Insects require adequate nutritious food for survival and metabolism and search for their food through migration or distribution. They are considered successful creatures because of their wide distribution and tremendous climatic adaptability. Food scarcity impacts insect growth and reproduction and can even result in mortality (Zhang et al., 2019). Crops are prone to attack by various insect pests, and it has been recorded that insect pests have a significant impact on the nutritional and economic values of agricultural produce. There is an urgent need to overcome pest infestation through the potential application of crop protection measures.

Several insecticides are commercially available and extensively applied to suppress pest density, but these synthetic agricultural chemicals negatively impact agro-ecosystem, crop productivity (Shahid and Khan, 2017, 2018, 2022; Shahid et al., 2017, 2018, 2019, 2021a,b; Khan et al., 2020), human health, and diverse beneficial non-target organisms. Moreover, the repeated application of the same group of pesticides leads to resistance development and pest resurgence of the insect pest population (Abrol and Shankar, 2016; Islam, 2017). Several successful attempts to manage insect pests have been made to reduce pesticide exposure by conserving the environment and preventing pollution (Haase et al., 2015). As a result, an alternative and ecologically sound strategy to improve the nutritional quality and quantity of agricultural produce for long-term crop protection has been adopted extensively (Yadav et al., 2018; Kiran Kumar et al., 2019; Hesham et al., 2021). Due to the adverse effects of insecticides, biopesticides are needed because of their eco-friendly, safe, highly effective, and quickly decomposable properties. Biopesticides manufactured from microbiomes have a non-toxic approach to controlling the insect pest population. Many agents, such as viruses, fungi, and bacteria, are used to manage the insect pest population (Rastegari et al., 2020). More than 100 pathogenic bacteria species are being extensively used in managing the insect pest population, with the entomopathogenic bacteria Bacillus thuringiensis most widely used. Despite the fact that B. thuringiensis has been demonstrated to be insightful for its entomopathogenic actions.

Apart from bacteria, fungi have also been recognized as potential candidates and are successfully employed in pest management programs. The most common entomopathogenic fungi (EPFs) are Beauveria, Metarhizium, Hirsutella, Verticillium, Lecanicillium, and Paecilomyces.

Entomopathogenic fungi have a wide host range, toxicity, and ability to suppress chewing and sucking insect pests, and have gained a significant position as a biocontrol agent (Khan et al., 2012). Entomopathogenic microbiomes are generally more eco-friendly, more suitable, specific, and less expensive. The effect of ENMs has been tested on non-target organisms, and human health demonstrated satisfactory results (Yadav et al., 2018; Kumari et al., 2020). Their use is justified, and they can survive in the natural ecosystem. Soil microbes, including bacteria and fungi, also help in the decomposition of organic matter and the reutilization of dead plant materials; plants quickly absorb nutrients from decomposing organic matter (Schmeisser et al., 2007). Fungal biocontrol agents are far more specific and have a distinct mechanism of infection. They penetrate their hosts directly through the cuticle, in contrast to bacteria and viruses, which require ingestion to thrive (Mejía et al., 2008; St. Leger and Wang, 2010). Fungi cause damage to the host by producing spores that germinate on the host's surface and subsequently proliferate inside the host's body, resulting in infection (Brivio and Mastore, 2020). The infection rate is determined by the species of fungus and the number of infectious spores (Lu and Leger, 2016). The fungus continues to develop new spores on the deceased host body after infection and death of the target organism. These spores will disperse and persist on new hosts (Jaber and Ownley, 2018). Several researchers have highlighted the various insect-pathogenic fungal species as natural colonizers/endophytes of a diverse variety of economically valuable crops, such as maize (Zea mays), coffee (Coffea arabica), potato (Solanum tuberosum), cotton (Gossypium spp.), tomato (Solanum lyocpersicum), and chickpea (Cicer arietinum) (Arnold and Lewis, 2005; Qayyum et al., 2015).

Entomopathogenic fungi are a diverse and systematized heterogeneous category with varying biology. The majority of EPF are pathogenic to insects (Shah and Pell, 2003; Scholte et al., 2004; Vega et al., 2009; Dash et al., 2018), with a high degree of efficacy in infecting their host and acting as a regulator for reducing the population of detrimental insects (Ortiz-Urquiza and Keyhani, 2013; Vidal and Jaber, 2015; Lu and Leger, 2016). Several bacterial species belonging to the family Bacillaceae have been investigated and recorded as pathogenic to invertebrates, especially insects (Castagnola and Stock, 2014; Ruiu, 2015). The most studied and extensively used bacterial species is B. thuringiensis. In bacteria, some crystal toxins (Cry and Cyt) are produced during the sporulation phase, and other toxins are also produced and released during the vegetative stage of growth. It is made by the genetic modification of bacteria, fungi, algae, viruses, protozoans, and entomopathogenic nematodes. These are produced toxins and damage the integument and gut of the insect. This review highlights the potential application of microbial formulations to reduce or suppress the pest population and enhance sustainable crop production. It is an innovative and ecological tactic, significantly employed to tackle the insect pest population, with the environment, ecosystem, and human health concerns regarded as biopesticides (Fang et al., 2014). The degree of acceptability and adoption of entomopathogens is growing substantially due to the overall performance in laboratory and field conditions; thus, investigation into their biology, ecology, and mechanism of action is gaining more scientific attention (Dong et al., 2016). Several microbial biopesticides have been discovered, developed, and commercialized in recent decades as a result of several academic and corporate research efforts. Despite their enormous potential in biocontrol operations, the application of ENMs has been neglected due to a lack of awareness among growers. In this context, this review analyzes the current state of knowledge on ENMs utilization and mechanisms as biological agents for plant growth promotion and pest suppression, consequently addressing the prospects and limitations of their adoption as alternatives to synthetic pesticides (Figure 1).

Most bacterial species belonging to the family Bacillaceae and their pathogenicity have been tested in invertebrates, especially insects (Ruiu, 2015). The most commercially used bacterial species are B. thuringiensis, a gram-positive and spore-forming bacterium. The sporulation process produces proteins with insecticidal properties in a parasporal crystal, comprising Cry and Cyt toxins, known as delta-endotoxin. Sometimes, cells release some compounds during vegetative development (De Maagd et al., 2001). Entomopathogenic bacteria are employed to suppress insect pests.

Entomopathogenic fungi are parasitic microorganisms that can infect and kill insects. They are mainly employed as biopesticides in ecological farming as a non-toxic alternative to toxic chemical insecticides, while some are used in biotechnological processes (Jaihan et al., 2016; Ríos-Moreno et al., 2016; Lovett and St. Leger, 2017). Generally, they have immobile chitinized cells (Badii and Abreu, 2006). Entomopathogenic fungi do not belong to a single phylogenetic group. To date, 12 Oomycetes species, 65 Chytridiomycota species, 339 Microsporidia species, 474 Entomophtoromycota species, 238 Basidiomycota species, and 476 Ascomycota species have been reported (Araújo and Hughes, 2016). Several biopesticides based on Metarhizium, Beauveria, Paecilomyces, Isaria, and Lecanicillium species are being applied worldwide. Because all these fungi have a broad range of action, they can infect a large range of arthropod species (Khan et al., 2012; Castro et al., 2016; Ríos-Moreno et al., 2016). They attack many insect and mite species, but particular species and fungal strains are distinct. Depending on the species of fungus and the number of infecting spores, host death might take 4–10 days. When the host dies, the fungus generates thousands of new spores on the corpse, spreading and completing the life cycle of new victims.

Beauveria bassiana was identified as the source of the destructive muscardine infection of silkworms by Agostino Bassi in 1835. It was essential in establishing the germ theory of disease. Elie Metchnikoff was among the first to suggest feasible microbial management of an insect crop pest in 1880 when he began testing the fungus Metarhizium anisopliae against grain beetles (Lord, 2005). In India, several research institutes are conducting experiments to determine entomopathogens' efficacy, including NBAIR in Bengaluru, Indian Institute of Oilseed Research (IIOR), Hyderabad, Central Plantation Crops Research Institute (CPCRI), Kayamkulam, and University of Agricultural Sciences, Dharwad. To date, studies have been conducted on species B. bassiana, Metarhizum anisoplie, Lecanicillium lecanii, Nomuraea rileyi, and Hirsutella spp. (Ramanujam et al., 2014).

As part of integrated pest management (IPM) techniques, there is increased interest in combining entomopathogenic fungi with other biological control agents. The compatibility of entomopathogenic fungi with other biological agents, including parasitoids, predators, and other microbes, has been the subject of some recent investigations. Here are a few current study examples: Wei et al. (2021) investigated the compatibility between the parasitoid wasp, Habrobracon hebetor, employed to manage several moth pests, and the entomopathogenic fungus, B. bassiana. They observed that the two plants, B. bassiana and H. hebetor, worked better together than alone to combat the moth pest, Plodia interpunctella. In research by Dubovskiy et al. (2020), the predatory mite Amblyseius swirskii and the entomopathogenic fungus Metarhizium robertsii were used to suppress the tomato russet mite (Aculops lycopersici). The scientists discovered that M. robertsii and A. swirskii worked synergistically to reduce the tomato russet mite population significantly. To manage the autumn armyworm (Spodoptera frugiperda), Wang B. et al. (2021) studied the compatibility of the entomopathogenic fungus I. fumosorosea with the bacterium B. thuringiensis. They determined that I. fumosorosea and B. thuringiensis worked together to suppress the autumn armyworm more effectively than each of them did by itself.

These studies highlight how entomopathogenic fungus may be applied with other biological pest control agents to boost pest control effectiveness and decrease dependency on chemical pesticides. However, the compatibility of various biological control agents in contexts of several pest management needs additional investigation.

An entomopathogenic virus is a tiny, essential infectious agent of nucleic acid (RNA or DNA) encased in a protective protein coat known as the capsid, which can multiply in a susceptible host cell. If a lipid bilayer envelops the nucleocapsid, it is a virion. The word virus means “poison,” derived from a Latin term.

Mathews and Shenk (1991) defined a virus as “a set of one or more nucleic acid template molecules, generally encased in a protective coat or coats of protein or lipoprotein, that can organize its replication only within suitable host cells.” Within such cells, virus replication is (i) dependent on the host's protein-synthesizing machinery, (ii) organized from pools of the required materials rather than by binary fission, (iii) located at sites that are not separated from the host cell contents by a lipoprotein bilayer membrane, and (iv) continually giving rise to variants through various kinds of change in the viral nucleic acid.

The virus nucleic acid (DNA or RNA) is crucial to the infection of the host cell and drives viral replication. These are classified into two genera, nucleopolyhedroviruses (NPVs) and granuloviruses (GVs). Both genera have a circular double-stranded DNA genome of ~80–180 kb packed within rod-shaped nucleocapsids and anticipated to encode 90–180 genes (Okano et al., 2006). Nucleopolyhedroviruses have polyhedral OBs, made of crystalline polyhedrin protein, which occludes massive virions. In contrast, granuloviruses have smaller granule-like OBs, made up of granulin protein. Every granulovirus OB contains a single virion. The Baltimore classification used to classify viruses is based on nucleic acids and capsid function (Sparks et al., 2010). When a virus enters a compatible host cell, its nucleic acid takes control of the metabolic process. It multiplies rapidly, producing new virus particles until the cell is deprived of its contents and dies; thus, a virus is classified as an obligatory parasite since it consumes the cell's material and its metabolic processes. Viruses cannot multiply “in vitro.” Although all viruses have the same fundamental structure and require host cells to reproduce, viruses in nature come in various shapes and sizes. All viruses have different morphologies, genomes, infectivities, and host ranges. Entomopathogenic viruses are both a potential and essential part of an integrated pest control approach. The few families of viruses show infectivity against the insects.

Moreover, entomopathogenic viruses belong to the Baculoviridae family. That family infests some orders, such as lepidoptera, hymenoptera, and diptera, as a natural host. These entomopathogenic viruses are managed by various agricultural and forest insect pest populations worldwide as biopesticides (Sun and Peng, 2007). Viruses are being used as a promising tool for managing economically important insect pests; various viruses, viz. nucleopolyhedrovirus (NPV), granulosis viruses (GVs), and cytoplasmic polyhedrosis viruses, are used for the management of insect pests throughout the world. The insects ingest the host-specific viral particles, and the virions infect the gut wall cells, fat body, and hemolymph, leading to the death of the insects. As a result, their application is innovative and environmentally friendly. They are non-hazardous to human health and the environment, offering novel industrial products and field uses (Sumathy et al., 1996). Baculoviruses have a single circular genome surrounded by rod-shaped nucleocapsids. To produce virions, enveloped by membrane singly or in groups, occluded in a protein matrix, forming the occlusion body (OB) (Williams et al., 2017).

Rao and Manjunath (1966) were the first to demonstrate that the Steinernema carpocapsae DD-136 strain was applied to target lepidopteran pests of rice, sugarcane, and apple in India. For scientific purposes, exotic strains of S. corpocapsae, Steinernema feltiae, Steinernema glaseri, and Heterorhabtidis bacteriophora were introduced later (Rahaman et al., 2000). Subsequently, extensive surveys were carried out to find out potent indigenous EPN species S. carpocapsae (Hussaini et al., 2001), Steinernema abbasi (Hussaini et al., 2003), Steinernema bicornutum (Hussaini et al., 2001), Steinernema thermophilum (Sudershan and Singh, 2000), Steinernema tami (Hussaini et al., 2001), and Steinernema riobrave (Ganguly et al., 2002). The product (Pusa Nemagel) has a long shelf life and inhibits white grubs, termites, and a variety of other lepidopteran pests (Ganguly et al., 2008). Steinernema thermophilum was the first EPN to be identified as pathogenic to lepidopteran eggs (Kalia et al., 2014). In Uttarakhand and Uttar Pradesh, large-scale EPN was tested in response to persistent white grub infestations in sugarcane. The Indian Agriculture Research Institute (IARI, New Delhi) undertook a biocontrol initiative, including the mass breeding and release of EPN-infested insect cadavers to counter the infestation (Mohan et al., 2017). Farmers were trained on in vivo rearing and field application as part of the research, which designed a low-cost EPN rearing method in Galleria mellonella.

Under the Insecticides Act of 1968 and the Insecticides Rules of 1971, the Central Insecticides Board and Registration Committee (CIBRC) is the regulating authority in India that governs biopesticides. This board advises the federal and state governments on technical issues concerning the manufacture, marketing, distribution, and use of insecticides, including biopesticides, to assure human health, the environment, and natural bioagents. After evaluating their formulations and validating data on efficacy, toxicity, and packaging supplied by the importer or manufacturer, the CIBRC's registration committee permits public and private enterprises for large-scale manufacturing, distribution, and sale of biopesticides stakeholders. Manufacturers can register new products under the Insecticides Act's section 9(3B) (provision registration for a novel active component used in India) or section 9(3) (regular registration).

Entomopathogens, which are microorganisms that may infect and kill insects, have been widely used in forestry and agriculture as biological control agents. However, there are a number of constraints in registering entomopathogens as pesticides, including lacking information on their effectiveness, safety, and environmental effects. The following are some of the major problems with entomopathogen registration and solutions.

Although microbial pesticides have significant advantages over traditional pest control agents, they have failed to gain widespread commercial development and implementation in India. Some factors constrain the Indian market for microbial pesticides. Product quality control concerns, such as low microbial count, which leads to poor field performance, a lack of large-scale production facilities, and the sale of unregistered items in the market, are listed as the main hurdles (Alam, 2000; Gupta and Dikshit, 2010; Arora et al., 2011; Mishra et al., 2014). According to NBAIR studies, 50%−70% of microbial biopesticide-based products in India had deficiencies such as lesser colony propagules specified on the label, excessive moisture content in solid formulations, or pollutants. It, therefore, failed to fulfill the declared CIBRC requirement (Ramasubramanian et al., 2014). Some microbial biopesticides have a short shelf life, and there is a challenge in rural locations where fresh products and refrigerated storage are scarce (Mishra et al., 2014). Microbial dry formulations have become ineffective as a result of poor storage conditions (Ramasubramanian et al., 2014). Additional obstacles in generating economically viable microbial pesticides include a slow rate of kill, lack of field persistence due to high UV radiation levels, and poor water solubility of some formulations (Aneja et al., 2016).

Although entomopathogenic fungi are being widely utilized as biocontrol agents for insect pest management, they have certain drawbacks, such as varying efficacy, uneven performance in the field, and expensive manufacturing and application costs. There are some instances of approaches that have been suggested to deal with these constraints.

Entomopathogens serve as promising biocontrol agents with the potential to become a significant component in sustainable pest management. According to a recent study, entomopathogens are preferable to chemical pesticides in several ways, including their ability to target specific pests, a lack of pesticide residue in foodstuffs, and the capacity to lower pesticide resistance (Roy et al., 2006). Genetic engineering and biotechnology advances have opened up new possibilities for developing novel entomopathogens with enhanced efficacy and specificity. For example, researchers are investigating RNA interference to target specific genes in insect pests and developing hybrid entomopathogens that combine the properties of multiple microorganisms (Agrawal et al., 2018). In addition, it can also be integrated with other pest management strategies, such as crop rotation, habitat management, and biological control agents, to enhance their effectiveness. For example, researchers have shown that combining entomopathogens with insect-repellent plants can improve the efficacy of pest management (Winkler et al., 2019). There is limited attention on biopesticides because rural farmers in India have the least agricultural education or training. Outreach activities, such as training and field demonstrations, are essential for promoting the use of entomopathogens as a sustainable pest control method. Outreach activities can provide individuals with a better understanding of the benefits and limitations of using entomopathogens as a pest control method. This knowledge can help individuals decide when and how to use these products. Moreover, proper use of entomopathogens requires specific knowledge and skills. Outreach activities can teach individuals how to apply and manage entomopathogens to maximize their efficacy properly. By promoting entomopathogens, outreach activities can help reduce pesticide use and promote sustainable pest management practices. Several studies have demonstrated the importance of outreach activities for promoting the use of entomopathogens. For instance, a study conducted in Ethiopia found that training workshops for farmers increased knowledge and adoption of entomopathogenic fungi for controlling the coffee berry borer (António et al., 2017). Similarly, a study in India found that field demonstrations and training workshops for farmers improved the adoption of entomopathogenic nematodes for controlling the insect pest root-knot nematode in tomatoes (Kumar et al., 2021). Outreach activities can help increase the adoption of entomopathogens as a pest control method. By educating individuals about the benefits and limitations of entomopathogens and providing them with the necessary knowledge and skills, outreach activities can help them feel more confident using these products. Organizations can offer training workshops, field demonstrations, and educational materials, such as brochures, manuals, and videos, to implement outreach activities for entomopathogen adoption. These activities can target farmers, extension agents, researchers, and other stakeholders involved in pest management. By doing so, we can promote using entomopathogens as a sustainable and effective pest management strategy.

There has been growing interest in employing biopesticides for protecting crops through sustainable approaches in developing countries. Because the indiscriminate application of synthetic molecules led to resistance development and excessively contaminate the environment. Entomopathogen-based biopesticides are mainly suggested to be incorporated for pest management because of their environmental safety, primarily due to their host specificity. In addition, the cost of formulation and registration of microbial pesticides is much lower than synthetic chemicals. Over the last number of decades, efforts have resulted in the potential application of entomopathogens against various insect pests. However, an investigation is needed to explore many more virulent strains identification, isolation, and formulation preparation to effectively apply them.

Additional work needs to be done to eliminate limitations, including sensitivity to UV rays, desiccation, and low colony counts. Although biopesticides possess several advantages, they are not popularized due to a lack of awareness. It is important to conduct outreach activities, such as training and field demonstrations, to promote the wider acceptance of biopesticide-based formulations among growers. Therefore, insightful information about the potential of microbial insecticides in pest suppression is provided. They have been proven to be an effective component of integrated pest management in managing pest populations below an economic threshold level.

I and MS conceived and designed the study and prepared the figures/artwork. I, MS, EH, and PR performed the literature search. I wrote the first draft of the manuscript. I, MS, AM, and EK edited the manuscript. I, MS, and EH formatted the reference list as per Journal's style. All authors have approved the final version of the manuscript.