of seasonal activity potential will need to be refined as more Australian-specific information is accumulated. This study has contributed to our early understanding of FAW movement and population dynamics in Australia. Importantly, the models established here provide a useful framework that will be available to other countries should FAW invade in the future. To increase the robustness of our model, field sampling to identify conditions under which population growth occurs, and the location of source populations for migration events is required. This will enable accurate forecasting and early warning to farmers, which should improve pest monitoring and control programs of FAW.

The 'migratory connectivity' concept, and its applicability to insect migrants

Migratory connectivity describes the degree of linkage between different parts of an animal's migratory range due to the movement trajectories of individuals. High connectivity occurs when individuals from one particular part of the migratory range move almost exclusively to another localized part of the migratory range with little mixing with individuals from other regions. Conversely, low migratory connectivity describes the situation where individuals spread over a wide area during migration and experience a large degree of mixing with individuals from elsewhere. The migratory connectivity concept is frequently applied to vertebrate migrants (especially birds), and it is highly relevant to conservation and management of populations. However, it is rarely employed in the insect migration literature, largely because much less is known about the migration circuits of most migratory insects than is known about birds. In this review, we discuss the applicability of the migratory connectivity concept to long-range insect migrations. In contrast to birds, insect migration circuits typically comprise multigenerational movements of geographically unstructured (non-discrete) populations between broad latitudinal zones. Also, compared to the faster-flying birds, the lower degree of control over movement directions would also tend to reduce connectivity in many insect migrants. Nonetheless, after taking account of these differences, we argue that the migratory connectivity framework can still be applied to insects, and we go on to consider postulated levels of connectivity in some of the most intensively studied insect migrants. We conclude that a greater understanding of insect migratory connectivity would be of value for conserving threatened species and managing pests.

Bioclimatic zonation and potential distribution of Spodoptera frugiperda (Lepidoptera: Noctuidae) in South Kivu Province, DR Congo

BACKGROUND

The fall Armyworm (FAW) Spodoptera frugiperda (JE Smith), is currently a devastating pest throughout the world due to its dispersal capacity and voracious feeding behaviour on several crops. A MaxEnt species distributions model (SDM) was developed based on collected FAW occurrence and environmental data's. Bioclimatic zones were identified and the potential distribution of FAW in South Kivu, eastern DR Congo, was predicted.

RESULTS

Mean annual temperature (bio1), annual rainfall (bio12), temperature seasonality (bio4) and longest dry season duration (llds) mainly affected the FAW potential distribution. The average area under the curve value of the model was 0.827 demonstrating the model efficient accuracy. According to Jackknife test of variable importance, the annual rainfall was found to correspond to the highest gain when used in isolation. FAWs' suitable areas where this pest is likely to be present in South Kivu province are divided into two corridors. The Eastern corridor covering the Eastern areas of Kalehe, Kabare, Walungu, Uvira and Fizi territories and the Western corridor covering the Western areas of Kalehe, Kabare, Walungu and Mwenga.

CONCLUSIONS

This research provides important information on the distribution of FAW and bioclimatic zones in South Kivu. Given the rapid spread of the insect and the climatic variability observed in the region that favor its development and dispersal, it would be planned in the future to develop a monitoring system and effective management strategies to limit it spread and crop damage.

Overwintering Distribution of Fall Armyworm (Spodoptera frugiperda) in Yunnan, China, and Influencing Environmental Factors

Simple Summary

The fall armyworm (Spodoptera frugiperda) is a nondiapausing insect pest capable of causing large reductions in the yield of crops, especially maize. Every year, the new generation of fall armyworms from Southeast Asia flies to East Asia via Yunnan, and some of them will grow, develop and reproduce in Yunnan since the geographical location and environmental conditions of Yunnan are very beneficial for the colonization of fall armyworms. This study explored the potential overwintering distribution of fall armyworms in Yunnan and the influence of environmental factors on its distribution. These results provide a basis for the precise prevention and control of fall armyworms by guiding management and decision-making and may facilitate meaningful reductions in pesticide application.

Abstract

The first fall armyworm (FAW; Spodoptera frugiperda) attack in Yunnan, China, occurred in January 2019. Because FAW lacks diapause ability, its population outbreaks largely depend on environmental conditions experienced during the overwinter months. Thus, there is an urgent need to make short-term predictions regarding the potential overwintering distribution of FAW to prevent outbreaks. In this study, we selected the MaxEnt model with the optimal parameter combination to predict the potential overwintering distribution of FAW in Yunnan. Remote sensing data were used in the prediction to provide real-time surface conditions. The results predict variation in the severity and geographic distribution of suitability. The high potential distribution shows a concentration in southwestern Yunnan that suitability continues to increase from January to March, gradually extending to eastern Yunnan and a small part of the northern areas. The monthly independent contributions of meteorological, vegetation, and soil factors were 30.6%, 16.5%, and 3.4%, respectively, indicating that the suitability of conditions for FAW was not solely dominated by the weather and that ground surface conditions also played a decisive role. These results provide a basis for the precise prevention and control of fall armyworms by guiding management and decision-making and may facilitate meaningful reductions in pesticide application.

The genetic characterization of fall armyworm populations in Ecuador and its implications to migration and pest management in the northern regions of South America

The fall armyworm (Spodoptera frugiperda) is a moth pest native to the Western Hemisphere that has recently become a global problem, invading Africa, Asia, and Australia. The species has a broad host range, long-distance migration capability, and a propensity for the generation of pesticide resistance traits that make it a formidable invasive threat and a difficult pest to control. While fall armyworm migration has been extensively studied in North America, where annual migrations of thousands of kilometers are the norm, migration patterns in South America are less understood. As a first step to address this issue we have been genetically characterizing fall armyworm populations in Ecuador, a country in the northern portion of South America that has not been extensively surveyed for this pest. These studies confirm and extend past findings indicating similarities in the fall armyworm populations from Ecuador, Trinidad-Tobago, Peru, and Bolivia that suggest substantial migratory interactions. Specifically, we found that populations throughout Ecuador are genetically homogeneous, indicating that the Andes mountain range is not a long-term barrier to fall armyworm migration. Quantification of genetic variation in an intron sequence describe patterns of similarity between fall armyworm from different locations in South America with implications for how migration might be occurring. In addition, we unexpectedly found these observations only apply to one subset of fall armyworm (the C-strain), as the other group (R-strain) was not present in Ecuador. The results suggest differences in migration behavior between fall armyworm groups in South America that appear to be related to differences in host plant preferences.

Temporal Variation in Genetic Composition of Migratory Helicoverpa Zea in Peripheral Populations

Migrant populations of Helicoverpa zea (Boddie) captured during 2002, 2005, 2016, and 2018 from Landisville and Rock Springs in Pennsylvania, USA were genotyped using 85 single nucleotide polymorphism (SNP) markers. Samples (n = 702) genotyped were divided into 16 putative populations based on collection time and site. Fixation indices (F-statistics), analysis of molecular variance, and discriminant analysis of principal components were used to examine within and among population genetic variation. The observed and expected heterozygosity in putative populations ranged from 0.317-0.418 and 0.320-0.359, respectively. Broad range of FST (0.0-0.2742) and FIS (0.0-0.2330) values indicated different genotype frequencies between and within the populations, respectively. High genetic diversity within and low genetic differentiation between populations was found in 2002 and 2005. Interestingly, high genetic differentiation between populations from two collection sites observed in 2018 populations was not evident in within-site comparisons of putative populations collected on different dates during the season. The shift of H. zea population genetic makeup in 2018 may be influenced by multiple biotic and abiotic factors including tropical storms. Continued assessment of these peripheral populations of H. zea will be needed to assess the impacts of genetic changes on pest control and resistance management tactics.