Fall armyworm
The fall armyworm (Spodoptera frugiperda J. E. Smith) is a moth species in the family Noctuidae, native to the tropical and subtropical Americas from the southern United States to Argentina, where its polyphagous larvae infest over 80 plant species, predominantly gramineous crops such as maize, sorghum, and rice.[1] The greenish-brown caterpillars, marked by longitudinal stripes and dark spots, grow to 34 mm in length across six instars and cause damage by skeletonizing foliage, boring into whorls, stems, and ears, often resulting in 5–20% yield reductions in maize at densities of 0.2–0.8 larvae per plant during the late whorl stage.[1] Adults, with a wingspan of 32–40 mm, are strong fliers capable of long-distance dispersal, lacking diapause to enable continuous generations and annual migrations northward in their native range, while larval aggregations exhibit marching behavior that consumes vegetation in their path, giving rise to the "armyworm" designation.[1] Since its initial detection in sub-Saharan Africa in 2016, S. frugiperda has rapidly invaded Asia and other continents, exacerbating global food security risks through its high fecundity, broad host range exceeding 350 species, and propensity for pesticide resistance, with annual economic losses from maize and other staples estimated at billions of dollars.[2][3]Taxonomy and Systematics
Classification and Strains
Spodoptera frugiperda, the fall armyworm, is classified within the insect order Lepidoptera and family Noctuidae. Its complete taxonomic hierarchy is Kingdom: Animalia; Phylum: Arthropoda; Class: Insecta; Order: Lepidoptera; Family: Noctuidae; Genus: Spodoptera; Species: S. frugiperda (J. E. Smith, 1797).[4][5] The species comprises two genetically distinct host strains: the corn strain (C-strain or sfC) and rice strain (R-strain or sfR), which are morphologically identical but differ in host plant preferences and certain physiological traits.[6][7] The C-strain predominantly attacks gramineous crops like corn (Zea mays), sorghum (Sorghum bicolor), and occasionally cotton (Gossypium spp.), showing optimal larval development on these hosts.[8] In contrast, the R-strain favors rice (Oryza sativa), millet (Pennisetum spp.), and forage grasses such as bermudagrass (Cynodon dactylon), with larvae exhibiting faster weight gain and shorter development times on these plants compared to corn.[9][10] These strains are distinguished genetically via mitochondrial DNA markers (e.g., COI gene haplotypes) and nuclear microsatellite loci, revealing limited gene flow despite occasional hybridization in overlapping ranges.[11][12] Cross-host experiments demonstrate strain-specific nutritional adaptations, as R-strain larvae on corn experience delayed development and reduced survival, while C-strain larvae perform suboptimally on rice.[9][10] Invasive populations of S. frugiperda in Africa, Asia, and Europe since 2016 are overwhelmingly C-strain, based on genomic analyses showing closer affinity to American C-strain references than R-strain.[13] This predominance may reflect greater dispersal capability or agricultural vulnerability in invaded regions dominated by corn cultivation.[13][6]Morphology
Eggs
Female Spodoptera frugiperda moths deposit eggs in clusters, with each mass typically containing 100 to 200 eggs, though variation occurs based on environmental conditions and female physiology. A single female produces an average of approximately 1500 eggs over her lifetime, with recorded maxima exceeding 2000.[1] These clusters are preferentially laid on the underside of host plant foliage, such as maize leaves, but high population densities can lead to oviposition on upper leaf surfaces, stems, or even non-host structures. The egg masses acquire a characteristic fuzzy, grayish-white appearance due to coverage by scales from the female's abdomen.[1] [14] Eggs are spherical, measuring about 0.4 mm in diameter, and initially creamy white or pale yellow in color, gradually darkening to light gray or brown as embryogenesis nears completion and hatching impends.[15] [16] The incubation period is temperature-dependent, lasting 2 to 4 days at 21–27°C under summer conditions, but extending to 7.5 days at 19°C or shortening to 2.0 days at 31°C; hatchability remains high across this range, often exceeding 95% in optimal lab settings.[14] [17] Upon eclosion, first-instar larvae consume the chorion (eggshell) remnants before ballooning via silk threads or crawling to adjacent plant tissues for initial feeding, a behavior that facilitates rapid dispersal within the mass.[1] Factors such as humidity and parasitoid pressure can influence egg viability, with desiccation reducing survival rates in arid environments.[16]Larvae
The larvae of Spodoptera frugiperda pass through six instars, with head capsule widths ranging from 0.35 mm in the first instar to 2.6 mm in the sixth, and body lengths increasing from 1.7 mm to 34.2 mm.[1] Newly hatched larvae appear greenish with a black head capsule that shifts to orangish in the second instar.[1] In later instars, the body develops a brownish or greenish hue, featuring white subdorsal and lateral longitudinal lines, a reddish-brown head mottled with white and marked by an inverted "Y" on the face, and four dark dorsal spots per abdominal segment arranged in a square pattern; a greenish form may exhibit pale spots instead.[1] The epidermis is rough-textured, aiding identification.[1] Developmental duration for the larval stage averages 14 days under summer conditions, extending to 30 days in cooler weather, with instar-specific times at 25°C of approximately 3.3, 1.7, 1.5, 1.5, 2.0, and 3.7 days, respectively.[1] [18] Larvae exhibit color polymorphism between typical brownish and rarer greenish morphs, potentially linked to host plant or environmental factors, though morphological traits remain consistent across strains.[1] Early instars feed gregariously on foliage, creating small holes and "windowpane" damage by skeletonizing leaves, while larger larvae become more solitary and cannibalistic, often reducing populations to one or two per plant.[1] They preferentially target tender tissues such as corn whorls, producing characteristic rows of perforations, and may burrow into growing points, stems, or ears, consuming kernels and causing defoliation or ragged leaves.[1] Feeding occurs primarily at night, with larvae concealing in plant crowns or soil during daylight to avoid predation.[1] This polyphagous behavior affects over 100 plant species, predominantly grasses and cereals like maize, sorghum, and rice, with maize supporting the fastest larval development.[1] Damage intensity peaks in mid- to late-whorl stages of crops, where larval densities of 0.2 to 0.8 per plant can yield 5 to 20% reductions in grain production.[1]Pupae
Mature larvae of Spodoptera frugiperda cease feeding and burrow into the soil to depths of 2 to 8 cm to initiate pupation, constructing a loose, oval-shaped earthen chamber approximately 20 to 30 mm long that serves as a cocoon.[1] This prepupal wandering and burrowing phase typically lasts 1 to 2 days before the larval skin is shed, revealing the pupa.[19] Pupation predominantly occurs in soil but may rarely happen in plant debris or other sheltered sites under field conditions.[16] The pupa is stout and reddish-brown, tapering slightly toward the posterior end, with a length of 14 to 18 mm and width of about 4.5 mm.[1] Female pupae are generally larger than males, averaging 17.2 mm versus 14.3 mm in length, respectively, with sexual dimorphism also evident in genital and anal structures.[20] [21] Pupal development duration varies with temperature, ranging from 8 to 14 days under laboratory conditions at 25–27°C, with a mean of about 9 to 10 days.[1] [19] At lower temperatures of 19°C, it extends to 21–24 days, while at 31°C, it shortens to 6–7 days; the minimum developmental threshold is approximately 13°C.[22] [23] Optimal development occurs between 28–30°C, beyond which survival may decline.[24] Pupal emergence rates typically range from 60% to 94%, influenced by environmental factors and strain.[20] Upon completion, adults eclose from the pupal case, leaving behind the exuviae in the soil chamber.[1]Adults
The adult stage of Spodoptera frugiperda consists of moths with a wingspan measuring 32 to 40 mm.[1] Males display sexual dimorphism in forewing coloration, featuring shades of gray and brown with distinct triangular white spots near the tip and center, whereas females possess more uniform gray forewings with faint, obscure lines and spots.[1] [25] Hindwings are predominantly white in both sexes, and the body length ranges from 20 to 25 mm.[1] Adults are nocturnal, concealing themselves during daylight in vegetation or debris, and exhibit courtship behaviors including parabolic, circular, and zigzag flight patterns, flapping, and crawling.[16] [26] Flight activity influences reproductive processes, with extended flights shortening pre-oviposition periods and enhancing egg-laying synchronization.[27] Females typically live about 14 days, during which they deposit 1,000 to 2,000 eggs in egg masses on host plants, prioritizing reproduction over substantial feeding on nectar.[28] Adult nutrition, such as honey, can elevate mating rates to 79.7% and fecundity to approximately 645 eggs per female while delaying male pheromone decay.[29] Mating induces sex-specific behavioral and transcriptional changes, modulating post-mating activities in both sexes.[30]Life Cycle
Developmental Stages
The fall armyworm (Spodoptera frugiperda) exhibits complete metamorphosis, progressing through four distinct developmental stages: egg, larva, pupa, and adult.[1] The duration of these stages is highly temperature-dependent, with optimal development occurring between 25–32 °C, where the full egg-to-adult cycle completes in approximately 23–30 days; lower temperatures (e.g., 20 °C) extend it to 50 days or more, while extremes below 14 °C or above 34 °C can halt or prevent development.[24][31] Larval development, in particular, shows strong thermal sensitivity, spanning 8–18 days across instars under favorable conditions.[31] Eggs are laid in clusters of 50–500 or more, typically on the undersides of leaves, and coated with a grayish-white layer of scales from the female's abdomen, providing camouflage and protection.[1] Hatching occurs after 1.5–4 days at 25–30 °C, with the embryonic period shortening to under 2 days at higher temperatures (e.g., 32 °C) and extending beyond 3 days below 20 °C; viability decreases sharply outside 18–32 °C.[32][33] Upon emergence, first-instar larvae are pale green and measure about 1.5 mm in length.[1] The larval stage, the primary feeding and damaging phase, consists of six instars, with total duration ranging from 9–14 days in warm conditions (e.g., 26–34 °C) to 30–82 days at cooler temperatures (e.g., 14–20 °C).[1][34] Early instars (1–3) are gregarious, light-colored with dark heads marked by a white inverted "Y," and feed skeletonizing leaves; later instars (4–6) grow to 30–50 mm, adopt solitary habits, develop longitudinal stripes, and bore into plant whorls or ears, consuming up to 85% of their total food intake in the final instar.[1][35] Instar durations progressively shorten with age and rising temperature, but host plant quality and density can modulate growth rates.[32] Pupation follows larval maturation, with individuals burrowing 5–10 cm into soil to form a reddish-brown, exarate pupa approximately 13 mm long.[1] This stage lasts 7–16 days at 20–32 °C, with shorter times (e.g., 7–9 days) at warmer optima and prolongation or mortality at extremes; soil moisture and texture influence survival, as dry conditions increase desiccation risk.[24][31] Adults emerge as nocturnal moths with a wingspan of 32–40 mm; males are darker gray with a white spot near the hindwing tip, while females are lighter with a tan band.[1] Adult longevity averages 10 days (range 7–21 days), during which females mate within 1–2 nights and oviposit up to 3,000 eggs in multiple clusters over 4–7 days; development from pupa to eclosion aligns with prior stages' thermal responses.[1][36] Multiple generations (up to 5–8 annually in tropical regions) arise from rapid cycling under suitable conditions.[37]| Stage | Typical Duration (25–30 °C) | Temperature Influence |
|---|---|---|
| Egg | 2–3 days | 1.5 days at 32 °C; >3 days at <20 °C[24] |
| Larva (6 instars) | 12–14 days | 9 days at 34 °C; 30+ days at 14–20 °C[34] |
| Pupa | 8–12 days | 7 days at 32 °C; 16+ days at 20 °C[31] |
| Adult | 7–14 days | Shorter at higher temps; oviposition peaks early[1] |