First report of natural infestation of Ovomermis sinensis (Nematoda: Mermithidae) parasitizing fall armyworm Spodoptera sp. (Lepidoptera: Noctuidae) in Himachal Pradesh, India

Spodoptera frugiperda invaded in India at the end of 2018 and has caused severe damage to maize including other crops. Several S. frugiperda naturally parasitized by nematodes were observed in Nauni, Himachal Pradesh, India. The morphological features based on the results of scanning electron microscopy indicated that the nematode belongs to the family Mermithidae. Furthermore, characters discovered that the nematode belongs to Ovomermis sinensis, a known entomoparasitic nematode. Our nding was the rst record that S. frugiperda, was naturally parasitized by O. sinensis. The results of this study are of great importance for potential biological control of S. frugiperda by indigenous natural benecial organisms, i.e., O. sinensis within an integrated pest management system.


Background
Spodoptera frugiperda (J. E. Smith), fall armyworm (FAW) is a pest native of tropical to subtropical regions of America and widely distributed throughout the American continents (Todd and Poole, 1998). The indigenous pest is highly polyphagous, causing economic damage to various crops viz., cotton, maize, beans and sorghum (Day et al., 2017). It has already invaded many countries in Africa and Asia and is causing substantial yield losses due to its voracity, high dispersal ability, wide host range, and high fecundity. . FAW causes up to $16 billion in crop losses across Africa annually (Harrison et al., 2019). FAW invaded Yunnan province, China in late 2018. FAW then, dramatically spread in China (Zhang et al., 2019;Li et al., 2019)  FAW has developed resistance to a variety of chemical pesticides in several countries due to intensive pesticide application, (Yu et al., 2003;Zhao et al., 2019), i.e., substantially high levels of resistance to omethoate was reported in FAW captured in China (Zhao et al., 2019). Based on the adverse effects of some chemicals on human health, the environment, and living organisms, researchers are focusing on potential biological control agents (Harrison et al., 2019). FAW is attacked by various natural biocontrol agents viz., parasitoids (Lopez et al., 2018), bacteria (del Valle Loto et al., 2019), fungi (Shylesha et al., 2018), nematodes (Ruiz-Nájera et al., 2013;Viteri et al., 2018) and virus (Souza et al., 2019). There is a dearth of information on natural enemies of FAW, in China, particularly because the insect only invaded the region within the past year. It is high time to develop biological control methods for FAW in China. Although assessing the virulence of commercial biological control agents is a crucial way to control FAW from the perspective of environmental protection, there is also a great need to understand, promote and maximize the effectiveness of indigenous populations of natural enemies. Nematodes are the most abundant metazoan on earth. Nematodes in numerous families can kill insects, known to be entomoparasitic nematodes. Mermithidae, Steinernematidae, and Heterorhabditidae have been studied a lot and some are considered as biological control agents for FAW (Huot et al., 2019;Ruiz-Najera et al., 2013;Tarla et al., 2015;Viteri et al., 2018).
Recently, we found FAW naturally parasitized by a mermithid nematode in a eld located in Nauni, Himachal Pradesh, India and that is the rst report of O. sinensis (Nematoda: Mermithidae) parasitizing FAW S. frugiperda (Lepidoptera: Noctuidae) in India. Our discovery can provide vital and useful information on policymaking for the control of FAW in India and Asia.

Field survey
From March 27, 2019, to September 20, 2020, the collection of FAW was carried out in the tomato and corn elds of Nauni, Solan, Himachal Pradesh. Large numbers of larvae were collected by a chessboard sampling method in maize elds in the above areas. The instar and number of FAW larvae obtained were recorded and all insects were brought back to the laboratory for feeding and observation. Digital images were obtained using Olympus BX63 and the nematodes were removed from the parasitized FAW.

Results
The prevalence of parasitic nematodes in parts of Nauni, Himachal Pradesh is shown in Table 1. According to the data of the survey, the parasitic rate of the nematodes in FAW is about 2%. FAW was found during the cultivation process and the survey distribution of nematode infection rate is shown in Table 1.

Diagnostic characters
Based on morphological characteristics, these nematodes obtained from Nauni Himachal Pradesh belong to the family Mermithidae. This nematode has a stylet on the anterior portion of the post-parasitic juvenile (Fig. 1A) and a post-parasitic tail appendage (0.62mm) on the posterior end of the post-parasitic juvenile (Fig. 1B). The mermithids are white and 225.3mm in length (Fig. 3). S. frugiperda parasitized by the nematodes is illustrated in (Figure 4).

Discussion
For our knowledge, this is the rst report that S. frugiperda was parasitized by O. sinensis in a natural context, an indigenous mermithid nematode in Himachal Pradesh, India. In a survey conducted in Mexico, mermithid nematode species from another genus, Hexamermis sp., was observed to be parasitic on FAW, causing a mortality rate of 8.42 % (Ruiz-Najera et al., 2013). S. frugiperda naturally parasitized by nematodes were recorded in Hainan Province, China (Sun et al., 2020). The present study con rms new associations of mermithid nematodes O. sinensis as natural enemies of S. frugiperda.
O. sinensis had been e ciently used to control pest insects from family Noctuidae, i.e. Spodoptera litura and Helicoverpa armigera (Li et al., 2003) as well as M. separata (Walker) (Chen et al., 1991;Zhang et al., 1992). S. frugiperda, also from Noctuidae, was found naturally parasitized by O. sinensis in the preset study. All these ndings con rmed the great potential of O. sinensis for control against the invasive pest insect S. frugiperda and other pest insects from Noctuidae. An intriguing nding in this study is that we can exploit the great potential to use O. sinensis to control FAW. Information on the occurrence and rates of parasitism of indigenous natural enemies is of paramount importance in designing a biological control program for S. frugiperda, either through the conservation of native natural enemies or the introduction of new species for augmentative release. Major eld releases of mermithids have been conducted for Anopheles population control (Kobylinski et al., 2012). Therefore, further study is needed to understand the biological and ecological relationship between O. sinensis nematodes and their hosts and to explore the approach to arti cially rear this nematode for potential eld application in the future. The authors declare that they have no con ict of interest.