Rod N. Nagoshi

BEHAVIOR AND DISTRIBUTION OF THE TWO FALL ARMYWORM HOST STRAINS IN FLORIDA

Abstract Fall armyworm is a significant agricultural pest in the United States, affecting most notably sweet corn and turf grass. While infesting much of North America, fall armyworms invading the eastern United States arise from annual migrations of populations wintering in southern Florida. It has long been noted that this seasonal geographical localization represents an opportunity for controlling this pest prior to its annual migration. However, such efforts have been hindered by the presence of two genetically distinct but morphologically identical strains that differ physiologically and behaviorally. The biology of the host strains is poorly understood and this lack of knowledge precludes accurate predictions of fall armyworm population behavior in the field. This paper reviews recent studies examining strain behavior and discusses the potential relevance of these results to the development of effective regional management strategies that can be used proactively to mitigate the economic impact of this pest.

FR Tandem-Repeat Sequence in Fall Armyworm (Lepidoptera: Noctuidae) Host Strains

Abstract The fall armyworm, Spodoptera frugiperda (J. E. Smith), consists of two sympatric strains (rice and corn) that are morphologically identical. To facilitate strain identification, a polymerase chain reaction (PCR)-based method was developed to detect FR, a reportedly strain-specific element found in the nuclear genome in large tandem-repeat clusters. The efficacy and accuracy of the PCR method were tested on inbred laboratory lines of both rice and corn strains. Although it was previously thought that FR sequences were exclusive to the rice strain, we were able to isolate homologous sequences in a subset of the corn strain population. Even in these cases, the PCR method detected strain-specific amplification patterns, indicating that the distinction between rice and corn strains is caused by differences in copy number and organization in the genome rather than simply the presence or absence of FR. A second polymorphism detectable by PCR was also found that distinguishes between rice and corn strain laboratory colonies. The availability of PCR-based methods for detecting strain- and population-specific molecular markers will facilitate studies on strain distribution in the field and the examination of strain-specific behavior.

Identification and Comparison of Fall Armyworm (Lepidoptera: Noctuidae) Host Strains in Brazil, Texas, and Florida

Abstract Fall armyworm, Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), is a major economic pest throughout the Western Hemisphere. Studies of populations in the southern United States and the Caribbean demonstrated the existence of two morphologically identical but genetically distinct host strains. These races can be distinguished by using polymorphisms in the mitochondrial cytochrome oxidase I gene that define two distinct maternal lineages that correlate with strain-specific behaviors in Florida populations. Although there is evidence of different biotypes in Brazil, it has not been demonstrated that these biotypes are equivalent to the U.S. strains. Sampling from Brazil indicates that its fall armyworm populations consist of the two strain-specific haplotypes found in Florida and also display the expected biases in plant host distribution. The same genetic markers also were present in samples from Texas, a major source of migrating fall armyworm in North America. These results indicate that the biology and behaviors of Brazilian fall armyworm populations are similar to those found in North America.

Comparison of Haplotype Frequencies Differentiate Fall Armyworm (Lepidoptera: Noctuidae) Corn-Strain Populations from Florida and Brazil

Abstract Fall armyworm,Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), is a major economic pest throughout the Western Hemisphere. Populations can be subdivided into two morphologically identical but genetically distinct strains (corn-strain and rice-strain) that differ in their host plant preferences. These strains can be distinguished by using polymorphisms in the mitochondrialcytochrome oxidase I gene. Additional sequence analysis of this locus identified two sites that were highly polymorphic in the corn-strain population and that produced four different haplotype subgroups. Comparisons of the frequency distribution of these haplotypes found no seasonal or plant host specificities, but they did demonstrate that the Brazil corn-strain population is different from corn-strain fall armyworm found in Florida. The development of a rapid means of distinguishing fall armyworm populations originating from Brazil versus Florida provides an opportunity for investigating and comparing the genetic complexity and long-range movements of this important agricultural pest.

Seasonal Distribution of Fall Armyworm (Lepidoptera: Noctuidae) Host Strains in Agricultural and Turf Grass Habitats

Abstract Male fall armyworm moths [Spodoptera frugiperda (J. E. Smith)] were captured in pheromone traps over a 16- to 24-mo period in selected sites in southern Florida. Molecular markers were used to determine whether individuals were of one of two host strains (historically designated “rice-strain” and “corn-strain”). Traps placed in agricultural areas showed a population peak in the spring (March–May) and fall (October–December), with a prolonged decline in numbers in summer (July–October) and a smaller reduction in mid-winter (January). The host strain distribution during these periods varied significantly, suggesting strain-specific and seasonal population patterns. Both strains were captured in substantial numbers during the spring peak, but surprisingly, only the rice-strain showed an increase in capture rates during the fall, despite the presence of sweet corn throughout this period. Trap captures in a sod (turfgrass) farm were composed almost entirely of the rice-strain and showed a bimodal seasonal distribution similar to that seen in the agricultural areas, with peaks in the spring and fall. These results represent the first indication that the two host strains might have substantially different population dynamics in the overwintering agricultural areas of Florida and suggest that the rice-strain is the predominant fall armyworm pest during the fall and winter growing periods. It further indicates that the two strains can display a markedly different response to seasonal environmental cues. The implications of these findings on our understanding of fall armyworm migration are discussed.

Effects of Fall Armyworm (Lepidoptera: Noctuidae) Interstrain Mating in Wild Populations

Abstract Fall armyworm is a significant agricultural pest in the United States, affecting most notably sweet corn and turfgrass. Two morphologically identical strains, rice strain (R-strain) and corn strain (C-strain), exist that differ physiologically and behaviorally and can be identified by mitochondrial haplotyping. Recent studies of overwintering populations in Florida indicate that the mitochondrial lineage associated with the R-strain itself consists of two genetically distinct subgroups, with one having molecular markers consistent with interstrain hybridization between R-strain females and C-strain males. To test this possibility and examine the ramifications of interstrain mating on population behavior and strain fidelity, larvae and adult males were tested for genetic marker combinations representative of the host strains and potential hybrids. These studies showed a sexually dimorphic distribution pattern for a sex-linked marker that is a predicted result of interstrain mating. Despite evidence of substantial interbreeding in the overwintering sites, both FR and the strain-diagnostic mitochondrial markers still showed the plant host and habitat biases associated with the host strains, indicating that strain integrity was largely maintained. However, there is evidence that the two R-strain subpopulations differ in habitat distribution in a manner suggestive of the “hybrid” genotype being less specific in its plant host preference. The existence of a genetically distinct hybrid subpopulation must be taken into account when evaluating fall armyworm population dynamics and infestation patterns in overwintering areas.

Using stable isotope analysis to examine fall armyworm (Lepidoptera: Noctuidae) host strains in a cotton habitat.

Abstract 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.

Genetic Variation in Neonate Behavior of Fall Armyworm (Lepidoptera: Noctuidae)

Abstract Bioassays were developed to test plant selection of fall armyworm (Spodoptera frugiperda (J. E. Smith)) host strains to corn (Zea mays L.) and stargrass, a forage grass closely related to bermudagrass (Cynodon nlemfuensis Vanderyst var. nlemfuensis). Neonate larvae from 3 corn strain and 3 rice strain colonies preferentially selected corn over stargrass in petri dish choice tests. However, bioassays of whole plants and whole plant volatiles showed that selection of a particular host was not clear and there were no significant differences in plant choice. Two additional bioassays were conducted to determine if larvae would continue to disperse once they came in contact with a plant source. One colony was always biased towards corn regardless of which plant was encountered first. For 4 colonies, the attraction to corn was reduced such that when stargrass was first contacted, equal or greater numbers of larvae stayed and did not migrate to corn. Finally, the attraction to corn was lowest in 1 colony in which significantly more larvae moved away from corn even when it was presented first. Results of our study, along with behavioral and feeding trials from other studies, suggest that there is a consistent attraction of neonates to damaged corn regardless of “strain” designation, but substantial variability in the strength of that attraction if they come in contact with another plant host first. These differences among colonies most likely reflect genetic and phenotypic variation in wild populations. More information at http://www.ars.usda.gov/pandp/people/people.htm?personid=11809

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

ABSTRACT 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.