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.”     

Morphological defense of the egg mass of Spodoptera frugiperda (Lepidoptera: Noctuidae) affects parasitic capacity and alters behaviors of egg parasitoid wasps

The fall armyworm, Spodoptera frugiperda, is a dangerous pest of cereals originating from the tropical and subtropical parts of the Americas. It has invaded over a hundred countries and is spreading rapidly throughout East Asia. Biological control programs are a “model strategy” for the control of invasive pests. Egg parasitoids Telenomus remus and Trichogramma pretiosum are viewed as candidates for the control of S. frugiperda. The aim of this study was to evaluate the behavioral patterns and parasitism efficacy of Te. remus, Tr. pretiosum, and Trichogramma dendrolimi on S. frugiperda egg masses with and without scales. The proportion of parasitism by Te. remus was significantly higher than that by Tr. dendrolimi and Tr. pretiosum. The proportion of parasitism by Tr. pretiosum and Tr. dendrolimi on egg masses with scales was significantly lower than that on egg masses without scales. However, Te. remus had a similar proportion of parasitism on egg masses with and without scales. Residence time, oviposition time, oviposition frequency, risk of host being found, and risk of parasitism by Te. remus were significantly higher than the corresponding parameters of Tr. pretiosum and Tr. dendrolimi. Tr. pretiosum females exhibited a higher residence time, oviposition time, oviposition frequency, risk of host being found, and risk of parasitism than those of Tr. dendrolimi females. In addition, Te. remus females often crept into the scale layer covering the egg masses, whereas Tr. dendrolimi and Tr. pretiosum females did not; they could only parasitize the eggs located on the periphery of the egg mass. Both Te. remus and Tr. pretiosum females had similar proportions of superparasitism, which were significantly higher than that of Tr. dendrolimi. Therefore, Te. remus is the dominant egg parasitoid of S. frugiperda and this has important implications for developing augmentative biological control strategies for S. frugiperda.     

Effect of cold acclimation and rapid cold-hardening on the survival of Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) under cold stress

Spodoptera frugiperda is a highly invasive pest species that recently invaded Africa and Asia causing severe economic losses, primarily related to corn and rice crops. Temperature is one of the most important environmental factors that influence the invasion of pests into new habitats. However, little is known regarding the thermal tolerance characteristics of invasive S. frugiperda. Thus, we investigated the response of four developmental stages of S. frugiperda (i.e., eggs, third and sixth instar larvae, and pupae) to cold acclimation (CA) and rapid cold-hardening (RCH). All individuals suffered high mortality with 24-h temperature treatments at <?5°C and >35 °C. The CA treatment significantly increased the survival rate of the eggs and third instar larvae, although it did not affect the sixth instar larvae and it decreased the pupation rate. The RCH treatment at 5 °C for 5 h or 2 °C for 2 h increased the cold tolerance capabilities of the third and sixth instar larvae, respectively. Thus, the larval stage appears to be crucial for the cold tolerance of S. frugiperda. Our findings improve the current understanding of the cold tolerance characteristics of S. frugiperda and indicate its potential for survival in the newly invaded temperate regions of Asia.     

草地贪夜蛾人工饲料及饲养条件的优化

改良草地贪夜蛾Spodoptera frugiperda(J.E Smith)人工饲料配方及优化饲养条件,为草地贪夜蛾室内批量饲养奠定基础。利用不同人工饲料配方及制作工艺、不同温湿条件、对草地贪夜蛾进行群体饲养,研究其对草地贪夜蛾生长发育及繁殖的影响,并开展不同产卵介质对草地贪夜蛾成虫产卵量及卵孵化率的影响。结果显示,在温度27℃±0.5℃、相对湿度70%±5%、光周期L∶D=16 h∶8 h的人工气候箱中,采用饲料配方4进行幼虫群体饲养的效果最好,其发育历期最短,仅为27.1 d,幼虫存活率高达83.8%,化蛹率为91.2%,雌雄性比为1∶1.08,单雌产卵量为836粒。另外,草地贪夜蛾幼虫最喜食加热20 min的饲料,成虫于粉色塑料纸上产卵量最大。该饲料配方非常适宜草地贪夜蛾室内饲养,优化的饲养条件也为其室内规模化饲养提供参考。     

3种人工饲料对草地贪夜蛾生长发育及繁殖力的影响

草地贪夜蛾Spodoptera frugiperda(J.E. Smith)是2019年入侵我国的重要农业害虫,筛选出合适的人工饲料可为规模化繁育提供基础。本文通过组建实验种群生命表的方法,研究了3种人工饲料对草地贪夜蛾生长发育、繁殖力以及种群增长潜能的影响,以玉米叶饲养的作为对照。结果表明,3种人工饲料对草地贪夜的饲养效果具有明显差异。其中,以饲料Ⅱ饲养的结果最理想,除幼虫-蛹发育历期(如雌性个体,23.63 d)比对照(20.98 d)略有延长外,幼期存活率(88.00%)、雌性蛹重(243.90 mg)、雄性蛹重(251.60 mg)、产卵量(845.90粒)等指标均优于对照,生命表参数中的净生殖力R_0(346.29)、内禀增长率r_m(0.1859)、周限增长率λ(1.2043)也高于对照。用饲料Ⅰ饲养时,草地贪夜蛾能够完成世代发育,但存在幼虫-蛹的发育历期(41.56 d)明显延长、幼期存活率(36.00%)、产卵雌成虫比例低(25.00%)等问题,难以达到室内大量繁育的要求。而以饲料Ⅲ饲养的草地贪夜蛾仅能生长发育至5龄幼虫,不能完成世代发育。综合相关生物学指标来看,饲料Ⅱ是饲养草地贪夜的适宜人工饲料。     

CO2浓度升高对草地贪夜蛾生长发育的直接影响

草地贪夜蛾自2019年1月入侵云南以来,已迅速扩散至我国20个省,对我国的农业生产造成了极大危害。本研究通过在两种CO_2浓度(400 ppm和800 ppm)下用离体玉米叶片饲养草地贪夜蛾,分析比较了CO_2浓度升高对草地贪夜蛾生长发育和繁殖力的直接影响。结果表明:在800 ppm CO_2浓度下,草地贪夜蛾各虫态发育历期均长于400 ppm CO_2浓度下的发育历期;其中,1龄、6龄幼虫发育历期,幼虫总发育历期和蛹期在两种CO_2浓度下差异均达到极显著水平(P0.01)。此外,相同龄期草地贪夜蛾幼虫体重在两个CO_2浓度之间差异不显著,但蛹重存在极显著性差异,分别为198.82±3.45 mg(400 ppm)和245.89±4.18 mg(800 ppm)(P0.01);高CO_2浓度下雌虫的产卵期显著长于正常CO_2浓度下的产卵期,分别为2.4±0.8 d(400 ppm)和6.0±0.9 d(800 ppm),高CO_2浓度下平均单雌产卵量是正常CO_2浓度下的2.43倍,差异显著(P0.05)。研究结果为预测草地贪夜蛾未来的发生趋势提供了理论依据。     

首次入侵广东的草地贪夜蛾迁入路径及天气背景分析

草地贪夜蛾 Spodoptera frugiperda (J. E. Smith)是联合国粮农组织全球预警的超级害虫,2019年1月入侵我国云南,并随季风远距离迁飞扩散,4月23日,广东省广州市增城区首次发现并确认该虫幼虫发生为害,对当地玉米等粮食作物构成严重威胁。本研究运用基于WRF模式的昆虫三维轨迹分析程序和GrADS气象图形软件,模拟了入侵广东的草地贪夜蛾的迁飞路径及天气背景场。结果表明:按照幼虫发育历期推算,首批草地贪夜蛾成虫迁入广东的时间可能为4月8日~13日;在此期间,越南北部及华南地区的西南低空急流可为草地贪夜蛾的迁飞提供运载气流,而风切变、降雨和下沉气流为草地贪夜蛾的迫降提供便利条件;4月9-10日存在草地贪夜蛾的有效迁飞路径,入侵广东省增城区草地贪夜蛾种群的有效虫源地分布在越南及老挝北部。本研究为华南地区草地贪夜蛾春季迁入种群的监测预警及防控提供了科学依据。     

Silicon application promotes rice growth and negatively affects development of Spodoptera frugiperda (J. E. Smith)

Silicon (Si) has been reported to enhance plant resistance against biotic and abiotic stressors and also benefit plant growth. These effects are more pronounced in grass species, especially with soil‐applied Si. This study investigated the effects of Si application on rice resistance to Spodoptera frugiperda development and plant vegetative growth. Effects of Si on rice were assessed via soil and foliar applications and compared with untreated plants (control). Si was soil‐ and foliar‐applied as 1% silicic acid solution at a dosage equivalent to 1.4 t Si per ha. After application, leaf material was collected from Si‐treated and untreated plants and placed in Petri dishes with individual S. frugiperda neonate larvae, where development was followed to adult emergence and biological parameters recorded. Vegetative growth parameters recorded in rice plants were the height, chlorophyll content, fresh and dry weights of shoots, and shoot Si content. No effects of Si application were observed on the durations of larval and pupal stages, larval and pupal survival, and sex ratio of S. frugiperda. Insects fed leaves from Si‐treated plants exhibited lower leaf consumption, larval and pupal weights, longevity of males and females, number of eggs, and egg viability. The negative effects were correlated with higher rice Si content. Si application to rice increased plant height, chlorophyll content and dry weight. Our study demonstrates that foliar‐applied Si is as efficient as soil‐applied Si in negatively affecting S. frugiperda development and providing beneficial effects on rice plant growth.     

Microbiome of the larvae of Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) from maize plants

Fall armyworm (FAW), Spodoptera frugiperda, is an invasive insect pest with a diverse host range threatening food security in the African continent by causing severe economic damage to staple maize crop. FAW like all other insects are associated with various microbiota that influence their several characteristics and activities. The microbiota of insects plays important roles in their growth, development and environmental adaptation to their host plants or animals and not much is known about microbiota of FAW in Nigeria. In this study, the microbial community of FAW larvae from maize plants in Nigeria was investigated using Illumina PE250 NovaSeq of 16S rRNA V3-V4 gene region. A total of 1,160,133 sequence reads that ranged from 83,794 to 145,475 were obtained. They span through 2 kingdoms: bacteria (99.59%) and archaea (0.37%), 37 phyla, 59 classes, 78 orders, 145 families and 285 genera. The dominant taxonomic group of bacteria across the larval samples were Proteobacteria (58.32%), Firmicutes (35.87%) and Bacteroidetes (4.02%). There were no significant differences in bacteria species diversity and richness among the individual fall armyworm larva and samples from different geographical regions. The relative abundance of Bacteria phyla and classes also showed no significant differences among the larval samples. Significant differences were documented in bacteria orders Acidobacteriales and Aeromonadales, family Acidobacteriaceae_ (Subgroup_1) and genera Novosphingobium and Pedobacter. The results indicate that the gut of FAW larvae harbours a diverse set of bacteria and archaea biota. Understanding their ecological and functional relevance would provide novel insight on potential approaches for their exploitation in the integrated management of FAW in the tropics.     

Compensatory responses of the fall armyworm (Spodoptera frugiperda) when fed water- and cellulose-diluted diets

Abstract Caterpillars of the noctuid moth, Spodoptera frugiperda (J. E. Smith), reared on artificial diets diluted with cellulose and water, increased fresh weight (fw) consumption 2.2–2.5-fold over those on the undiluted diet. At moderate levels of water- or cellulose-dilution, the increased consumption, combined with increased digestion and absorption of nutrients (ADNU), sufficiently compensated for the reduced nutrient intake to achieve pupal biomass equivalent to that of pupae from the undiluted diet. At higher levels of water- and cellulose-dilution, fw consumption and ADNU increased further but pupal dry weight declined on the water-diluted diets. At each level of dilution fw consumption and ADNU increased similarly on the water- and cellulose-diluted diets, but biomass gain was reduced on the water- compared with the cellulose-diluted diets. This was due in part to lowered food conversion efficiency on the water-diluted diets, possibly caused by increased costs of metabolizing the wetter diets. Our data support the hypothesis that consumption rates are regulated by a nutrient feedback mechanism. Our data also suggest that digestive enzyme activity is correlated with consumption. Furthermore, the cost of increased consumption rates on diets of reduced energetic value may constitute a substantially greater energy expenditure than previously believed. However, this cost was insufficient to reduce relative growth rates but was apparently manifested in lowered lipid accumulation.     

Effects of citronella oil (Cymbopogon winterianus Jowitt ex Bor) on Spodoptera frugiperda (J. E. Smith) midgut and fat body

The armyworm, Spodoptera frugiperda, is the principal pest of corn in Brazil. Control is achieved primarily by synthetic insecticides, which cause problems for the agro-ecosystem. Alternative methods of control are under investigation and citronella (Cymbopogon winterianus) essential oil appears to be a promising agent. We investigated the effects of citronella oil using histological, histochemical and immunohistochemical methods. The midgut of larvae treated with citronella exhibited altered epithelium including cytoplasmic protrusions, columnar cell extrusion, pyknotic nuclei, and increased periodic acid-Schiff positive granules. Regenerative cells in the epithelium of the midgut increased in number, which facilitated subsequent regeneration of this tissue. After exposure to citronella, trophocytes, the principal cell type of the fat body, possessed enlarged vacuoles and mitotic bodies, and contained reduced amounts of glycogen, lipid, and protein. Citronella oil caused morphological changes of the midgut and reduction of stored resources in the fat body, which may adversely affect insect reproduction and survival.     

广东省草地贪夜蛾应急防控体系的构建

草地贪夜蛾Spodoptera frugiperda(Smith)是一种农业重大有害生物,自2019年1月份入侵我国云南以来,迅速席卷国内多个省市,对我国粮食安全构成严重威胁。广东省自发现草地贪夜蛾入侵以来,在政府部门领导、组织或支持下,制定了工作方案,组建了专家队伍,迅速开展该虫监测预警、基因检测、生物学生态学研究、防控药剂筛选、生物防治试验、诱杀技术试验、调查监测与防控方案制定、示范推广、宣传培训等各项工作,形成了一套协调、高效的草地贪夜蛾应急防控体系。     

广东省草地贪夜蛾冬季发生特征及周年繁殖区域研究

广东省是草地贪夜蛾入侵我国的桥头堡和主要的北迁虫源地之一,明确该虫在广东省的冬季发生特征及越冬存活情况,对广东省乃至全国草地贪夜蛾的预测预报及源头治理意义重大。为准确掌握广东省草地贪夜蛾的周年繁殖区范围及冬季发生为害情况,2020年1-3月在粤东、粤西、粤北及珠三角地区,利用成虫性诱、挖土查蛹、幼虫密度及植株为害率普查等方法,分析广东省草地贪夜蛾冬前、冬后种群发生为害情况及冬季发生特征。调查结果表明:(1)广东省冬玉米种植区主要分布在湛江、茂名、阳江、惠州等地,冬玉米种植区均发现草地贪夜蛾幼虫为害,主要为害冬玉米,极少为害甘蔗;(2)不同地区冬种玉米上草地贪夜蛾的发生程度差异较大,湛江、茂名、阳江发生为害较为严重,平均为害率30%左右,而珠三角及粤东地区发生较轻,为害率低于10%;较冬前调查,冬后草地贪夜蛾发生量和为害程度出现不同程度的下降;(3)广东省大部分地区的冬种玉米田和空闲地均可持续诱捕到草地贪夜蛾成虫,而挖土调查发现草地贪夜蛾蛹密度较低。本调查明确了草地贪夜蛾在北回归线以南的冬玉米区可以周年繁殖,无明显的滞育越冬现象,粤西茂名、阳江以南至雷州半岛一带为典型冬种玉米区,草地贪夜蛾发生为害较为严重,珠三角及粤东大部分地区草地贪夜蛾种群数量相对较低,调查结果为广东省乃至全国草地贪夜蛾的早期预警和精准防控提供了重要参考。     

不同生育期玉米上草地贪夜蛾的发生为害规律

2019年在广东湛江、江门调查了玉米上草地贪夜蛾Spodoptera frugiperda (Smith)发生为害动态,分析了不同生育期玉米上该虫发生为害差异规律。发现各个地区草地贪夜蛾的虫口数量与玉米受害程度存在较大差异,呈现不均衡性;该虫对玉米苗期为害最严重,其次为大喇叭口期;建立了2个地区玉米6个生育期受害率与虫口数量之间的线性回归模型。研究结果可为草地贪夜蛾监测预警及防控提供参考。     

Lethal and sublethal effects of methoxyfenozide on the development, survival and reproduction of the fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae)

The lethal and sublethal effects of the ecdysone agonist methoxyfenozide on the fall armyworm, Spodoptera frugiperda (J. E. Smith), were investigated by feeding a methoxyfenozide-treated diet to fifth instars until pupation in doses corresponding to the LC10 and LC25 for the compound. Larval mortality reached 8% and 26% in the low and high concentration groups, respectively, on the seventh day of the experiment. A progressive larval mortality of 12% for the LC10 and 60% for the LC25 was observed before pupation. Treated larvae exhibited lower pupal weights, higher pupal mortality, presence of deformed pupae, and more deformed adults than untreated larvae. The incorporation of methoxyfenozide into the diet had a significant effect on the timing of larval development. The development period for males and females was about seven days longer than the controls for both concentrations tested. In contrast, the compound affected neither pupae nor adult longevity. Finally, S. frugiperda adults that resulted from fifth instars treated with methoxyfenozide were not affected in their mean cumulative number of eggs laid per female (fecundity), nor percentages of eggs hatched (fertility), or the sex ratio. Our results suggest that the combination of lethal and sublethal effects of methoxyfenozide may have important implications for the population dynamics of the fall armyworm.     

RFLP analysis of genetic variation in North American populations of the fall armyworm moth Spodoptera frugiperda (Lepidoptera: Noctuidae)

A high level of genetic variability was detected in North American fall armyworm (FA W), Spodoptera frugiperda (J. E. Smith), populations by restriction fragment length polymorphism (RFLP) analysis of genomic DNA. In nearly all cases individual larvae could be differentiated using 22 probe-enzyme combinations. Laboratory colonies formed distinct groups based on RFLP analysis. Individuals from a colony of the previously determined ‘rice strain’ formed a group showing obvious deviation in RFLP patterns from the other five populations, which included a population from a ‘com strain’ colony. The results were consistent with previous studies using allozymes which indicated that there are two genetically differentiated forms of the fall armyworm. Diagnostic markers were also found that distinguish these two groups.     

Effects of a nucleopolyhedrovirus in mixtures with azadirachtin on Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) larvae and viral occlusion body production

Abstract

The combined and alone effects of azadirachtin (AZA) and Spodoptera frugiperda multicapsid nucleopolyhedrovirus (SfMNPV) on the mortality of S. frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) were evaluated in the laboratory. For this, diet surface contamination bioassays were performed on S. frugiperda in the third instar. LC₅₀ values for SfMNPV alone were determined to be 430 and 373 viral occlusion bodies (OBs)/mm² at 192 and 216 h after treatment, respectively. LC₅₀ values for AZA alone were estimated for two periods of continuous exposure (4 or 5 days). In this case, LC₅₀ values were 45.5 and 16.8 mg L^?1 at 216 h after treatment (4 or 5 days of larval exposition to insecticide, respectively). We observed that although the interaction of AZA with SfMNPV increased viral pathogenicity, such improvements were of greater magnitude and more consistent at the lower OB concentration used (177 OBs/mm²). Application of SfMNPV (430 OBs/mm²), AZA (26.4 mg L^?1) or SfMNPV–AZA mixtures resulted in a significant reduction in the mean weight of larvae treated in the third instar across the experiment, by 23–41%, 17–95% and 26–97%, respectively, compared to control. The duration of larval development during the third and fourth instars increased significantly in larvae exposed to SfMNPV–AZA mixtures and AZA alone compared to SfMNPV alone and control treatments. The yield of OBs/mg weight of larvae treated with SfMNPV alone was 1.8-fold higher than OB yields from insects inoculated with SfMNPV–AZA mixtures. We conclude that AZA + SfMNPV mixtures are unlikely to be useful for the mass production of this virus and laboratory observations on the value of AZA + SfMNPV mixtures as a potentiator of biological insecticides require validation in field studies under commercial growing conditions.