Robert L. Meagher

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.

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.

Susceptibility of Field Populations of the Fall Armyworm (Lepidoptera: Noctuidae) from Florida and Puerto Rico to Purified Cry1f Protein and Corn Leaf Tissue Containing Single and Pyramided Bt Genes

ABSTRACT Larval survival of Cry1F-susceptible (FL), -resistant (PR and Cry1F-RR), and -heterozygous (FL x PR and Cry1F-RS) populations of the fall armyworm, Spodoptera frugiperda (J. E. Smith) to purified Cry1F protein and corn leaf tissue of 7 Bacillus thuringiensis (Bt) corn hybrids and 5 non-Bt corn hybrids was evaluated in the laboratory. The 7 Bt corn hybrids represent 5 Bt corn traits: Herculex®I, which expresses a single Bt protein (Cry1F), and Genuity® VT Double Pro™, VT Triple Pro™, SmartStax™, and Agrisure® Viptera® 3111, which contain ≥ 2 pyramided Bt genes. The original FL and PR populations were collected from corn fields in 2011 in Florida and Puerto Rico, respectively. Diet-incorporation bioassays showed that FL was susceptible to Cry1F protein with a LC50 value of 0.13–0.23 µg/g, while PR was highly resistant to Cry1F protein (> 137-fold). FL was also susceptible to all 7 Bt corn hybrids with a 7-day mortality of > 95%, while PR and a backcrossed and reselected population, Cry1F-RR, were highly resistant to Cry1F corn leaf tissue. The resistance was recessive or incompletely recessive. None of the 5 populations of S. frugiperda could survive on Viptera™ 3111, suggesting this Bt corn trait can completely overcome the resistance and thus should provide a means of managing Cry1F resistance in S. frugiperda. However, Cry1F-RR exhibited a significant cross-resistance to the leaf tissue of the other 3 pyramided Bt corn traits. The possible cross-resistance between single-gene and pyramided Bt corn products suggest that careful selection of Bt genes is essential in use of gene pyramiding strategy for resistance management.

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.

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.

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

ABSTRACT n 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.

Life Table Studies of Rachiplusia nu (Guenée) and Chrysodeixis (= Pseudoplusia) Includens (Walker) (Lepidoptera: Noctuidae) on Artificial Diet

ABSTRACT Rachiplusia nu (Guenée) and Chrysodeixis (= Pseudoplusia) includens (Walker) (Lepidoptera: Noctuidae) are 2 economically important species in soybean in northern Argentina. Life cycle, reproductive and population parameters of R. nu and C. includens reared on artificial diet were determined under controlled environmental conditions. Fecundity increased from d 2 to 3 with values of 67.6 eggs per female for R. nu and 75.7 eggs for C. includens. An average of 79.9% (R. nu) and 74.4% (C. includens) of individuals reached the larval stage and 71.1% (R. nu) and 71.4% (C. includens) of individuals reached the adult stage. The life expectancy (ex) curve showed 4 and 3 periods of mortality. The survivorship curves of the 2 species resembled the theoretical type I. Life table analysis determined that R. nu and C. includens have the potential to quickly increase their populations under controlled rearing conditions. These results provide important information that will be used to improve artificial rearing procedures contributing with biological studies towards to develop management programs of both species.

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.

Monitoring for Exotic Spodoptera Species (Lepidoptera: Noctuidae) in Florida

Abstract Trapping studies were conducted in 2 Florida locations to determine if 3 Old World Spodoptera Guenée species were present. Commercially-produced lures for S. exempta (Walker), S. littoralis (Boisduval), and S. litura (F.), plus a S. litura lure made by the USDA-APHIS-CPHST laboratory at Otis ANGB in Massachusetts, were used with plastic Unitraps and placed near 2 orchid nurseries in Lake and Miami-Dade counties. One S. litura male moth was identified from collections made in Apr 2007; no other exotic species were found in either location. However, thousands of resident species were collected, including S. albula (Walker) (= S. sunia Guenée), S. dolichos (F.), S. eridania (Stoll), S. exigua (Hübner), (J. E. Smith), and S. pulchella (Herrich-Schäffer). This study exposed the amount of labor and level of technical knowledge needed for scientists involved in finding exotic Spodoptera species.

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, developmental time, and survivability when grown on different lines of grasses. These results indicate that the selection of pasture grasses made by growers can significantly and differentially affect the population densities of these grass defoliators.