Insect adaptation: unveiling the physiology of digestion in challenging environments

Abstract

Insect’s resilience to adverse conditions poses a significant challenge for integrated pest control. This has resulted in huge economic losses to agriculture and forestry production as well as a range of severe ecological issues. As a physiological mechanism of insects, digestive physiology plays an important role in the process of adaptation to stress factors. However, there has been no systematic review of what stresses insects can adapt to through digestive physiology and how digestive physiology is involved in insect adaptation to stresses. In this review, the potential link between digestive physiology and adaptation of insects to biotic and abiotic stresses, including plant defense mechanisms, chemical insecticides, and entomopathogenic microorganisms, is analyzed. We point to that digestive physiology composed of digestive enzymes and gut microbial communities is an important strategy for insects to resist plant physical defense (e.g., hemicellulose, pectin, and microfibers), chemical defense (e.g., azadirachtin, diterpenoid acids, and phenolic glycosides), chemical insecticide stress, and entomopathogenic microorganism infection. In addition, the primary function of the digestive physiology in insects is to ensure energy supply during biotic and abiotic stress, assist in the metabolism of exogenous toxins (e.g., anti-insect proteins, primary metabolites, secondary metabolites, and insecticides), and improve their innate immunity against entomopathogenic microorganisms. This review is helpful to elucidate the mechanism of pest adaptation to adversity, and provide a breakthrough point for analyzing the causes of pest outbreaks.

Graphical abstract

Introduction

Insects are a vast and ubiquitous animal species worldwide. Currently approximately 6 million insect species worldwide represent half of the total number of living organisms on Earth. Over 50% of insects in the ecosystem are phytophagous [1]. These diverse species of phytophagous insects have a major impact on the health and productivity of trees in many countries and regions, and even cause a large number of tree deaths. For example, according to the census results of the National Forestry and Grassland Administration of China, a total of 5,030 kinds of forest pests are harmful to forestry in China, and the annual pest occurrence area is up to 10 million hm² [2]. With the change of global environment and climate, many phytophagous insects are spreading from their native habitats to high latitudes or high altitudes, and the frequency and degree of occurrence of phytophagous insects are continuously increasing [3, 4]. Upon analyzing global pest occurrence data, it was found that the United States, India, China, Italy and Japan had 1,365, 1,074, 1,070, 855 and 737 forest pest incidents in 2020, respectively [5]. The large-scale outbreak of pests has caused significant economic losses and ecological problems to agriculture and forestry worldwide. Based on incomplete data, the global crop loss caused by pests exceeds 20% annually, resulting in economic losses of up to 40 billion of dollars [6].

This occurrence trends of insects are closely associated with their remarkable ability to adapt to challenging environments [7]. The occurrence of pests is limited by the bottom-up forces of plants, which mainly includes phytochemical defense and physical defense [8]. However, over a long period of natural selection, insects have developed a set of effective counter-defense strategies to adapt to plants [9, 10]. This makes the host spectrum of some high-hazard insects very broad and is the basis for their spread and outbreak. For example, the Hyphantria cunea, a globally significant quarantine pest, has been reported to have over 600 species of host plants [11]. The Lymantria dispar, a common pest in East Asia, also have more than 500 species of host plants [12]. Various chemical and biological pest control techniques have been developed to control these harmful insects. However, owing to natural selection or improper use, insects have also gradually developed the resistance/tolerance to insecticides or entomopathogenic microorganism [12, 13]. The Leptinotarsa decemlineata, is able to rapidly evolve resistance to over 50 insecticides in a short period of time [14]. The Cydia pomonella has developed resistance to a variety of insecticides and pathogenic microorganisms, including organophosphates, neonicotinoids, hydrazines, benzoylureas, and C. pomonella granulovirus [15]. Adaptation to control measures must be another important cause of pest outbreaks.

The adaptive mechanism of insects to biological and abiotic stresses is reflected in migration behavior, feeding behavior, reproductive behavior, physiological and metabolic pathways [16]. Digestive physiology, as an important part of physiological and metabolic pathway, plays an important role in insect adaptation to the environment. Gut serves as a vital means for insects to interact with their surroundings. It not only facilitates the digestion and absorption of nutrients but also acts as an innate barrier to maintain the homeostasis of insects [17]. The insect gut harbors a diverse array of digestive enzymes and microflora, which are strongly associated with the host organism [18]. The digestive enzymes produced in insects mainly include protease, amylase and lipase family. Their main function is to break down the protein, sugar and fat nutrients or the indigestible components of cellulose contained in food into small molecules that can be absorbed [19, 20]. Some gut microbes co-exist in the digestive tract of insects, helping to digest food, decompose plant toxins, and detoxify pesticides [21]. Some phytophagous insects can even obtain digestive enzymes from gut microbes through horizontal gene transfer, thus achieving the purpose of breaking down food and absorbing nutrients [22]. Insect digestive physiology, composed of digestive enzymes and gut microbiota, is actively involved in a number of important functions, including assisting insects in degrading plant cell wall components, providing channels and energy for detoxifying plant secondary substances and pesticides, and stimulating insect immune systems in response to entomopathogenic microbial infection [23,24,25].

In recent years, with the deepening of research in the field of digestive physiology, multi-omics technology, gene editing technology and gene recombination technology have been used to comprehensively analyze the biological functions of insect digestive enzymes and gut microbiota [26, 27]. However, there has been no systematic review of what stresses insects can adapt to through digestive physiology and how digestive physiology is involved in insect adaptation to stresses. In this review, the core point is to define digestive physiology (digestive enzymes and gut microflora) as a powerful physiological factor that regulates pest adaptation to the environment (Table 1). First, by focusing on the ability of digestion physiology to degrade and metabolize key components of plant physical and chemical defense, we analyzed how digestion physiology participated in insect adaptation to plant defense. Subsequently, we elucidated the potential relationship between digestive physiology and insect tolerance or resistance to insecticides in terms of energy supply and degradation. Finally, we discussed the role of digestive physiology in insect response to pathogenic microorganisms, including the degradation of microbial toxins and the activation of innate immunity. The present review can clearly elucidate the mechanism of pest adaptation to adversity from the perspective of digestive physiology, and provide a breakthrough point for analyzing the causes of pest outbreaks.

Table 1 Role of digestive physiology in insect’s adaptation to adverse environments

Digestive physiology of insects and plant anti-insect defense

Plant defense

Phytophagous insects often acquire ample nutrients from host plants through feeding. Host plants have evolved complex defense mechanisms against phytophagous insects, including physical defense and chemical defense [28]. Physical defense, also known as mechanical defense, is a plant defense mechanism that uses morphological structures to improve insect resistance [29, 30]. The alteration of leaf characteristics, such as thickness, toughness, vein structure, and the presence of trichoid or gland structures in plants can minimize damage caused by phytophagous insects [31]. Chemical defense refers to a plant defense strategy that inhibits the growth and development of harmful insects [32,33,34]. It involves physiological and biochemical processes, such as increasing secondary metabolites, improving the activity of defense proteases and enzyme inhibitors, adjusting nutrient content, and attracting predatory or parasitic natural enemies using volatiles [35,36,37,38]. Furthermore, various signaling pathways in plants have been implicated in defense responses triggered by insect feeding. The initial plant signal response involves membrane potential depolarization, Ca²⁺ flow alteration, MAPK cascade reaction, and reactive oxygen species generation. The response of plant hormone signals involves the activation and inhibition of jasmonic acid, ethylene, salicylic acid, and abscisic acid signal pathways [39,40,41,42,43].

Insect digestive enzymes and plant defense

To adapt to the host plant, phytophagous insects first need to overcome the physical defense of the plants (Fig. 1). The plant cell wall serves as a physical defense mechanism against insect feeding. The primary components of plant cell walls are hemicellulose, pectin, proteins, and microfibers within the matrix [44,45,46,47]. Phytophagous insects degrade plant cell walls using digestive enzymes, such as proteases, amylases, pectinases and glucanases found in their oral secretions or gut. There are eleven cellulases and fourteen pectinases detected in the midgut of Eucryptorrhynchus scrobiculatus, aids its adaptation to plant cell walls and improves feeding efficiency [48]. Similarly, the endo-β-1, 4-glucanase (NlEG1) in the saliva of Nilaparvata lugens can break down cellulose in the rice cell wall, allowing the oral needle of N. lugens to reach the phloem tissue for feeding [49]. Glucosidase has been reported to be involved in the hydrolysis and utilization of cellulose by insects, such as Nasutitermes takasagoensis, Anoplophora glabripennis, Periplaneta americana, and Myzus persicae [50,51,52,53]. The main function of this enzyme is to convert insoluble cellulose into small oligosaccharides that are soluble and easy to use.

Fig. 1

Role of digestive physiology in insect counter-defense

The second step in the counter-defense strategy of phytophagous insects is overcoming the chemical defense of plants (Fig. 1). Phytophagous insects can regulate digestive enzyme activity in response to plant defense proteins. Plant defense components include chitinase, lectins, and enzyme inhibitors [54,55,56]. The midgut trypsin activity of Ephestia kuehniella larvae decreased after treatment with Inga vera trypsin inhibitor, but chymotrypsin-like activity increased [57]. Tenebrio molitor improves digestion by enhancing cathepsin activity, counteracting the negative effects of trypsin inhibitors [58]. Similarly, H. cunea treated with α-amylase inhibitors showed a decrease in midgut α-amylase activity and an increase in trypsin activity [59]. Primary metabolites are crucial in assessing insect resistance in plants. An imbalance in primary metabolite digestion and utilization in phytophagous insects can lead to malnutrition and growth retardation. The most abundant primary metabolites involved in gut digestion and metabolism in insects are proteins, lipids, and carbohydrates [60]. Phytophagous insects use the adaptive regulation of digestive enzyme activity to cope with plant primary metabolites [61, 62]. This ensures an optimal state of digestion in phytophagous insects. For example, Helicoverpa armigera larvae can efficiently use proteins, carbohydrates, and lipids from the host plant Ocimum kilimandscharicum by adjusting the activities of protease, amylase, and lipase [63]. In addition, some insects use a homeostatic strategy for digestive enzyme activity to cope with plant secondary metabolites. Azadirachtin, a secondary metabolite, is the primary active ingredient in Azadirachta indica extract. It can disrupt the physiology and digestion of insects. Azadirachin stress significantly inhibited the growth of Bactrocera dorsalis but was accompanied by a notable increase in larval cathepsin activity [64]. The increased cathepsin activity in B. dorsalis may be caused by the digestive response of the larvae to meet the energy demands for metamorphosis in the presence of plant secondary metabolic substances. Based on the above reasons, we conclude that digestive enzymes can assist insects in coping with phytochemical defenses from three aspects: plant defense proteins, primary metabolites, and secondary metabolites.

Insect gut microbes and plant defense

The gut microbiota of phytophagous insects aids in the digestion of tough plant cell wall components, such as cellulose, pectin, and lignin. Cellulose is a key component of the physical defense mechanism and inhibits insect digestion [65]. Insect gut microbiota can break down cellulose into sugar [66, 67]. For example, Streptomyces and Amylostereum areolatum, which are present in the gut of Sirex noctilo, provide cellulase to the host insects. This enables cellulose breakdown and enhances host nutrient absorption [68]. Pectin in plant cell walls carries cellulose and hemicellulose and plays a crucial role in anti-insect defense [69]. Insects must effectively digest pectin to feed on plant leaves. Some studies have shown that pectin degradation also relies on insect gut bacteria. For example, Stammera, the gut symbiont of tortoise leaf beetles Cassida rubiginosa, possesses genes related to pectin digestion, such as polygalacturonase [70]. Similarly, in insects that rely heavily on pollen for nutrients, accessing the nutrients in the pollen involves overcoming obstacles in the pollen wall. The inner layer of pollen is primarily composed of pectin [71]. The gut bacteria Gilliamella apicola in honey bee Apis mellifera have genes involved in pectin degradation, including pectin lyase and pectin debranching enzyme [72]. Its pectin degradation was confirmed using in vitro culture assays. Phytophagous insects also depend on the gut microbes to degrade lignin. For example, three dominant strains of Heterapoderopsis bicallosicollis (Penicillium sp., Aspergillus sp. and Cladosporium sp.) can reduce lignin levels [73].

Gut microbes aid insect adaptation to plant secondary metabolites. For example, they help insects degrade defensive allelochemicals, such as toxic alkaloids, terpenes and polyphenols [74]. The gut bacteria of Hylobius abietis not only aids in the metabolism of diterpenoid acids from Picea abies, but can also use diterpenoids as a carbon source to nourish H. abietis [75]. Curculio chinensis is an obligate seed pest that relies on gut microbes to break down saponins, which are plant secondary metabolites [76]. A bacterial community capable of degrading plant phenolic glycosides was found in the gut of L. dispar [77]. The dominant bacterium found in the gut of Brithys crini is Enterococcus casseliflavus strain, which degrades alkaloids [78]. A dominant bacterium Pseudomonas fulva in the Hypothenemus hampei, breaks down caffeine to provide its own source of carbon and nitrogen [79]. Bifidobacterium wkB204, which can completely degrade amygdalin, has been found in the gut of A. mellifera. This strain secretes arbohydrate-degrading enzymes, one of which belongs to the lycoside hydrolase family 3(GH3) and helps the host metabolize amygdalin [80]. Gut symbiotic bacteria Enterobacter sp. EbPXG5 mediate the degradation of kaempferol by Plutella xylostella [81]. The Dendroctonus valens relies mainly on the symbiotic fungus Leptographium procerum and two strains of bacteria, Erwinia and Erwinia, solated from the microbiota to degrade d-pinitol from Pinus tabuliformis. These studies highlight the significant role of the gut microbiome of phytophagous insects in overcoming plant defenses [82].

Gut microbes can also directly affect plant defense. Phytophagous insects can transmit gut bacteria to plants through various pathways, including saliva, reflux, feces, egg-laying, and honeydew, which can affect plant defenses [83]. Plant hormone-mediated direct defense is an entry point for the gut bacteria of phytophagous insects that affect plant-induced defense. For example, Leptinotarsa decemlineata carries symbiotic bacteria in its gut, which enhance the SA signaling pathway and inhibit JA-mediated tomato defense [84]. Chilo suppressalis larvae inhibit JA signal-related defenses in rice plants using bacteria secreted from their mouths [85]. Furthermore, the activation of indirect defense mechanisms in plants is influenced by insect gut bacteria. The bacteria present in the honeydew of N. lugens can strongly induce indirect defense in rice [86]. These bacteria facilitate the release of organic compounds in the leaves, which can be used to synthesize attractants for predators of phytophagous insects.

Insect digestive physiology and chemical insecticides

Insecticide resistance of insects

Insecticides are widely used in agriculture and forestry to control harmful insects because of their high efficiency, broad spectrum, and convenient use. In recent years, the tolerance of insects to certain insecticides has increased significantly owing to their widespread use. This led to a noticeable increase in resistance [87]. Insecticide resistance refers to the ability of insects to tolerate a certain amount of a drug that would normally kill most individuals in a population. This ability can be retained and developed within the population [88]. The emergence of insecticide resistance necessitates increased insecticide use for effective control. However, the increased use of insecticides not only increases the input, but also causes the residue of insecticides to pollute soil and water sources, destroy the diversity of agricultural ecosystems and seriously threaten the environment and human health [89]. Therefore, avoiding or slowing down the development of insecticide resistance is of utmost significance for the control of agricultural and forestry pests and protection of the ecological environment.

Resistance to insecticides of insects is a complex phenomenon. It is influenced by genetic, physiological, behavioural and ecological factors, as well as the amount of insecticides used [90]. Insects have evolved over time to develop several mechanisms to resist insecticide resistance. This includes metabolic, target, behavioral, and penetration resistance [91]. Metabolic resistance refers to resistance resulting from the rapid degradation of insecticides owing to the increased activity of detoxification enzymes. Target resistance refers to a reduction in insect sensitivity to insecticides due to mutations or changes in the expression level of the target molecule that interacts with the insecticides [92]. Behavioral resistance refers to the phenomenon by which insects alter their behavior in response to insecticide exposure and relocate to different areas. This enables the preservation of individuals with survival-conducive behaviors and facilitates behavioral changes in the entire insect population. Penetration resistance is the alteration in the structure and composition of an insect osmotic barrier, which reduces the permeability and residual amount of insecticides and alleviates the pressure of the detoxification system [93]. Recent research advances have also pointed out that epigenetic regulatory mechanisms, epitranscriptomic mechanisms, and transcription factor regulatory mechanisms can promote the development of insect resistance to insecticides [94,95,96,97]. Insecticide stress induces epigenetic regulation of gene expression through histone modification and DNA methylation, which is passed on to offspring. This prompts insects to rapidly evolve resistance and resistance to insecticides. For example, in the L. decemlineata larvae, DNA methylation modifies gene expression without altering the underlying genetic code, which contributes to the emergence of a resistant phenotype [98, 99]. N⁶-methyladenosine (m6A)-mediated epitranscriptomic regulation is critical for a variety of physiological processes in eukaryotes, including gene expression, alternative splicing, mRNA stability, and microRNA biogenesis. These functions have also been reported to correlate with insect tolerance to insecticides. For example, N⁶-methyladenosine (m6A) regulates the expression of the cytochrome P450 gene in Bemisia tabaci and leads to the development of resistance to thiamethoxam [95]. Transcription factors can regulate many stress response genes and detoxification metabolism genes, and help insects develop resistance to insecticides [100, 101]. Common transcriptional pathways associated with insecticide resistance include MAPK/ERK-p38/CREB, AhR/ARNT, and CncC/Maf signaling pathways.

Regulation of insecticide resistance by insect digestive enzymes

The response of insects to insecticide stress requires a significant amount of energy. This is commonly associated with lower body weight, longer developmental lifespan, and decreased reproductive ability [102]. In other words, insect resistance might result from a trade-off effect on fitness loss. It is an important survival strategy for insects to cope with adverse conditions. The trade-off effect is characterized by an energy deficiency. When exposed to insecticides, insects lack sufficient energy to meet both detoxification and growth requirements. Metabolic resistance is a crucial mechanism of insect resistance. It involves the activation of energy-demanding detoxification defense systems. Improving energy metabolism may be crucial for insects to survive insecticidal stress [103]. Insects maintain energy homeostasis by digesting carbohydrates, fat, and proteins. In this process, digestive enzymes play a decisive role. Multiple studies have demonstrated that alterations in digestive enzyme activity serve as a defense mechanism for insect energy metabolism in response to unfavorable conditions. For example, Spodoptera frugiperda larvae can increase the expression levels of trypsin and chymotrypsin genes to promote growth under the influence of digestive enzyme inhibitors [104]. D. melanogaster can cope with azadirachtin stress by increasing its larval lipase activity. Research has shown that digestive enzymes are involved in insect response to insecticide stress. For example, when a resistant strain of Sitophilus zeamais was exposed to the cypermethrin, lipase and trehalase activity levels were maintained to generate energy during insecticides metabolism [105]. A study on the Ceratitis capitata sensitive strain screened in the laboratory revealed that female flies can cope with cyhalothrin stress by improving α-amylase activity [106]. These findings indicate that digestive physiology plays a role in the development of resistance, in addition to existing mechanisms of resistance formation. We propose the hypothesis of “the energy-supply effects of insecticide resistance”. It means that digestive enzymes-dominated digestion physiology can provide energy support for the activation of various detoxification strategies of insect in response to insecticide stress and assist the formation of insect resistance and tolerance to insecticides (Fig. 2).

Fig. 2

Role of digestive physiology in the tolerance of insects to insecticides

Regulation of insecticide resistance by gut microbes in insects

Insect gut microbes contribute to insect resistance by adjusting microbial population abundance, improving fitness, and regulating detoxification metabolism [107,108,109]. Insecticide resistance in insects leads to variations in the populations of resistance-related gut microbes. This variation occurs between insect species and within the same host insect species depending on the type of insecticide used. The abundance of microbial communities in resistant strains differ from that in sensitive strains [110]. For example, the dominant bacteria in P. xylostella are Enterobacteriales, Vibrionales and Lactobacillales. In comparison to sensitive strains, P. xylostella midgut had lower abundance of Lactobacillales. In addition, following insecticide treatment, the abundance of Lactobacillales in the midgut of both the chlorpyrifos-resistant strain and the fipronil-resistant strain increased [111]. The 16s augmentation sequencing analysis revealed that the abundance of gut symbiont Sphingomonas in imidacloprid-resistant strains of Aphis gossypii was significantly higher than that in the sensitive strain. The removal of Sphingomonas from the gut of imidacloprid-resistant strains could increase susceptibility to imidacloprid [112]. Insecticide resistance adversely affects the life history and metabolism of insects. In this scenario, gut microbes assist the host in improving its fitness. Compared with the sensitive strains, the abundance of Pseudomonas and Stenotrophomonas in P. xylostella, which had anti-fungal and insecticide-degrading activity, and Serratia marcescens, which produced chitinase synthetase, were significantly increased in the prothiofos-resistant strains. This increased the growth and development rates of host insects [113]. Certain gut microbes have been reported to assist the hosts in insecticides metabolism (Fig. 2). Chlorpyrifos (CP) is an organophosphorus insecticide that causes acute neurotoxicity by inhibiting acetylcholinesterase in various species. Studies have shown that Lactobacilli in the gut of Drosophila melanogaster can greatly enhance the resistance of sterile D. melanogaster to CP [114]. A study on insecticide resistance in the midgut bacteria of P. xylostella showed that Enterococcus sp. bacteria improved their resistance to CP [115]. The gut symbiotic bacteria of S. frugiperda can degrade insecticides, such as cyhalothrin and lufenuron [116].

Insect digestive physiology and entomopathogenic microorganisms

Microbiologic control

Entomopathogenic microorganisms are an important natural regulator of insect population [117]. The field of pest control has long used microbes to control outbreaks of harmful insects. Microbial control involves the use of live pathogenic microorganisms and their metabolites for pest control. Compared with chemical insecticides, microbial insecticides derived from entomopathogenic microorganisms offer advantages, such as eco-friendliness, large-scale production, and sustainable pest control, and have been widely used in the integrated pest management of current agro-forestry ecosystems [118,119,120]. Entomopathogenic microorganisms infect insects and cause diseases that can spread among insect populations [121]. The main way entomopathogenic microorganisms infect insects is through horizontal transfer. Horizontal transfer refers to the horizontal transfer of entomopathogenic microorganisms from an individual exposed to the pathogen to an individual not exposed to the pathogen [122]. Insect interactions, interactions between predators and host insects, and transmission in food sharing are the main ways of transmission of entomopathogenic microorganisms [123]. Currently, pathogenic microorganisms that infect insects are primarily bacteria, fungi, and viruses [124]. The primary entomopathogenic bacteria include Bacillus thuringiensis (Bt), B. opilliae, Clostridium malacosomae, B. cereus, Smarcescens, Pseudomonas aeruginosa and Pseudomonas fluorescens. Bt is an efficient and excellent insecticide and it is widely used worldwide to suppress forest insect and crop pest outbreaks. In the control of Lepidoptera and Coleoptera larvae, the Bt kurstaki and Bt aizwai species are considered to be the strongest pathogenic microorganisms [125, 126]. In addition, the cultivation of transgenic seeds of Bt has also become a trend in modern agriculture. Pathogenic entomopathogenic fungi are widely used as biological insecticides to control pests and play an important role in the work of biological networks. The main pathogenic fungi include Beauveria bassiana, Metarhizium anisopliae, M. favoviride, M. brunneum, and Aspergillus flavus [127, 128]. The most extensive research among these is on pest control using B. bassiana and M. anisoplariae. Insect viruses, including nuclear, granular, and cytoplasmic polyhedrosis viruses of the baculovirus family. Insect viruses play a crucial role in controlling outbreaks of many pest populations, such as P. xylostella, S. exigua, H. armigera, and Argyrogramma agnata [129]. Outbreaks of the L. dispar in the United States, Japan, and Spain have been reported to have been successfully controlled by the host-specific virus, the isolated L. dispar nucleopolyhedrovirus [130].

Insect digestive enzymes and entomopathogenic microorganisms

The guts of phytophagous insects are a complex environment for nutrient breakdown. Digestive enzymes in the gut aid food digestion and protect against the invasion of pathogenic bacteria. Bt is a gram-positive bacterium that produces insecticidal proteins known as parasporal crystal proteins. These proteins have specific insecticidal activities against various insects, including Lepidoptera, Diptera, Coleoptera, and Hymenoptera [131]. When insects consume Bt insecticidal proteins, the Bt prototoxin is activated and hydrolyzed into active toxins under the action of midgut proteases. The active toxin binds to specific receptors in the midgut of insects, inducing the formation of oligomerized toxins. These oligomers insert into the cell membrane of the midgut epithelium, eventually forming holes that result in cell expansion, lysis, and ultimately death of the insect [132]. Prolonged insecticide uses or constant exposure to insecticidal proteins expressed in genetically modified crops causes pests to develop resistance to insecticides or insecticidal proteins. Among them, trypsin is the most abundant digestive enzyme in insects, which can cause incomplete or excessive hydrolysis of Bt and destroy the toxicity of Bt toxin. This is a significant contributor to pest resistance to Bt [133]. Several studies have confirmed that trypsin is involved in the insect detoxification of Bt Cry toxin. For example, in a study of Ostrinia furnacalis, four trypsin enzyme genes were consistently upregulated after feeding on Cry1Ab protoxin [134]. In S. frugiperda, trypsin SfT6 knockout reduced larval sensitivity to Cry1Ca1 [135]. Protease activity in S. exigua was significantly increased by Vip3Aa, a new insecticidal poison protein of Bt [136]. In addition to these positive responses, insect digestive enzymes can also serve as entry points for pathogenic microorganisms to exert pathogenicity. The entomopathogenic bacterium Xenorhabdus nematophila produces three metabolites that inhibit the activity of phospholipase A2, the digestive enzyme of S. exigua [137]. Transcriptome analysis revealed that the entomogenic fungus Mucor hiemalis BO-1significantly inhibited the levels of key digestive enzymes (protease, α-amylase, lipase and cellulase) in Bradysia odoriphaga larvae [138]. The pathogenic bacterium Serratia marcescens significantly reduced the digestive enzyme activity in Curculio dieckmanni [139].

Insect gut microbes and entomopathogenic microorganisms

The microbiota of insects regulate the interactions between insect–pathogenic microbes by regulating the immune and physiological conditions of the gut epithelium [140]. It can also directly or indirectly release microbial components. Symbiotic microorganisms in insects can protect their hosts against various pathogens by secreting antimicrobial substances (Fig. 3). For example, Pantoea agglomerans, a gut symbiotic bacterium in Schistocerca gregaria, produces phenolics that inhibit the gut proliferation of M. anisopliae [141]. A lipase from Bombyx mori gut bacterium Bacillus pumilus SW41 showed potent antiviral effects against germination virions of the silkworm nucleopolyhedrovirus [142]. Certain symbiotic microorganisms have been found to possess the ability to directly inhibit infections by pathogenic microorganisms. Citrobacter freundi and other six symbiotic microorganisms isolated from Delia antiqua, inhibits the growth of mycelia and spores of B. bassiana [143]. Insects lack acquired immunity like vertebrates but have evolved innate immunity, including cellular and humoral immunity. Insects rely primarily on innate immunity to resist pathogenic microorganisms [144]. Extensive research on insect symbiotic microorganisms has revealed that the symbiotic microbial community can regulate the proliferation and differentiation of insect host hemocytes. This improves the cellular immune response of the host (Fig. 3). The tsetse Glossina morsitans morsitans primary endosymbiont Wigglesworthia causes severe damage to the immune system of tsetse flies, which is characterized by lack of phagocytic hemocytes and abnormal expression of immune-related genes [145]. Insect symbiotic microorganisms can induce humoral immunity in the host against pathogenic microorganisms (Fig. 3). The Gilliamella and Lactobacillus in honey bee A. mellifera larvae can trigger an immune response resembling that of bacterial infection. The immune response in A. mellifera larvae leads to increased production of antimicrobial peptides and defensins, which enhances the immune capacity of the host and improves defense against pathogen infection [146]. However, not all symbiotic microorganisms exert a positive protective effect on host survival. Some symbiotic viruses in insects can be pathogenic or even lethal to the host. For example, overexpression of Spiroplasma poulsonii in D. melanogaster induces death of the male and massive apoptosis even neural defects [147]. In Spodoptera exempta, gut microorganism Wolbachia sp. has been found to increase host sensitivity to karyotype polyhedrosis viruses [148].

Fig. 3

Role of digestive physiology in the tolerance of insects to entomopathogenic microorganisms

Conclusion

Digestive physiology is an important part of the counter-defense strategy in insects to deal with the physical defense and chemical defense of host plants. Digestive enzymes have been shown to degrade plant cell wall components, such as cellulose and pectin. Adaptive regulation or homeostasis of digestive enzyme activity is an important strategy for insects to cope with anti-insect proteins, primary metabolites, and secondary metabolites. Gut microbiota can simultaneously degrade difficult-to-digest stubborn polymers, such as cellulose, pectin, and lignin in plant cell walls, as well as secondary metabolites with insecticidal activity in phytochemical defense. Digestive enzymes are involved in the detoxification metabolism of insecticides by insects in a way that provides energy. At this point, we propose the hypothesis of “the energy-supply effects of insecticide resistance”. Insecticide stress can drive changes in gut microbial community structure. The abundance of microorganisms capable of degrading insecticides was higher in insecticide resistant strains than in sensitive strains. Digestive enzymes are involved in the degradation of microbial toxins by insects, but have also been shown to be an entry point for pathogenic microorganisms to exert pathogenicity. Gut microbiota can directly protect the host insects against various pathogenic microbial stresses by secreting antimicrobial defense substances. Furthermore, gut microbiota have been found to regulate the proliferation and differentiation of host insect blood cells to improve the cellular immune response of the host insect, or activate the humoral immunity of host insects to improve their ability to resist pathogenic microorganisms.

Digestive physiology is an important strategy for insect adaptation to stress, encompassing both biological (such as plant defense and microbial infection) and abiotic factors (such as chemical insecticides). This is crucial for the survival and continuity of insect populations under adverse conditions. At the same time, this is also a significant reason for the inefficiency of chemical and microbial control measures against insects. Therefore, the disruption of insect digestive physiology is a key factor in improving pest control efficiency. RNA interference (RNAi) is a widespread post-transcriptional mRNA silencing mechanism in eukaryotes that can specifically degrade homologous mRNA sequences. Since the discovery of RNAi in Caenorhabditis elegans, RNAi has become a powerful tool for studying gene function in insects [149, 150]. It can silence key target genes in insects through the exogenous introduction of dsRNA- or shRNA-expressing constructs [151]. In recent years, nanoparticles have received extensive attention as RNA interference delivery carriers. Nanoparticles are particle dispersions or solid particles with a particle size between 10 and 1000 nm. Nanocarriers can protect RNAi molecules from enzymatic degradation and immune recognition, and have higher transport efficiency across cell membranes than other carriers [152, 153]. Common nanocarriers are cationic star polymer nanoparticles, zeolitic imidazolate framework 8, and chitosan [154,155,156]. Biopesticides made from nanocarriers coated with dsRNA or siRNA are defined as nucleic acid pesticides. This new class of pesticides can silence the expression of important functional genes in targeted insects, causing stunting or death of insects and achieving sustainable pest management. Nucleic acid pesticides based on RNAi technology have the advantages of strong specificity, rapid response, minimal residue, and limited impact on non-target organisms [157]. RNAi interference experiments on key digestive enzyme genes of insects have been extensively carried out and these genes are proved to be an important target for the construction of nucleic acid pesticides [158]. The manipulation or use of gut symbionts can alter the growth and population of host insects and is therefore a potential target for pest control. Regulating intestinal symbionts through RNAi can also improve the efficiency of pest control and effectively disrupt their digestive physiology [159, 160]. This two types of nucleic acid pesticide, which targets key digestive enzymes and gut symbionts, can significantly reduce insect resilience to adverse conditions and provide a novel strategy for efficient and safe pest control.