The fall armyworm Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), is one of the most aggressive pests worldwide, and causes considerable losses in host crops. The fall armyworm comprises two strains, the rice strain, which feeds upon rice and other grass plants, and the maize strain, which refers to maize and sorghum (Abbas et al., 2022; Kumar et al., 2022). When fall armyworm attacks in huge numbers, it causes a long-term threat and damaging to many crops (Baudron et al., 2019; Makgoba et al., 2021; Mutyambai et al., 2022). The synthetic chemical insecticides are still the most effective and regularly utilized technique; however, these chemicals are risky to the ecosystem and have adverse effects, e.g. the development of resistance in the insect pests, which increased the inevitability of search for safe and long-term, target-specific, and cost-effective alternatives. Several studies reported the resistance of the fall armyworm insect to most of the active synthetic insecticides; therefore biological control is considered an interesting option for insect pest management (Ahissou et al., 2021). Several species of the fall armyworm natural enemies were reported. Sisay et al. (2018) encountered 150 parasitoid species attack the fall armyworm. It is also attacked by diverse taxa of natural enemies (Harrison et al., 2019; Koffi et al., 2020) and entomopathogens such as viruses, bacteria, and fungi (Shylesha et al., 2018). The entomopathogenic fungi are thought to be an essential bio-control agent for this invasive pest. Recently, some entomopathogens were isolated from dead fall armyworm insect, in Africa and Asia, such as Spodoptera frugiperda Nuclear Polyhydrosis Virus, Bacillus thuringiensis, B. bassiana, and M. anisopliae (Chinwada, 2018; Shylesha et al., 2018; Sharanabasappa et al., 2019; Firake & Behere, 2020). Microbial insecticides neither induce resistance in their host insects nor deposit toxic residues in the environment (Qadri et al., 2020; Sato et al., 2021). The public awareness for healthful environment increased the attention on microbial bio-control agents (Paddock et al., 2021).
Mortality of the fall armyworm caused by mycopathogens
In the current investigation, the used entomopathogenic isolates showed high virulence against the early instars of the fall armyworm larvae. However, the fifth instar of the fall armyworm larvae showed the lowest sensitivity to the two entomopathogenic fungi. Idrees et al. (2021) evaluated the effect of five fungal isolates against immature instars of S. frugiperda larvae. The results showed significant effects on larvae mortality as B. bassiana ZK-5 reduced the feeding activity of first to third stage of S. frugiperda larvae. Sutanto et al. (2021) reported that the larvae hatched from treated eggs were decreased by the entomopathogenic fungi at 1x109 conidia ml− 1. The hatchability was significantly reduced by 26.84% in B. bassiana and 46.48% in M. anisopliae. Conidial concentrations of M. anisopliae, B. bassiana, and Isaria fumosorosea which ranged from 1 x 105 to 1 x 108 conidia ml− 1 demonstrated virulence on the S. litura eggs with mortality of 48.2–71.6% (Afandhi et al., 2020). Among the five tested fungal strains, they found that C. tenuissimum SE-10, B. bassiana ZK5, and P. citrinum CTD24 showed significant effects on the insect mortality (Idrees et al., 2021). In this study, conidial concentrations of B. bassiana AUMC3563 and M. anisopliae AUMC2605 demonstrated virulence on the S. frugiperda larvae with mortality ranged from 10.0 to 80.0% under lab conditions.
B. bassiana AUMC3563 strain at conidial concentrations ranged from 1.05 × 107 to 5.6× 107 conidia ml-1 achieved 80% mortality in the early instars of the fall armyworm larvae. Also, using dipping technique the fungal pathogen strain M. anisopliae AUMC2605 was effective on the fall armyworm with larvae mortality of 70.66%. Indeed, entomopathogenic fungi are common in maize fields and naturally involved in the suppression of several crop pests (Vega, 2018). After the outbreak of the fall armyworm, many larvae infected with mycopathogens were found in maize fields (Chinwada, 2018). B. bassiana is one of the most common used biological control agents globally (Dannon et al., 2020). Recently, B. bassiana demonstrated brilliant pathogenicity against eggs and early instar larvae of the fall armyworm (Gao et al., 2022). Some commercial products, based on B. bassiana or M. anisopliae are already available and in use West Africa (Rwomushana et al., 2018). Ullah et al. (2022) demonstrated that M. anisopliae was more virulent to the fall armyworm larvae than B. bassiana. However, B. bassiana isolate was more lethal to the Myzus persicae nymphs than M. anisopliae. At concentration of 1.0 × 109 conidia ml-1M. anisopliae caused 88% mortality in S. frugiperda and 65% mortality in M. persicae. B. bassiana exhibited 76% mortality in S. frugiperda and 94% mortality in M. persicae at the same concentration. Rajula et al. (2021) isolated the entomopathogenic fungus M. rileyi during field survey of the fall armyworm S. frugiperda. This fungal species has demonstrated 95% mortality of the third larvae instar of the fall armyworm larvae. This finding could be evidence enough to start production of entomopathogenic fungus as a potential bio-pesticide and gain the farmers conviction for application to reduce the insect pest damage (Rajula et al., 2021). Recently, many insecticidal microbial strains have been isolated from soil and effectively used by limited number of farmers as bio-insecticides spray (Kumar et al., 2021). The use of the entomopathogenic fungi B. bassiana and M. anisopliae in the integrated pest management of the fall armyworm is promising because these fungi can establish endophytic habitat in maize plants (Ramos et al. 2020). Their colonization as endophytes in maize plant caused 100% mortality on the second instar larvae, while 75% and 87% mortality were detected on the forth instar larvae inoculated with M. anisopliae and B. bassiana, respectively (Ramos et al. 2020). These two entomopathogenic fungi used, caused lethal infection to the different life stages of S. frugiperda. In this present data, the fifth instar of S. frugiperda larvae showed the lowest susceptibility to the entomopathogenic fungi. The early larval instars particularly the first instar was the most susceptible to B. bassiana and M. anisopliae (Sutanto et al., 2021). The thick cuticle of pupae may serves as barrier to fungal invasion; therefore, this stage of insect life is seldom being infected by fungal pathogens (Elya et al., 2021). Thus, the virulence of a microbial pathogen not only depends on the insect species but also their developmental stage. Bio-control approaches based on B. bassiana and Metarhizium anisopliae don’t just depend upon the interactions between pathogen and host but also on the ambient environmental conditions to which mycopathogens are exposed (Xu et al. 2016).
Botanical Insecticidal indices of in the field
Several plant species can produce a wide variety of secondary compounds that possess pesticidal properties and toxic to insect pests (Jaoko et al.,2020). Yarou et al. (2017) listed twenty plants that comprise insecticidal properties to control arthropod pests of vegetable crops. These bio-pesticides are expected to be more environmentally friendly with more diverse range of bioactive substances compared to synthetic chemical insecticides (Jaoko et al., 2020). Several studies reported that these botanicals were effective against fall armyworm (Siazemo et al., 2020; Kardinan et al., 2021; Rioba et al, 2020). Botanicals derived from plant extracts in Africa were effective in controlling fall armyworm with similar efficacy like chemical pesticides and also increased the production (Akeme et al., 2021). Sisay et al. 2019 reported seven potential plant extracts that were used in the control of the fall armyworm with mortality higher than 75%. Neem was probably the most effective among these pesticidal plants (Yarou et al., 2017). Moreover, the highest larval mortalities were detected with Nicotiana tabacum L. The aqueous extracts of Cassia nigricans reduced the fall armyworm infestation in maize by only 13% (Kambou & Millogo, 2019). Botanical insecticides based on capsaicin, neem, orange oil, and other aromatic herbs are commercially available in several countries (Bateman et al., 2018). Some botanical pesticides caused high mortality under laboratory conditions. For example, neem extracts demonstrated 70% reduction in the fall armyworm (Matova et al., 2020). Eucalyptus urograndis was found to possess properties to protect maize from pests (Andrade et al., 2016). The Carica papaya seed powder was detected as an efficient bio-insecticide (Ogbonna et al., 2021). Neem oil at concentration of 0.17–0.33% reduced the fall armyworm damage in maize (Abbas et al., 2022). Botanical insecticides are non-hazardous, target-specific, and environmentally safe for natural enemies (Ahmed et al., 2021). In this investigation, Jojoba oil was probably the most effective among the pesticidal plant oils; the highest larval mortalities were detected at 3% v/v of the Jojoba oil (100% mortality). However, the lowest effective botanical insecticide was Nigella sativa oil. The botanical pesticide efficacy is most likely due to the secondary metabolites in the plant such as amides, isobutyl amides, natural lipophilic, and piperine which act as an anti-feedant, deterrent, and neurotoxin (Akeme et al., 2021). Presently, the pesticides market is dominated by the synthetic chemicals despite the development of various botanical pesticides and entomopathogens (Ngegba et al., 2022). Actually, alternative strategy to chemical insecticides still plays a little role in the sustainable agriculture and in integrated pest management. Extracts of neem plant biopesticides are not available on a large commercial scale due to the lack of financial support, materials, and appropriate equipment (Yarou et al., 2017). Our study provides important information of natural products for the field application of biological control of the fall armyworm. Nevertheless, more researches are needed to confirm the active property of these natural products as insecticides responsible for protection. Thus, the potential effective botanical insecticides should be communicated and made available for sustainable controlling of the invasive fall armyworm (Deguine et al. (2021). Furthermore, tests must be conducted in greenhouse and field experiments to assess the efficacy of these microbial and botanical pesticides, which can be detected by the reduction in damage and yield increase.