Genetic basis of allochronic differentiation in the fall armyworm

Two host-plant strains in the fall armyworm

The fall armyworm (Spodoptera frugiperda) is one of the major pest insects damaging diverse crops including cotton, corn, rice, and sorghum. Fall armyworms have been identified as two morphologically indistinguishable strains, the corn strain, and the rice strain, named after their preferred host-plants. Although initially recognized as host-plant strains, there has been an ongoing debate regarding whether the corn and rice strains should be considered as such. In this article, we present arguments based on recent population genomics studies supporting that these two strains should be considered to be host-plant strains. Furthermore, host-plant adaptation appears to be a driving evolutionary force responsible for incipient speciation in the fall armyworm.

Regulation of feeding dynamics by the circadian clock, light and sex in an adult nocturnal insect

Animals invest crucial resources in foraging to support development, sustenance, and reproduction. Foraging and feeding behaviors are rhythmically expressed by most insects. Rhythmic behaviors are modified by exogenous factors like temperature and photoperiod, and internal factors such as the physiological status of the individual. However, the interactions between these factors and the circadian clock to pattern feeding behavior remains elusive. As Drosophila, a standard insect model, spends nearly all its life on food, we rather chose to focus on the adults of a non-model insect, Agrotis ipsilon, a nocturnal cosmopolitan crop pest moth having structured feeding activity. Our study aimed to explore the impact of environmental cues on directly measured feeding behavior rhythms. We took advantage of a new experimental set-up, mimicking an artificial flower, allowing us to specifically monitor feeding behavior in a naturalistic setting, e.g., the need to enter a flower to get food. We show that the frequency of flower visits is under the control of the circadian clock in males and females. Feeding behavior occurs only during the scotophase, informed by internal clock status and external photic input, and females start to visit flowers earlier than males. Shorter duration visits predominate as the night progresses. Importantly, food availability reorganizes the microstructure of feeding behavior, revealing its plasticity. Interestingly, males show a constant number of daily visits during the 5 days of adult life whereas females decrease visitations after the third day of adult life. Taken together, our results provide evidence that the rhythmicity of feeding behavior is sexually dimorphic and controlled by photoperiodic conditions through circadian clock-dependent and independent pathways. In addition, the use of the new experimental set-up provides future opportunities to examine the regulatory mechanisms of feeding behavior paving the way to investigate complex relationships between feeding, mating, and sleep-wake rhythms in insects.

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.

The Spodoptera frugiperda Host Strains: What They Are and Why They Matter for Understanding and Controlling This Global Agricultural Pest

Fall armyworm (Spodoptera frugiperda [J.E. Smith]) is a moth native to the Western Hemisphere where it is a major pest of corn (maize) and multiple other crops. It is now a global threat with its recent discovery and dissemination in the Eastern Hemisphere. Its broad host range is in part due to two subpopulations denoted as 'host strains' that differ in host plant specificity. Therefore, identifying the strains present in a location is critical to assessing what crops are at risk of infestation. However, much remains uncertain about how the strains differ and even on the fundamental issue of how they are identified. Complicating factors include the host strains are morphologically indistinguishable, the defining behavior of the strains (host plant specificity) is variable, and the existence of significant differences between geographical populations and laboratory colonies that are independent of strain identity. These factors contribute to substantial disagreements in the literature on presumptive strain differences. This paper presents a summary of strain characteristics and suggests the criteria that should be met before concluding a trait is 'strain-specific'. The intent is to facilitate a common understanding of what the strains represent and to develop a more consistent experimental framework for studies on strain phenotypes. Evidence is summarized that supports a primary role for Z-linked genes in strain identity, which has potential implications for genetic approaches to define the strains, and we discuss the possibility that the strains arose from allopatric (rather than sympatric) speciation processes.

The evolutionary process of invasion in the fall armyworm (Spodoptera frugiperda)

The fall armyworm (FAW; Spodoptera frugiperda) is one of the major agricultural pest insects. FAW is native to the Americas, and its invasion was first reported in West Africa in 2016. Then it quickly spread through Africa, Asia, and Oceania, becoming one of the main threats to corn production. We analyzed whole genome sequences of 177 FAW individuals from 12 locations on four continents to infer evolutionary processes of invasion. Principal component analysis from the TPI gene and whole genome sequences shows that invasive FAW populations originated from the corn strain. Ancestry coefficient and phylogenetic analyses from the nuclear genome indicate that invasive populations are derived from a single ancestry, distinct from native populations, while the mitochondrial phylogenetic tree supports the hypothesis of multiple introductions. Adaptive evolution specific to invasive populations was observed in detoxification, chemosensory, and digestion genes. We concluded that extant invasive FAW populations originated from the corn strain with potential contributions of adaptive evolution.

Host-plant adaptation as a driver of incipient speciation in the fall armyworm (Spodoptera frugiperda)

Background

Divergent selection on host-plants is one of the main evolutionary forces driving ecological speciation in phytophagous insects. The ecological speciation might be challenging in the presence of gene flow and assortative mating because the direction of divergence is not necessarily the same between ecological selection (through host-plant adaptation) and assortative mating. The fall armyworm (FAW), a major lepidopteran pest species, is composed of two sympatric strains, corn and rice strains, named after two of their preferred host-plants. These two strains have been hypothesized to undergo incipient speciation, based on (i) several lines of evidence encompassing both pre- and post-zygotic reproductive isolation, and (ii) the presence of a substantial level of genetic differentiation. Even though the status of these two strains has been established a long time ago, it is still yet to be found whether these two strains indeed exhibit a marked level of genetic differentiation from a large number of genomic loci. Here, we analyzed whole genome sequences from 56 FAW individuals either collected from pasture grasses (a part of the favored host range of the rice strain) or corn to assess the role of host-plant adaptation in incipient speciation.

Results

Principal component analysis of whole genome data shows that the pattern of divergence in the fall armyworm is predominantly explained by the genetic differentiation associated with host-plants. The level of genetic differentiation between corn and rice strains is particularly marked in the Z chromosome. We identified one autosomal locus and two Z chromosome loci targeted by selective sweeps specific to rice strain and corn strain, respectively. The autosomal locus has both increased DXY and FST while the Z chromosome loci had decreased DXY and increased FST.

Conclusion

These results show that the FAW population structure is dominated by the genetic differentiation between corn and rice strains. This differentiation involves divergent selection targeting at least three loci, which include a locus potentially causing reproductive isolation. Taken together, these results suggest the evolutionary scenario that host-plant speciation is a driver of incipient speciation in the fall armyworm.

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.

Two Microsatellite Types Within NAD6 Gene Help to Distinguish Populations and Infer the Migratory Route of the Invasive Fall Armyworm, Spodoptera frugiperda, (Lepidoptera, Noctuidae) in China

Spodoptera frugiperda is a major agricultural pest that has invaded China since January 2019. Given that most of the individuals present in China carried the diagnostic rice-strain mtDNA (COI-RS), there was no efficient method to distinguish populations of S. frugiperda. In this study, we identified and characterized two variant microsatellite alleles in the mitochondrial NAD6 gene of S. frugiperda retrieved from the National Center for Biotechnology Center GenBank. We then sequenced partial NAD6 genes containing the microsatellite region and the diagnostic COI barcoding gene (used to distinguish the corn-strain and the rice-strain) of 429 invasive S. frugiperda individuals that were collected from the main infested regions in China during 2019-2020. Our data indicates that two kinds of interrupted repeat sequences, (ATA)4T(ATA)3 and (ATA)5T(ATA)3, exist in the microsatellite region which we defined as the deletion type (NAD6-D), and the insertion type (NAD6-I) based on the repeat units' differentiation, respectively. The presence of these two microsatellite types in the mtDNA genome of S. frugiperda was further confirmed with the sequencing results in 429 samples. Moreover, NAD6-I and NAD6-D types were both present in individuals with COI-RS, while only NAD6-D type was detected in the COI-CS individuals. Interestingly, the two microsatellite types suggested a possible geographic distribution: the western migratory route (Yunan and Chongqing) was comprised exclusively of NAD6-I type, while both NAD6-I and NAD6-D types were identified in the predicted eastern migration trajectories (Hainan, Guangxi, Shandong, etc.). These results suggested that NAD6-D and NAD6-I types may be useful in distinguishing between populations, analyzing the evolutionary mechanism of mtDNA microsatellite polymorphism, inferring the migratory route of S. frugiperda in China, and developing precise and integrated control strategies for S. frugiperda.

Knockouts of positive and negative elements of the circadian clock disrupt photoperiodic diapause induction in the silkworm, Bombyx mori

Diapause is one of the most important traits that have sustained insects to thrive. To survive harsh seasons, most insects can arrest their development and enter diapause. The photoperiod is the signal that indicates insects the proper timing to enter diapause. Circadian clock genes are shown to be involved in photoperiodic diapause induction in various insect species. The silkworm, Bombyx mori, enters diapause at the embryonic stage. In bivoltine strains, diapause determination is under maternal control and affected by temperature and photoperiodic conditions that mothers experienced during embryonic and larval stages. Two independent studies showed that knocking out the core clock gene, period, perturb photoperiodic diapause induction in B. mori. However, whether the circadian clock as whole or individual clock genes are responsible for the photoperiodic diapause induction remains unknown. In this study, using CRISPR/Cas9 we knocked out negative (period and timeless) and positive elements (Clock and cycle) in p50T, a bivoltine strain which exhibits photoperiodic diapause induction during both embryonic and larval stages. The temporal expression patterns of clock genes changed in each core clock gene knockout strain, suggesting disruption of normal feedback loops produced by circadian clock genes. Furthermore, the ability of female moths to appropriately produce diapause or non-diapause eggs in response to photoperiod in both embryonic and larval stages was lost in all knockout strains. Our results indicate the involvement of circadian clock in photoperiodic diapause induction in B. mori.

Molecular Oscillator Affects Susceptibility of Caterpillars to Insecticides: Studies on the Egyptian Cotton Leaf Worm-Spodoptera littoralis (Lepidoptera: Noctuidae)

The molecular oscillator is the core of the biological clock and is formed by genes and proteins whose cyclic expression is regulated in the transcriptional-translational feedback loops (TTFLs). Proteins of the TTFLs are regulators of both their own and executive genes involved in the control of many processes in insects (e.g., rhythmic metabolism of xenobiotics, including insecticides). We disrupted the clock operation in S. littoralis larvae by injecting the dsRNA of clock genes into their body cavity and culturing the larvae under continuous light. As a result, the daily susceptibility of larvae to insecticides was abolished and the susceptibility itself increased (in most cases). In the fat body, midgut, and Malpighian tubules (the main organs metabolizing xenobiotics) of the larvae treated with injected-dsRNA, the daily activity profiles of enzymes involved in detoxification-cytochrome P450 monooxygenases, Glutathione-S-transferase, and esterase-have changed significantly. The presented results prove the role of the molecular oscillator in the regulation of larvae responses to insecticides and provide grounds for rational use of these compounds (at suitable times of the day), and may indicate clock genes as potential targets of molecular manipulation to produce plant protection compounds based on the RNAi method.

Incipient speciation between host-plant strains in the fall armyworm

Background

Recent advancement in speciation biology proposes that genetic differentiation across the whole genome (genomic differentiation, GD) may occur at the beginning of a speciation process and that GD itself may accelerate the rate of speciation. The fall armyworm (FAW, Spodoptera frugiperda) has been used as a model species to study the process of speciation between diverging host-plant strains. We showed in a previous study that GD between the host-plant strains occurred at the beginning of a speciation process based on a population genomics analysis from a population in Mississippi (USA), providing empirical support for the theoretical prediction. In a recent paper, however, panmixia was reported in FAW based on the genomic analysis of 55 individuals collected from Argentina, Brazil, Kenya, Puerto Rico, and the mainland USA. If panmixia is true, the observed differentiation in Mississippi could be at most a phenomenon specific to a geographic population, rather than a status during a speciation process. In this report, we reanalyzed the resequencing data to test the existence of population structure according to host plants using different bioinformatics pipelines.

Results

Principal component analysis, F_ST statistics, and ancestry coefficient analysis supported genetic differentiation between strains regardless of the used bioinformatics pipelines. The strain-specific selective sweep was observed from the Z chromosome, implying the presence of strain-specific divergence selection. Z chromosome has a particularly high level of genetic differentiation between strains, while autosomes have low but significant genetic differentiation. Intriguingly, the re-sequencing dataset demonstrates the spread of Bacillus thuringiensis resistance mutations from Puerto Rico to the US mainland.

Conclusions

These results show that a pair of host-plant strains in FAW experience genomic differentiation at the beginning of a speciation process, including Z chromosome divergent selection and possibly hitchhiking effect on autosomal sequences.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-022-02008-7.

Enterococcal symbionts of caterpillars facilitate the utilization of a suboptimal diet

Bacterial gut symbionts of insect herbivores can impact their host through different mechanisms. However, in most lepidopteran systems we lack experimental examples to explain how specific members of the gut bacterial community influence their host. We used fall armyworm (Spodoptera frugiperda) as a model system to address this objective. We implemented axenic and gnotobiotic techniques using two semi-artificial diets with pinto bean and wheat germ-based components. Following an initial screen of bacterial isolates representing different genera, larvae inoculated with Enterococcus FAW 2-1 exhibited increased body mass on the pinto bean diet, but not on the wheat germ diet. We conducted a systematic bioassay screening of Enterococcus isolated from fall armyworm, revealing they had divergent effects on the hosts' usage pinto bean diet, even among phylogenetically similar isolates. Dilution of the pinto bean diet revealed that larvae performed better on less-concentrated diets, suggesting the presence of a potential toxin. Collectively, these results demonstrate that some gut microorganisms of lepidopterans can benefit the host, but the dietary context is key towards understanding the direction of the response and magnitude of the effect. We provide evidence that gut microorganisms may play a wider role in mediating feeding breadth in lepidopteran pests, but overall impacts could be related to the environmental stress and the metabolic potentials of the microorganisms inhabiting the gut.

Patterns of genomic and allochronic strain divergence in the fall armyworm, Spodoptera frugiperda (J.E. Smith)

Speciation is the process through which reproductive isolation develops between distinct populations. Because this process takes time, speciation studies often necessarily examine populations within a species that are at various stages of divergence. The fall armyworm, Spodoptera frugiperda (J.E. Smith), is comprised of two strains (R = Rice & C = Corn) that serve as a novel system to explore population divergence in sympatry. Here, we use ddRADSeq data to show that fall armyworm strains in the field are largely genetically distinct, but some interstrain hybridization occurs. Although we detected F1 hybrids of both R‐ and C‐strain maternal origin, only hybrids with R‐strain mtDNA were found to contribute to subsequent generations, possibly indicating a unidirectional barrier to gene flow. Although these strains have been previously defined as “host plant‐associated,” we recovered an equal proportion of R‐ and C‐strain moths in fields dominated by C‐strain host plants. As an alternative to host‐associated divergence, we tested the hypothesis that differences in nightly activity patterns could account for reproductive isolation by genotyping temporally collected moths. Our data indicates that strains exhibit a significant shift in the timing of their nightly activities in the field. This divergence in phenology creates a prezygotic reproductive barrier that likely maintains the genetic isolation between strains. Thus, we conclude that it may be ecologically inaccurate to refer to the C‐ and R‐ strain as “host‐associated” and they should more appropriately be considered “allochronic strains.”

Novel real-time PCR based assays for differentiating fall armyworm strains using four single nucleotide polymorphisms

The fall armyworm, Spodoptera frugiperda, is a polyphagous global pest with a preference for gramineous crops such as corn, sorghum and pasture grasses. This species is comprised of two morphologically identical but genetically distinct host strains known as the corn and rice strains, which can complicate pest management approaches. Two molecular markers are commonly used to differentiate between strains, however, discordance between these markers can lead to inconclusive strain identification. Here, we used double digest restriction site associated DNA sequencing to identify diagnostic single nucleotide polymorphisms (SNPs) with alleles unique to each strain. We then used these strain-specific SNPs to develop four real-time PCR based TaqMan assays to rapidly and reliably differentiate between strains and interstrain hybrids. These assays provide a new tool for differentiating between strains in field-collected samples, facilitating future studies on strain population dynamics and interstrain hybridization rates. Understanding the basic ecology of S. frugiperda strains is necessary to inform future management strategies.

Timing strains of the marine insect Clunio marinus diverged and persist with gene flow

Genetic divergence of populations in the presence of gene flow is a central theme in speciation research. Theory predicts that divergence can happen with full range overlap - in sympatry - driven by ecological factors, but there are few empirical examples of how ecologically divergent selection can overcome gene flow and lead to reproductive isolation. In the marine midge Clunio marinus (Diptera: Chironomidae) reproduction is ecologically restricted to the time of the lowest tides, which is ensured through accurate control of development and adult emergence by circalunar and circadian clocks. As tidal regimes differ along the coastline, locally adapted timing strains of C. marinus are found in different sites across Europe. At the same time, ecologically suitable low tides occur at both full and new moon and twice a day, providing C. marinus with four nonoverlapping temporal niches at every geographic location. Along the coast of Brittany, which is characterized by a steep gradient in timing of the tides, we found an unusually large number of differentially adapted timing strains, and the first known instances of sympatric C. marinus strains occupying divergent temporal niches. Analysis of mitochondrial genotypes suggests that these timing strains originated from a single recent colonization event. Nuclear genotypes show strong gene flow, sympatric timing strains being the least differentiated. Even when sympatric strains exist in nonoverlapping temporal niches, timing adaptations do not result in genome-wide genetic divergence, suggesting timing adaptations are maintained by permanent ecological selection. This constitutes a model case for incipient ecological divergence with gene flow.

The Fall Armyworm Spodoptera frugiperda Utilizes Specific UDP-Glycosyltransferases to Inactivate Maize Defensive Benzoxazinoids

The relationship between plants and insects is continuously evolving, and many insects rely on biochemical strategies to mitigate the effects of toxic chemicals in their food plants, allowing them to feed on well-defended plants. Spodoptera frugiperda, the fall armyworm (FAW), accepts a number of plants as hosts, and has particular success on plants of the Poaceae family such as maize, despite their benzoxazinoid (BXD) defenses. BXDs stored as inert glucosides are converted into toxic aglucones by plant glucosidases upon herbivory. DIMBOA, the main BXD aglucone released by maize leaves, can be stereoselectively re-glucosylated by UDP-glycosyltransferases (UGTs) in the insect gut, rendering it non-toxic. Here, we identify UGTs involved in BXD detoxification by FAW larvae and examine how RNAi-mediated manipulation of the larval glucosylation capacity toward the major maize BXD, DIMBOA, affects larval growth. Our findings highlight the involvement of members of two major UGT families, UGT33 and UGT40, in the glycosylation of BXDs. Most of the BXD excretion in the frass occurs in the form of glucosylated products. Furthermore, the DIMBOA-associated activity was enriched in the gut tissue, with a single conserved UGT33 enzyme (SfUGT33F28) being dedicated to DIMBOA re-glucosylation in the FAW gut. The knock-down of its encoding gene reduces larval performance in a strain-specific manner. This study thus reveals that a single UGT enzyme is responsible for detoxification of the major maize-defensive BXD in this pest insect.

Positive selection alone is sufficient for whole genome differentiation at the early stage of speciation process in the fall armyworm

BACKGROUND

The process of speciation involves differentiation of whole genome sequences between a pair of diverging taxa. In the absence of a geographic barrier and in the presence of gene flow, genomic differentiation may occur when the homogenizing effect of recombination is overcome across the whole genome. The fall armyworm is observed as two sympatric strains with different host-plant preferences across the entire habitat. These two strains exhibit a very low level of genetic differentiation across the whole genome, suggesting that genomic differentiation occurred at an early stage of speciation. In this study, we aim at identifying critical evolutionary forces responsible for genomic differentiation in the fall armyworm.

RESULTS

These two strains exhibit a low level of genomic differentiation (FST = 0.0174), while 99.2% of 200 kb windows have genetically differentiated sequences (FST > 0). We found that the combined effect of mild positive selection and genetic linkage to selectively targeted loci are responsible for the genomic differentiation. However, a single event of very strong positive selection appears not to be responsible for genomic differentiation. The contribution of chromosomal inversions or tight genetic linkage among positively selected loci causing reproductive barriers is not supported by our data. Phylogenetic analysis shows that the genomic differentiation occurred by sub-setting of genetic variants in one strain from the other.

CONCLUSIONS

From these results, we concluded that genomic differentiation may occur at the early stage of a speciation process in the fall armyworm and that mild positive selection targeting many loci alone is sufficient evolutionary force for generating the pattern of genomic differentiation. This genomic differentiation may provide a condition for accelerated genomic differentiation by synergistic effects among linkage disequilibrium generated by following events of positive selection. Our study highlights genomic differentiation as a key evolutionary factor connecting positive selection to divergent selection.

Functional analysis of Pdp1 and vrille in the circadian system of a cricket

Circadian rhythms are generated by a circadian clock for which oscillations are based on the rhythmic expression of the so-called clock genes. The present study investigated the role of Gryllus bimaculatus vrille (Gb'vri) and Par domain protein 1 (Gb'Pdp1) in the circadian clock of the cricket Gryllus bimaculatus. Structural analysis of Gb'vri and Gb'Pdp1 cDNAs revealed that they are a member of the bZIP transcription factors. Under light/dark cycles (LD) both genes were rhythmically expressed in the clock tissue, the optic lobes, whereas the rhythm diminished under constant darkness (DD). Gb'vri and Gb'Pdp1 mRNA levels were significantly reduced by RNA interference (RNAi) of Gb'Clk and Gb'cyc, suggesting they are controlled by Gb'CLK/Gb'CYC. RNAi of Gb'vri and Gb'Pdp1 had little effect on locomotor rhythms, although their effects became visible when treated together with Gb'cyc^RNAi. The average free-running period of Gb'vri^RNAi/Gb'cyc^RNAi crickets was significantly shorter than that of Gb'cyc^RNAi crickets. A similar period shortening was observed also when treated with Gb'Pdp1^RNAi/Gb'cyc^RNAi. Some Gb'Pdp1^RNAi/Gb'cyc^RNAi crickets showed rhythm splitting into two free-running components with different periods. Gb'vri^RNAi and Gb'Pdp1^RNAi treatments significantly altered the expression of Gb'Clk, Gb'cyc, and Gb'tim in LD. These results suggest that Gb'vri and Gb'Pdp1 play important roles in cricket circadian clocks.

Sexual communication of Spodoptera frugiperda from West Africa: Adaptation of an invasive species and implications for pest management

The pest species Spodoptera frugiperda, which is native to North and South America, has invaded Africa in 2016. The species consists of two strains, the corn-strain and rice-strain, which differ in their sexual communication. When we investigated populations from Benin and Nigeria, consisting of corn-strain and rice-corn-hybrid descendants, we found no strain-specific sexual communication differences. Both genotypes exhibited the same pheromone composition, consisting of around 97% (Z)-9-tetradecenyl acetate (Z9–14:Ac), 2% (Z)-7-dodecenyl acetate (Z7–12:Ac), and 1% (Z)-9-dodecenyl acetate (Z9–12:Ac), they had similar electrophysiological responses, and all mated around three hours into scotophase. However, we found geographic variation between African and American populations. The sex pheromone of African corn-strain and hybrid descendant females was similar to American rice-strain females and showed higher percentages of the male-attracting minor component Z7–12:Ac. In addition, African males exhibited the highest antennal sensitivity towards Z7–12:Ac, while American males showed highest sensitivity towards the major pheromone component Z9–14:Ac. Increasing the production of and response to the critical minor component Z7–12:Ac may reduce communication interference with other African Spodoptera species that share the same major pheromone component. The implications of our results on pheromone-based pest management strategies are discussed.

Differences in gene regulation in a tephritid model of prezygotic reproductive isolation

The two tephritid fruit fly pests, Bactrocera tryoni and Bactrocera neohumeralis, are unusually well suited to the study of the genetics of reproductive isolating mechanisms. Sequence difference between the species is no greater than between a pair of conspecific Drosophila melanogaster populations. The two species exist in close sympatry, yet do not hybridize in the field, apparently kept separate by a strong premating isolation mechanism involving the time of day at which mating occurs. This spurred us to search for key genes for which time of day expression is regulated differently between the species. Using replicated, quantitative transcriptomes from head tissues of males of the two species, sampled in the day and night, we identified 141 transcripts whose abundance showed a significant interaction between species and time of day, indicating a difference in gene regulation. The brain transcripts showing this interaction were enriched for genes with a neurone function and 90% of these were more abundant at night than day in B. tryoni. Features of the expression patterns suggest that there may be a difference in the regulation of sleep-wake cycles between the species. In particular several genes, which in D. melanogaster are expressed in circadian pacemaker cells, are promising candidates to further explore the genetic differentiation involved in this prezygotic reproductive isolation mechanism.

Host population related variations in circadian clock gene sequences and expression patterns in Chilo suppressalis

The rice stem borer, Chilo suppressalis Walker, is one of the most important global agricultural pests. C. suppressalis has distinct rice and water-oat host populations. Asynchrony in sexual activity is thought to be the main factor maintaining reproductive segregation between these populations, particularly the obvious difference in the circadian rhythm of female calling activity between populations. However, the mechanism responsible for this difference in the timing of female calling is poorly understood. The circadian clock is an essential regulator of daily behavioral rhythms in insects, including female calling. We investigated the variation in circadian clock genes of the rice and water-oat populations of C. suppressalis. We did this by comparing deduced amino acid sequences and the expression patterns of seven circadian clock genes (clock, cycle, period, timeless, timeout, cryptochrome1, and cryptochrome2) between females from each population. We found that the two populations had different variants of the timeout and cryptochrome1 genes and differed in the expression of period, timeless and timeout. This suggests that population-related variation in the circadian clock genes period, timeless, timeout and cryptochrome1 could be responsible for the different circadian rhythms of female calling in these host population of C. suppressalis. These results provide new insights into the molecular mechanisms underlying asynchronous sexual activity in insect populations and suggest new topics for future research on the origins and maintenance of population differentiation in insects.

Spodoptera frugiperda Smith (Lepidoptera: Noctuidae) in Cameroon: Case study on its distribution, damage, pesticide use, genetic differentiation and host plants

Maize farmers in sub-Saharan Africa recently experienced unusual damage in their farms, attributed to the fall armyworm (FAW) Spodoptera frugiperda (J. E. Smith). This pest was first recorded in Africa in 2016, but detailed information on its distribution and damage and farmer’s response in invaded areas are largely lacking. In this study, we determined FAW distribution, genetic diversity, host plants, crop damage, and farmers’ responses. S. frugiperda was recorded in the 10 regions of Cameroon. Average percentage of infested plants and damage severity (on a scale of 1 to 5) were lowest—20.7 ± 7.4% and 2.1 ± 0.1 respectively—in the Sahelian regions and greatest—69.0 ± 4.3% and 3.1 ± 0.1 respectively—in the Western Highlands. Altitude did not influence FAW incidence and severity and its larvae infrequently co-occurred with maize stemborers on the same plants, suggesting possible direct and/or indirect competition between the two groups of maize pests. In response to this new threat to maize production, farmers have opted for the application of synthetic pesticides. Although our experiments were not designed to determine pesticide efficacy, as parameters such as time since application were not considered, our observations suggest lack of a drastic effect on S. frugiperda infestations on maize. There were two haplotypes of FAW co-occurring in Cameroon corresponding to the rice and corn strains and separated by 1.7% sequence divergence, which does not support the existence of cryptic species. S. frugiperda larvae were also recorded on Sorghum bicolor (L.) Moench (10.6%), Solanum tuberosum L. (2.8%), Ipomoea batatas (L.) Lam. (1.9%), Saccharum officinarum L (0.8%), Phaseolus vulgaris L. (0.4%) and Gossypium hirsutum L. (1.9%). This study show that two strains are present in all agroecological zones in Cameroon, and probably in neighboring countries of central Africa sharing the same agroecologies. Management options should therefore consider the use of specific natural enemies and an informed decision of intervention based on strain capture and damage threshold, to avoid pesticide resistance that may arise from inadequate use of chemicals. Further studies should also be undertaken to assess the response of the two S. frugiperda strains to biopesticides and botanical insecticides.

Forecasting the global extent of invasion of the cereal pest Spodoptera frugiperda, the fall armyworm

Fall armyworm, Spodopterafrugiperda, is a crop pest native to the Americas, which has invaded and spread throughout sub-Saharan Africa within two years. Recent estimates of 20–50% maize yield loss in Africa suggest severe impact on livelihoods. Fall armyworm is still infilling its potential range in Africa and could spread to other continents. In order to understand fall armyworm’s year-round, global, potential distribution, we used evidence of the effects of temperature and precipitation on fall armyworm life-history, combined with data on native and African distributions to construct Species Distribution Models (SDMs). We also investigated the strength of trade and transportation pathways that could carry fall armyworm beyond Africa. Up till now, fall armyworm has only invaded areas that have a climate similar to the native distribution, validating the use of climatic SDMs. The strongest climatic limits on fall armyworm’s year-round distribution are the coldest annual temperature and the amount of rain in the wet season. Much of sub-Saharan Africa can host year-round fall armyworm populations, but the likelihoods of colonising North Africa and seasonal migrations into Europe are hard to predict. South and Southeast Asia and Australia have climate conditions that would permit fall armyworm to invade. Current trade and transportation routes reveal Australia, China, India, Indonesia, Malaysia, Philippines and Thailand face high threat of fall armyworm invasions originating from Africa.

Assessing Genomic Admixture between Cryptic Plutella Moth Species following Secondary Contact

Cryptic species are genetically distinct taxa without obvious variation in morphology and are occasionally discovered using molecular or sequence data sets of populations previously thought to be a single species. The world-wide Brassica pest, Plutella xylostella (diamondback moth), has been a problematic insect in Australia since 1882, yet a morphologically cryptic species with apparent endemism (P. australiana) was only recognized in 2013. Plutella xylostella and P. australiana are able to hybridize under laboratory conditions, and it was unknown whether introgression of adaptive traits could occur in the field to improve fitness and potentially increase pressure on agriculture. Phylogenetic reconstruction of 29 nuclear genomes confirmed P. xylostella and P. australiana are divergent, and molecular dating with 13 mitochondrial genes estimated a common Plutella ancestor 1.96 ± 0.175 Ma. Sympatric Australian populations and allopatric Hawaiian P. xylostella populations were used to test whether neutral or adaptive introgression had occurred between the two Australian species. We used three approaches to test for genomic admixture in empirical and simulated data sets including 1) the f3 statistic at the level of the population, 2) pairwise comparisons of Nei's absolute genetic divergence (dXY) between populations, and 3) changes in phylogenetic branch lengths between individuals across 50-kb genomic windows. These complementary approaches all supported reproductive isolation of the Plutella species in Australia, despite their ability to hybridize. Finally, we highlight the most divergent genomic regions between the two cryptic Plutella species and find they contain genes involved with processes including digestion, detoxification, and DNA binding.

Pest risk assessment of Spodoptera frugiperda for the European Union

EFSA was asked for a partial risk assessment of Spodoptera frugiperda for the territory of the EU focussing on the main pathways for entry, factors affecting establishment, risk reduction options and pest management. As a polyphagous pest, five commodity pathways were examined in detail. Aggregating across these and other pathways, we estimate that tens of thousands to over a million individual larvae could enter the EU annually on host commodities. Instigating risk reduction options on sweetcorn, a principal host, reduces entry on that pathway 100-fold. However, sweetcorn imports are a small proportion of all S. frugiperda host imports, several of which are already regulated and further regulation is estimated to reduce the median number entering over all pathways by approximately 10%. Low temperatures limit the area for establishment but small areas of Spain, Italy and Greece can provide climatic conditions suitable for establishment. If infested imported commodities are distributed across the EU in proportion to consumer population, a few hundreds to a few thousands of individuals would reach NUTS 2 regions within which suitable conditions for establishment exist. Although S. frugiperda is a known migrant, entry directly into the EU from extant populations in sub-Saharan Africa is judged not feasible. However, if S. frugiperda were to establish in North Africa, in the range of thousands to over two million adults could seasonally migrate into the southern EU. Entry into suitable NUTS2 areas via migration will be greater than via commercial trade but is contingent on the establishment of S. frugiperda in North Africa. The likelihood of entry of the pest via natural dispersal could only be mitigated via control of the pest in Africa. If S. frugiperda were to arrive and become a pest of maize in the EU, Integrated Pest Management (IPM) or broad spectrum insecticides currently used against existing pests could be applied.

Non-Pleiotropic Coupling of Daily and Seasonal Temporal Isolation in the European Corn Borer

Speciation often involves the coupling of multiple isolating barriers to produce reproductive isolation, but how coupling is generated among different premating barriers is unknown. We measure the degree of coupling between the daily mating time and seasonal mating time between strains of European corn borer (Ostrinia nubilalis) and evaluate the hypothesis that the coupling of different forms of allochrony is due to a shared genetic architecture, involving genes with pleiotropic effects on both timing phenotypes. We measure differences in gene expression at peak mating times and compare these genes to previously identified candidates that are associated with changes in seasonal mating time between the corn borer strains. We find that the E strain, which mates earlier in the season, also mates 2.7 h earlier in the night than the Z strain. Earlier daily mating is correlated with the differences in expression of the circadian clock genes cycle, slimb, and vrille. However, different circadian clock genes associate with daily and seasonal timing, suggesting that the coupling of timing traits is maintained by natural selection rather than pleiotropy. Juvenile hormone gene expression was associated with both types of timing, suggesting that circadian genes activate common downstream modules that may impose constraint on future evolution of these traits.

Pest categorisation of Spodoptera frugiperda

The European Commission requested EFSA to conduct a pest categorisation of Spodoptera frugiperda (Lepidoptera: Noctuidae) a pest with hosts in 27 plant families. Favoured hosts include maize, rice and sorghum (Poaceae). Hosts also include crops within the Brassicaceae, Cucurbitaceae, Solanaceae, Rutaceae and other families. S. frugiperda is a taxonomic entity with reliable methods for identification. It is regulated in the EU as a harmful organism whose introduction into the EU is banned. It is native to tropical and subtropical regions of the Americas and migrates to temperate regions in North and South America during the summer. Establishment in temperate areas is prevented by its inability to overwinter. S. frugiperda has been intercepted on plant produce entering the EU. Phytosanitary measures are available to impede entry via traded commodities. In 2016, S. frugiperda was reported damaging maize in Africa. Subsequent reports indicate that it continues to spread severely damaging maize and other crops. If S. frugiperda spreads into north Africa, the likelihood of adults migrating into the temperate EU increases. Within the scope and level of analysis appropriate for pest categorisation, the EFSA Plant Health Panel concludes that S. frugiperda could establish in a small area of the southern EU from where it is likely to enter more northern regions forming transient summer populations, particularly in maize growing regions where impacts on yield could occur. However, uncertainties regarding establishment remain. Considering the criteria within the remit of EFSA to assess as regards status as a potential Union quarantine pest (QP) or as a potential regulated non‐quarantine pest (RNQP), S. frugiperda satisfies the criteria to be regarded a Union QP but does not meet the criteria of (i) occurring in the EU territory, and (ii) plants for planting being the principal means of spread, criteria required for RNQP status.

Genetic basis of allochronic differentiation in the fall armyworm

BACKGROUND

Very little is known on how changes in circadian rhythms evolve. The noctuid moth Spodoptera frugiperda (Lepidoptera: Noctuidae) consists of two strains that exhibit allochronic differentiation in their mating time, which acts as a premating isolation barrier between the strains. We investigated the genetic basis of the strain-specific timing differences to identify the molecular mechanisms of differentiation in circadian rhythms.

RESULTS

Through QTL analyses we identified one major Quantitative trait chromosome (QTC) underlying differentiation in circadian timing of mating activity. Using RADtags, we identified this QTC to be homologous to Bombyx mori C27, on which the clock gene vrille is located, which thus became the major candidate gene. In S. frugiperda, vrille showed strain-specific polymorphisms. Also, vrille expression differed significantly between the strains, with the rice-strain showing higher expression levels than the corn-strain. In addition, RT-qPCR experiments with the other main clock genes showed that pdp1, antagonist of vrille in the modulatory feedback loop of the circadian clock, showed higher expression levels in the rice-strain than in the corn-strain.

CONCLUSIONS

Together, our results indicate that the allochronic differentiation in the two strains of S. frugiperda is associated with differential transcription of vrille or a cis-acting gene close to vrille, which contributes to the evolution of prezygotic isolation in S. frugiperda.