INTRODUCTION
The fall armyworm, Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), is a polyphagous migratory pest that feeds on over 80 species of plants, mostly known as a major pest of maize and various cereals (Montezano et al., 2018; Spark, 1979). There are two morphologically indistinguishable host-plant specific strains ofS. frugiperda , the “corn strain” and the “rice strain”. The corn strain prefers to feed on corn, cotton, and sorghum, while the rice strain prefers rice and various pasture grass (Dumas et al., 2015). These two strains have been hypothesized to start diverging about 2 Myr ago and continue to diversify (Gouin et al., 2017a).
The females lay eggs on abaxial leaf surfaces of host leaves and lay up to 200 eggs in an egg mass on a single host (Kumela et al., 2019). At high densities, S. frugiperda larvae can completely defoliate their host plants and can cause severe yield losses in many economically important crops. In Brazil, attacks from S. frugiperda can reduce the corn grain yield up to 34%, causing losses of 400 million US dollars annually (Lima, Silva, Oliveira, Silva, & Freitas, 2010).
S. frugiperda is native to the tropical and subtropical Americas but was first detected in western and central Africa in early 2016 (Goergen, Kumar, Sankung, Togola, & Tamo, 2016) and has spread rapidly across almost all sub-Saharan countries. Subsequently, it was verified in several Asian countries including India (Sisay et al., 2018), Thailand, Myanmar, and Bangladesh in 2018, and recently China (Jing et al., 2019). In Korea, the first observation of S. frugiperda was made in cornfields at Jeju in early June 2019 (Seo et al., 2019). By the end of August 2019, S. frugiperda larvae were detected in cornfields of eight provincial regions in South Korea. In most cases, at least two or more other lepidopteran pests, such as S. litura(Fabricius), S. exigua (Hübner), Mythimna separata(Walker) and Helicoverpa armigera (Hübner), were observed in the same cornfields. Egg mass morphology, larval morphology and plant damage may be similar for different species. Therefore, for effective pest management and monitoring of invasive S. frugiperda , an effective identification method is required to distinguish it from similar species.
Molecular diagnostics to distinguish the two host strains of S. frugiperda are based on conventional PCR, restriction digestion, and sequencing of part of the mitochondrial COI gene (Cock, Beseh, Buddie, Cafa, & Crozier, 2017; Otim et al., 2018) and sequencing of part of the nuclear-encoded, sex-linked Tpi gene (Jing et al., 2019). Species identification utilizes random amplification polymorphic DNA (RAPD)- polymerase chain reaction (PCR) (Martinelli, Barata, Zucchi, Silva-Filho Mde, & Omoto, 2006), amplified fragment length polymorphism (AFLP) (Clark et al., 2007; Martinelli et al., 2007) and real-time PCR based on the mitochondrial cytochrome b gene to distinguish among S. eridania , S. frugiperda , S. littoralis , and S. litura (Van de Vossenberg & Van der Straten, 2014) . However, these molecular tools are expensive, time consuming, and depend on specialized equipment. A simpler technique, termed loop-mediated isothermal amplification assay (LAMP), is also widely used for the rapid identification of pest species (Blaser, Diem, von Felten, Gueuning, Andreou, Boonham, Tomlinson, Muller, Utzinger, Frey, Frey, et al., 2018; Blaser, Diem, von Felten, Gueuning, Andreou, Boonham, Tomlinson, Muller, Utzinger, Frey, & Buhlmann, 2018; Hsieh, Wang, Chen, & Ko, 2012; Y. H. Kim, Hur, Lee, Choi, & Koh, 2016). The LAMP assay was developed for rapid, simple, effective and specific amplification of DNA. It is performed under isothermal conditions that require a set of four primers, a strand-displacing DNA polymerase, and a water bath or heat block to maintain the temperature at about 65 ºC following a one-time denaturation at 95 ºC (Notomi et al., 2000) or one step incubation at about 65 ºC (Nagamine, Hase, & Notomi, 2002).
Following the first infestation of S. frugiperda in Korea, there is great demand from agricultural research, extension services, and farmers for diagnostic methods for these species. Therefore, we present a method based on LAMP based on specimens collected in Korea and other sequences from GenBank. This method should be useful in assisting effective pest management of S. frugiperda.
MATERIALS AND METHODS