Rodney N. Nagoshi

Use of DNA barcodes to identify invasive armyworm Spodoptera species in Florida.

Abstract A critical component for sustaining adequate food production is the protection of local agriculture from invasive pest insects. Essential to this goal is the ability to accurately distinguish foreign from closely related domestic species, a process that has traditionally required identification using diagnostic morphological “keys” that can be both subtle and labor-intensive. This is the case for the Lepidopteran group of insects represented by Spodoptera, a genus of Noctuidae “armyworm” moths that includes several important agricultural pests. Two of the most destructive species, Spodoptera littoralis (Boisduval) (Lepidoptera: Noctuidae) and S. litura (F.) are not yet established in North America. To facilitate the monitoring for these pests, the feasibility of using DNA barcoding methodology for distinguishing between domestic and foreign Spodoptera species was tested. A DNA barcoding database was derived for a subset of Spodoptera species native to Florida, with an emphasis on those attracted to pheromone blends developed for S. litura or S. littoralis. These were then compared to the barcode sequences of S. litura collected from Taiwan and S. littoralis from Portugal. Consistent discrimination of the different species was obtained with phenetic relationships produced that were generally in agreement with phylogenetic studies using morphological characteristics. The data presented here indicate that DNA barcoding has the potential to be an efficient and accurate supplement to morphological methods for the identification of invasive Spodoptera pests in North America.

Molecular Characterization of Host Strains of Spodoptera frugiperda (Lepidoptera: Noctuidae) in Southern Brazil

Abstract Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae) is a Neotropical species found from Argentina to the United States of America. In its distribution area, two genetically distinct strains are found that differ in their plant host distribution. The strains are morphologically identical, and they can only be reliably distinguished by molecular markers. In this study, we characterized caterpillars collected in corn, Zea mays L., and rice, Oryza sativa L., plantations in the state of Rio Grande do Sul, Brazil. The mitochondrial haplotype typical of the rice strain was found in 100% of the caterpillars collected from that crop, whereas 83% of the caterpillars obtained from corn were of the corn strain haplotype. A sex-linked tandem repeat element called FR was previously shown to have a strain-biased distribution in North American populations. We demonstrated that the same element is found in Brazilian fall armyworm and that it also organized in large tandem repeat clusters that are most frequently observed in caterpillars collected from rice and that display the rice strain mitochondrial haplotype. These results indicate that the Brazilian and North American fall armyworm populations are virtually indistinguishable with respect to the known genetic markers that are diagnostic of strain identity. Furthermore the data are consistent with other studies suggesting substantial gene flow between these widely separated populations. The strain-biased distribution of the FR element makes it a potentially useful marker to estimate interstrain hybridization frequencies of this species in Brazil.

Review of Fall Armyworm (Lepidoptera: Noctuidae) Genetic Complexity and Migration

Abstract The fall armyworm, Spodoptera frugiperda (J. E. Smith) is a significant economic pest in the western hemisphere, causing substantial losses in corn, sorghum, forage, and turf grasses. Although fall armyworm does not survive severe winters, it infests most of the central and eastern United States and Canada because of annual migrations from overwintering sites in Florida and Texas. A detailed description of these movements is a prerequisite for identifying the factors that determine the timing and direction of migration and for developing models that can predict the severity of infestations at the migratory destinations. Complicating this effort is genetic heterogeneity within the species, which increases phenotypic variability. Particularly important are 2 “host strains”, defined by a preferential association with either large grasses (designated corn-strain), such as corn and sorghum, or smaller grasses (designated rice-strain), such as rice and bermudagrass. This paper reviews recent studies examining the genetic complexity of fall armyworm populations, including characteristics of the 2 strains and the possibility of subgroups within strains. The use of this information to monitor short and long distance movements is discussed.

LARVAL DEVELOPMENT OF FALL ARMYWORM (LEPIDOPTERA: NOCTUIDAE) ON DIFFERENT COVER CROP PLANTS

Abstract A series of laboratory and field experiments were conducted to compare larval development, feeding behavior, and ovipositional preference of fall armyworm (Spodoptera frugiperda) on a standard host plant, a standard cover crop plant, and two candidate cover crop plants. The results indicate that larvae from different rearing cultures and host strains developed comparably on corn and sorghum-sudangrass, but generally developed poorly on cowpeas and sunnhemp. Larval and ovipositional experiments also suggested a preference for either corn or sorghum-sudangrass. Field plantings of cowpeas and sunnhemp in two locations were ignored by fall armyworm in favor of corn. These studies suggest that cowpeas and sunnhemp have the potential to reduce stepping stone nursery populations of fall armyworm by lengthening developmental time and increasing larval mortality.

Population dynamics and occurrence of Spodoptera frugiperda host strains in southern Florida

Abstract.  1. The development of an area‐wide management programme for the migratory pest fall armyworm [Spodoptera frugiperda (J. E. Smith)] requires knowledge of its preferred habitats throughout the year and a detailed description of seasonal changes in population numbers.

Inferring the annual migration patterns of fall armyworm (Lepidoptera: Noctuidae) in the United States from mitochondrial haplotypes

Spodoptera frugiperda (J. E. Smith) or fall armyworm is an important agricultural pest of a number of crops in the western hemisphere. In the United States, infestations in corn acreages extend from the Mexican to the Canadian border. Because fall armyworm does not survive prolonged freezing, the infestations annually affecting most of North America are migrants from southern Texas and Florida, where winter temperatures are mild and host plants are available. A haplotype method was developed that can distinguish between these two geographically distant overwintering populations, with the potential to delineate the associated migratory pathways. Several years of collections from major corn-producing areas in the southern, central, and eastern United States were used to map the geographical distribution of the fall armyworm haplotypes. From these haplotype profiles, it was possible to develop the most detailed description yet of the annual northward movements of fall armyworm. The consistency of these results with past studies and the implications on our understanding of fall armyworm biology are discussed. A better understanding of fall armyworm populations and their movement is critical for the development of strategies to predict infestation levels and eventually control this pest in the United States.

Using Haplotypes to Monitor the Migration of Fall Armyworm (Lepidoptera: Noctuidae) Corn-Strain Populations from Texas and Florida

Abstract Fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), infestations in most of North America north of Mexico arise from annual migrations of populations that overwinter in southern Texas and Florida. A comparison of the cytochrome oxidase I haplotype profiles within the fall armyworm corn-strain, the subgroup that preferentially infests corn (Zea mays L.) and sorghum (Sorghum vulgare Pers.), identified significant differences in the proportions of certain haplotypes between the Texas and Florida populations. These proportional differences were preserved as the populations migrated, providing a molecular metric by which the source of a migrant population could be identified. The migratory pattern derived from this method for several southeastern states was shown to be consistent with predictions based on analysis of historical agricultural and fall armyworm infestation data. These results demonstrate the utility of haplotype proportions to monitor fall armyworm migration, and they also introduce a potential method to predict the severity of cotton crop infestations in the short term.

Puerto Rico Fall Armyworm Has Only Limited Interactions with Those from Brazil or Texas but Could Have Substantial Exchanges with Florida Populations

ABSTRACT Fall armyworm, Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), is an important agricultural pest that is endemic to Puerto Rico and the rest of the Caribbean islands. Relatively little is known about the population movements of fall armyworm in the Caribbean and the magnitude of genetic interactions, if any, with populations from North, South, and Central America. To address this issue, a novel method involving mitochondrial haplotype ratios currently being used to study the migration of fall armyworm in North America was applied to populations in Puerto Rico. The results indicate limited interactions between Puerto Rico fall armyworm and those from Brazil or Texas but the potential for significant exchanges with populations in Florida.

Texas is the Overwintering Source of Fall Armyworm in Central Pennsylvania: Implications for Migration Into the Northeastern United States

ABSTRACT Fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), infestations in most of North America arise from annual migrations of populations that overwinter in southern Texas and Florida. Cytochrome oxidase I haplotype profiles within the fall armyworm corn strain, the subgroup that preferentially infests corn (Zea mays L.), can differentiate the Texas and Florida populations. We use this molecular metric to show that fall armyworms in central Pennsylvania originate from Texas, indicating the existence of a migratory pathway from Texas to the northeastern United States. These results were compared with historical trapping data for fall armyworm and another migratory noctuid, corn earworm Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae), in the Pennsylvania and Maryland corn-producing areas to better define lepidopteran migratory pathways.

The fall armyworm triose phosphate isomerase (Tpi) gene as a marker of strain identity and interstrain mating.

ABSTRACT Fall armyworm, Spodoptera frugiperda (J.E. Smith), is a significant agricultural pest in the United States, affecting most notably sweet corn, Zea mays L., and turfgrass. Two strains exist that differ physiologically and behaviorally but are morphologically indistinguishable. Polymorphisms within the fall armyworm Triose phosphate isomerase gene (Tpi; EC 5.3.1.1) were characterized and used to develop a polymerase chain reaction-based method for discriminating fall armyworm strains. The Tpi markers were used in multilocus analysis to estimate interstrain hybrid frequency and their distribution in populations from the United States and Brazil. The results were compared with previous studies using different marker combinations to test hypotheses about interstrain mating behavior and to explain earlier observations of unusual strain distribution patterns. The importance of understanding the frequency and consequences of interstrain hybridization to deciphering the mechanism of strain divergence and the distribution of fall armyworm subpopulations in different habitats are discussed.