Fall Armyworm Host Strains Research Articles

Identification of Spodoptera frugiperda (Lepidoptera: Noctuidae) and its two host strains in China by PCR-RFLP.

The fall armyworm (FAW) Spodoptera frugiperda was first found in China in 2018. In other countries, FAW has evolved corn and rice strain biotypes. It is not possible to identify these strains based on morphology. In addition, FAW is very similar in appearance to several other common pests. These situations bring great challenges to the population management of FAW. In this study, we developed a rapid identification method based on PCR-RFLP to distinguish the two FAW strains and the FAW from other lepidopteran pests. A 697 bp mitochondrial cytochrome c oxidase I (COI) was cloned and sequenced from FAW, Spodoptera litura, Spodoptera exigua, and Mythimna separata. The COI fragments of these species revealed unique digestion patterns created by three enzymes (Tail, AlWN I, and BstY II). Thus, these four species can be distinguished from each other. The enzyme Ban I recognized a unique SNP site on a 638 bp triosephosphate isomerase (Tpi) fragment of the corn strain FAW. The Tpi fragment of the corn strain was cut into two bands. However, the rice strain could not be digested. Using this method, all 28 FAW samples collected from different host plants and locations in China were identified as the corn strain. This suggests that the rice strain has not yet invaded China. This method allows discrimination of FAW from other Lepidopteran pests and distinguishes the two FAW host strains.

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.

Cry1 Bt Susceptibilities of Fall Armyworm (Lepidoptera: Noctuidae) Host Strains.

The fall armyworm, Spodoptera frugiperda (Smith; Lepidoptera: Noctuidae), is a highly polyphagous, multivoltine pest of commercial crops including corn (Zea mays L.), cotton (Gossypium spp. L.), rice (Oryza sativa L.), and pasture grasses. Fall armyworm has become a growing concern in agricultural communities across the Americas as field populations in many locales have evolved resistance to several Cry1 toxins derived from the bacterium Bacillus thuringiensis Berliner (Bt). An often overlooked aspect of fall armyworm biology is the existence of two host strains, the 'rice' and 'corn' strains. There has been little research devoted to the characterization of fall armyworm host strains, although there is evidence that the rice and corn-strains may differ in their tolerances to Bt toxins expressed by transgenic plants. In this study, diet-based bioassays were conducted to compare the susceptibilities of one rice-strain, two corn-strains, and one rice-corn hybrid population to Cry1Ab, Cry1Ac, and Cry1F protein. Results indicate that the corn-strains and hybrid populations are more tolerant to the Bt toxins, especially to Cry1F, than the rice-strain population. Results from this study, when combined with existing techniques for host strain identification, may aid in the development of regional insect resistance management programs for fall armyworm.

Demonstration and Quantification of Restricted Mating Between Fall Armyworm Host Strains in Field Collections by SNP Comparisons.

Gene introgression between related pest populations is an important component in the assessment of how rapidly economically important traits, such as pesticide resistance, can spread within a region. An example of this is provided by the noctuid moth Spodoptera frugiperda (J.E. Smith), or fall armyworm, which is composed of two 'host strains' that differ in their host plant preferences. Resistance to a Bacillus thuringiensis toxin has been observed in some populations and there is concern about its spread throughout the Western Hemisphere. If this trait is easily transmitted between strains, it would expand the range of plants affected and make control efforts more difficult. A complicating factor is that the strains are morphologically indistinguishable and can only be identified by a small number of genetic markers. As a result, little is known about the frequency of interstrain hybridization in the wild. This study uses a novel strategy involving comparisons between two single-nucleotide polymorphisms (SNPs) to quantify the frequency of interstrain mating in field populations. The results demonstrate that hybridization between strains is 4- to 5-fold reduced compared to that within strains. In addition, it appears that directional interstrain mating biases observed in laboratory studies are probably not a major factor in determining the distribution of hybrid genotypes in field populations. The differential SNP technique is a significant improvement over current methods for identifying interstrain hybrids and should facilitate our understanding of fall armyworm strain and hybrid distributions in the field and the frequency of genetic exchanges between strains.

Quantitative proteomic analysis of the fall armyworm saliva

Lepidopteran larvae secrete saliva on plant tissues during feeding. Components in the saliva may aid in food digestion, whereas other components are recognized by plants as cues to elicit defense responses. Despite the ecological and economical importance of these plant-feeding insects, knowledge of their saliva composition is limited to a few species. In this study, we identified the salivary proteins of larvae of the fall armyworm (FAW), Spodoptera frugiperda; determined qualitative and quantitative differences in the salivary proteome of the two host races—corn and rice strains—of this insect; and identified changes in total protein concentration and relative protein abundance in the saliva of FAW larvae associated with different host plants. Quantitative proteomic analyses were performed using labeling with isobaric tags for relative and absolute quantification followed by liquid chromatography-tandem mass spectrometry. In total, 98 proteins were identified (>99% confidence) in the FAW saliva. These proteins were further categorized into five functional groups: proteins potentially involved in (1) plant defense regulation, (2) herbivore offense, (3) insect immunity, (4) detoxification, (5) digestion, and (6) other functions. Moreover, there were differences in the salivary proteome between the FAW strains that were identified by label-free proteomic analyses. Thirteen differentially identified proteins were present in each strain. There were also differences in the relative abundance of eleven salivary proteins between the two FAW host strains as well as differences within each strain associated with different diets. The total salivary protein concentration was also different for the two strains reared on different host plants. Based on these results, we conclude that the FAW saliva contains a complex mixture of proteins involved in different functions that are specific for each strain and its composition can change plastically in response to diet type.

Distributional patterns of fall armyworm parasitoids in a corn field and a pasture field in Florida

An assessment of parasitoids and their selective patterns among Spodoptera frugiperda corn and rice host strains was performed from August 2008 to August 2010 in a corn crop and a grass pasture in northern Florida under different seasonal conditions (spring and fall). Sentinel larvae from our laboratory were placed in both habitats. Results obtained in the corn crop differed from those in the pasture due to poor recovery rate of larvae in the pasture habitat. In corn fields, we found that the parasitoid community was composed of eight species of Hymenoptera and one Diptera species, which represented four families and two guilds. The more abundant parasitoid species in the corn field included Aleiodes laphygmae, Meteorus autographae, Cotesia sp., Euplectrus platyhypenae and Ophion flavidus while the Diptera species constituted the most abundant parasitoid in pastures. Among the parasitoid species collected in pastures were three Diptera species and four hymenoptera species.Parasitoid species did not discriminate among fall armyworm host strains, however they varied among the different habitats in the corn field. The ectoparasitoid Euplectrus platyhypenae and an unknown ichneumonid species were found primarily at the edges of the crop (in areas adjacent to the woods); while the endoparasitoids Cotesia sp. and Meteorus sp. were recovered more often in the main corn crop. Meteorus and Ophion were more abundant in the spring while the unknown ichneumonid was more abundant in the fall. Predator species differ among corn and pasture, which might contribute to the dynamics of parasitoid species in these fields.

Captures and Host Strains of Fall Armyworm (Lepidoptera: Noctuidae) Males in Traps Baited with Different Commercial Pheromone Blends

Traps baited with 4 different commercial sex pheromone lures that contained 2, 3, or 4 components were used to capture male fall armyworm [Spodoptera frugiperda (J. E. Smith)] in Alabama, Georgia, Florida, and Texas from 2006–2009. A subset of the moths collected was analyzed for their host strain to determine if there was a difference in attraction to these lures. Chemical analysis of the lures was completed to identify the pheromone components released. Each lure released the number of components expected, but the Trécé lure released relatively higher amounts of the minor component Z7-12:Ac and at a higher percentage of its blend, than the other lures. The 4 lures attracted similar numbers of moths in Alabama, Georgia, and Texas, and there was only a difference among lures in the Florida 2006 trial. More moths were captured in fall 2007 than fall 2008 in Alabama and Georgia. The southern region in Alabama and Georgia averaged more than 13 moths per night, compared to 8.5 in the central region, and 1.9 in the northern region. Lures attracted both host strains of moths, but across years and locations (n = 4546), all lures attracted more corn strain than rice strain males (>55% of moths analyzed were corn strain). However, traps baited with Trécé lures captured a 5% lower percentage of corn strain moths than Scenturion-baited lures. Geographic location and time of season appeared to be much more important in determining host strain identity than the specific commercial lure used. Results from these trials suggest that any of the commercial lures tested will attract the numbers of fall armyworm moths necessary for genetic and migration analysis, and that site location (away from trees and in open areas) and periodic trap maintenance (removal of spiders and frogs from clogging the funnel or eating trap catch) are also important in capturing the highest number of moths.

Allochronic Separation versus Mate Choice: Nonrandom Patterns of Mating between Fall Armyworm Host Strains

Assortative mating may result from intrinsic individual mating preferences or from assortment traits not requiring expression of preferences. Assortment traits are phenotypes expressed in both sexes that enhance the probability of encountering individuals possessing similar trait values. In the noctuid moth Spodoptera frugiperda, it has been suggested that nonrandom mating between two host strains is caused by a temporal assortment trait-that is, differential timing of calling and copulation during the night. By experimental manipulation of this trait in controlled mate-choice experiments, we investigated whether mating by same-strain individuals is enhanced mainly by the allochronic shift of mating activity or is also affected by time-independent intrinsic mating preferences. The observed patterns suggest that nonrandom mating between the two host strains in the laboratory is shaped by an interaction of both effects that is dominated by mating preferences during the first encounter night. This interaction changes over time as the preferences become weaker on subsequent nights. Males were less restricted than females with regard to both the time shift in mating activity and mate preferences. Although the nature of the mate-preference mechanism remains elusive, its restriction to females suggests that male-produced close-range pheromones emitted during courtship play a role.

Using Stable Isotope Analysis to Examine Fall Armyworm (Lepidoptera: Noctuidae) Host Strains in a Cotton Habitat

Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), or fall armyworm, is an important agricultural pest of several crops in the Western Hemisphere, including cotton (Gossypium L.). Two morphologically identical host strains of fall armyworm exist that differ in plant host use and habitat distribution. The corn-strain is a primary pest of corn, Zea mays L., whereas the rice-strain is the majority population infesting rice (Oryza spp.) and turfgrass (Cynodon spp.). With the increased use of Bacillus thuringiensis (Bt) toxin-expressing cotton varieties and the necessity of ensuring adequate refuge areas to prevent the spread of Bt toxin resistance, it is crucial to identify the alternative plant hosts available for the fall armyworm population infesting cotton. Stable isotope analysis combined with the molecular analysis of strain-specific markers was used to investigate whether one or both strains routinely develop on cotton grown in the Mississippi delta. We found that the majority of fall armyworm adults present during the early cotton growing season arose from C4 plants (e.g., corn and sorghum, Sorghum vulgare Pers.) and that the only strain likely to be developing on cotton (a C3 plant) in substantial numbers was the corn-strain. The population distribution patterns observed were consistent with corn providing an important refuge for the fall armyworm strain infesting cotton and suggested that late season populations in the Mississippi delta may be migrants from more northern corn areas.

CATERPILLAR (LEPIDOPTERA: NOCTUIDAE) FEEDING ON PASTURE GRASSES IN CENTRAL FLORIDA

Abstract Stargrasses (Cynodon nlemfuensis Vanderyst var. nlemfuensis) and bermudagrasses (C. dactylon (L.) Persoon) are important warm-season forage grasses, with several cultivars developed for conditions found in central and southern Florida. Major insect pests of these grasses include grass loopers (Mocis spp.) and fall armyworm (Spodoptera frugiperda (J. E. Smith)), which annually may impose economic losses for beef cattle and hay producers. Population studies conducted during a 3-year period showed that both species had similar profiles with respect to larval population seasonality but not abundance. Plot studies with 4 stargrass and 4 bermudagrass lines showed that higher grass looper populations were found in stargrasses than bermudagrasses. Laboratory studies found grass loopers and fall armyworm larvae generally developed faster with larger weights on lines of stargrass than lines of bermudagrass. The two fall armyworm host strains also can differ substantially in their larval weight, development...

Multilocus Genetic Analysis of Host Use, Introgression, and Speciation in Host Strains of Fall Armyworm (Lepidoptera: Noctuidae)

Two genetically differentiated forms of fall armyworm, Spodoptera frugiperda (J. E. Smith), use different graminaceous host plants, coexist sympatrically throughout their ranges, yet seem to hybridize. To address the taxonomic status of the two forms, determine extent and directionality of hybridization, and examine host specificity, we compiled multilocus genotypes consisting of mitochondrial DNA (mtDNA) haplotypes, an esterase locus, and eight amplified fragment-length polymorphism (AFLP) loci in moths collected across a broad geographic range. Multilocus analyses indicated 16% of individuals sampled were potentially hybrids with a minority being F1 in origin. Analysis of host use indicated asymmetries in host specificity with one strain specific to corn, Zea mays L., and the other strain predominating on pasture grasses and rice, but occasionally using corn. Location of hybrids in nature was biased toward cornfields, the habitat used by both strains. To assess genetic divergence of each gene, we calculated their relative strain discriminating ability. Eight AFLP loci collectively had the greatest discriminating power (98%), followed by a single AFLP locus (93%) and mtDNA (91%). Esterase exhibited 89% discrimination. Esterase is X-linked along with an assortative mating trait, suggesting esterase differentiation may be maintained by association with strain-specific fitness genes. Despite strong discrimination of these genes, most of the genome surveyed was not distinct. Cytonuclear comparisons provided evidence for unidirectional matings consistent with mate preference studies. Collectively, these data support introgressive hybridization between recently evolved species that are not completely reproductively isolated. Genetic divergence in the presence of gene flow may be a common phase in the speciation process, especially in taxa whose ranges have been altered dramatically by humans.

IDENTIFYING HOST STRAINS OF FALL ARMYWORM (LEPIDOPTERA\ NOCTUIDAE) IN FLORIDA USING MITOCHONDRIAL MARKERS

Two molecular techniques were used to identify host strains of fall armyworm, Spodoptera frugiperda (J. E. Smith) from male moths captured in pheromone-baited traps in north-central and central Florida. Moths collected in 1998 were analyzed using direct detection of mitochondrial DNA (mtDNA) RFLPs generated from restriction endonuclease digestion of total DNA, while moths collected in 2000 and 2001 were analyzed using a mitochondrial cytochrome oxidase subunit I (COI) gene PCR-RFLP marker. Both techniques could distinguish between rice and corn strain moths, however, the COI PCR-RFLP marker was more robust as indicated by a time interval experiment that showed that moths held for up to 15 days in a “bucket trap” could still be used for strain diagnosis. In a field study, our strategy gave results consistent with expectations. Rice strain moths were common in habitats with large areas of small grasses, corn strain moths were common in large areas planted to corn, and habitats with mixed large- and small-grass plantings contained both strains. Our methodology of combining pheromone traps with PCR-RFLP analysis will provide a valuable sampling system to determine the population ecology habits and strain isolating mechanisms of fall armyworm populations in numerous habitats, including overwintering areas of southern Florida.

Host-Plant Adaptation in Fall Armyworm Host Strains: Comparison of Food Consumption, Utilization, and Detoxication Enzyme Activities

The fall armyworm, Spodoptera frugiperda (J. E. Smith), consists of two genetically different strains. The corn strain feeds primarily on corn, and the rice strain feeds primarily on forage grasses and rice. Possible causes for differences in larval performance between the strains were examined. The objectives were to establish whether behavioral or physiological factors could explain differences in performance on corn. A comparison between strains in food consumption and utilization indicated that poorer performance on corn by the rice strain is, in part, caused by a lower consumption rate. In addition, the rice strain has a lower efficiency of converting digested food into biomass than the corn strain when reared on corn. When larvae were reared on either corn or bermudagrass, the rice strain had lower mixed-function oxidase (MFO) activities than die corn strain. In the corn strain, MFO activity was higher when larvae were reared on corn than on bermudagrass. Corn and bermudagrassreared larvae from the rice strain expressed similar MFO activities. Esterase activity did not differ between strains or between com and bermudagrass-reared larvae. Thus, the results indicate that both behavioral and physiological factors associated with host use differ between the strains. Moreover, MFOs may play an important role in host-plant adaptation in the fall armyworm.

Corrections: Quantitative Genetics of Fall Armyworm Host Strains

Corrections: Quantitative Genetics of Fall Armyworm Host Strains

Correction: Quantitative Genetics, Development and Physiological Adaptation in Host Strains of Fall Armyworm

Correction: Quantitative Genetics, Development and Physiological Adaptation in Host Strains of Fall Armyworm

Current Status of Fall Armyworm Host Strains

Research on two host strains of the fall armyworm (FAW), Spodoptera frugiperda [J. E. Smith], is reviewed. They differ at allozyme loci and mtDNA restriction enzyme profiles, in their development on a variety of plants, and in their resistance to insecticides. Data on interstrain matings indicate unidirectional incompatibilities in the laboratory and strong preferences among males for females of their own strain in nature. Further study is required to clarify precise strain affinities for the long list of FAW host records. Information on phenologies and geographic distributions is presented but clearly in need of in-depth study. Those studying FAW are recommended to collect from a single host, to keep host records with laboratory colonies, to replenish colonies with individuals from the same host, and to establish a set of voucher specimens for each collection or study.

QUANTITATIVE GENETICS, DEVELOPMENT, AND PHYSIOLOGICAL ADAPTATION IN HOST STRAINS OF FALL ARMYWORM.

Two genetically differentiated host strains of fall armyworm were reared on their own and each other's host plants, rice and corn, to determine whether they were physiologically adapted to their natural hosts and whether they exhibited genetically based differences in development. Larval host had a greater impact on development in the rice strain than in the corn strain, indicating that physiology could have facilitated specialization in one strain but not the other. Consequently, behavioral mechanisms are also likely to be important in the maintenance of host specificity. Comparisons between strains indicated significant differences in one trait, the rate at which larvae gained weight. Because this character had consistently high heritabilities, genetic differentiation in development is indicated. An analysis of genotype-by-environment interactions within each strain detected significant interactions for three of five traits, suggesting that genotypic performance on one host was not indicative of performance on the other. Each strain thus exhibited genetic variation that would facilitate host-associated divergence and adaptation if coupled with a mechanism that reduced gene flow between hosts. Finally, significant genetic correlations between several characters were detected when strains were reared on their natural hosts but not when they were reared on nonnatural hosts. Apparently, feeding on novel hosts caused developmental uncoupling of characters. Release from genetic constraints could provide a mechanism for physiological adjustments to newly occupied habitats.

Quantitative Genetics, Development and Physiological Adaptation in Host Strains of Fall Armyworm

Quantitative Genetics, Development and Physiological Adaptation in Host Strains of Fall Armyworm