The complex interactions between nutrition, immunity and infection in insects

Insects are the most diverse lineage of multicellular organisms on the planet (Engel and Grimaldi, 2004). Insects are found in nearly all ecosystems and are essential to ecosystem functioning. For these reasons, understanding the effects of environmental changes on insect populations – effects that have been quite dramatic in the last few centuries – can be considered a defining challenge for entomological and ecological research in the 21st century (Boggs, 2016; Halsch et al., 2021). Changes in the environment include modifications of the nutritional landscape that can have important implications for insect populations.

The field of nutritional immunology captures the complexity of the relationships between the nutritional environment, infection and immunity (Ponton et al., 2013). Nutrition is a multi-dimensional problem. Food sources can be abundant, scarce, diverse, unique, easy or difficult to access, balanced, unbalanced or contaminated. Responses to infection are also complex. Innate immune responses have been traditionally described as broad and non-specific to pathogen strains; however, recent research suggests that insect innate immunity (see Glossary) is also capable of providing highly specific immune responses to fight infections in a similar way to adaptive immune responses found in vertebrates (Cooper and Eleftherianos, 2017; Schulenburg et al., 2007). Responses to infections also include processes such as tolerance to infection (see Glossary), which allows individuals to maintain their fitness while infected (Ayres and Schneider, 2012). In this Review, we focus on the recent research and questions that link the nutritional environment with individual and population health, as well as the different factors that may define the framework of nutritional immunology in insects. We explore the links (i.e. connections) and relationships (i.e. the manner in which components may be connected) between dietary composition and innate immunity, resistance to infection (see Glossary) and tolerance of insects when confronted with different types of parasites. Intracellular and extracellular symbionts can interfere with host metabolism and immunity, and we discuss how they affect the relationship between nutrition and immunity. Insects successfully occupy multiple ecological niches thanks to the different morphological and physiological phases they express during their development. Here, we consider the links between nutrition and immunity throughout development but also across generations. Finally, the great diversity of lifestyles of insects, from solitary to eusocial, adds another layer of complexity with implications for the interactions between nutrition and immunity. Understanding how changes in the nutritional environment affect insect health and fitness is a complex challenge, but it is relevant to issues such as pest management, disease vector control, insect conservation, and the use of insects as food and feed. Although most of the work in eco-immunology has made use of insect model species, in the future it will be important to explore other systems to obtain a wider and more integrative understanding of this field of research. Further, understanding the importance of experimental nutritional conditions is essential, as it can influence the outcome of experiments as well as their reproducibility.

Insects are rather uniform in their nutritional requirements because their chemical compositions and metabolic capabilities are broadly similar (Chapman, 2013). However, there is still variation in nutritional requirements among insect species that arises from adaptations to specific nutritional environments. For example, plant-feeding insect species generally require approximately equal amounts of protein and carbohydrates as adults, with larvae usually needing a high protein intake to support their growth. By contrast, predatory insects have high protein and fat requirements relative to carbohydrates. Insects feeding on high-carbohydrate foods, such as grains, have a high requirement for carbohydrates relative to proteins.

An insect's nutritional state influences its immune status and resistance to infection. Table 1 summarises the studies that have investigated the interactions between nutrition, immunity and resistance in insects. These studies have generally investigated the effects of food restriction and starvation on the expression of immune and life history traits (i.e. reproduction and longevity/survival) in the absence of infection (i.e. immuno-competence) and post-infection. Starvation and limited food accessibility can both have negative effects on fitness and immune parameters (however, see discussion on positive effects of caloric restriction in Partridge et al., 2005). When food limitation is severe, humoral and cellular immunity (see Glossary) are altered, and mortality post-infection is increased (Kangassalo et al., 2015; Ariani et al., 2015; Adamo et al., 2016; Kutzer and Armitage, 2016; McKay et al., 2016; Ebrahimi and Ajamhassani, 2020; Meshrif et al., 2022). However, total food limitation is not always what individuals face in nature, as some nutrients might be more limited than others, leading to imbalanced diets. In this context, the immune effects of deficiencies in specific macro- or micro-nutrients (see Glossary) – such as protein, alkaloids, ascorbic acid or selenium – have been studied in different insect species (Popham et al., 2005; de Roode et al., 2008; Popham and Shelby, 2009; Howick and Lazzaro, 2014; Anthony et al., 2015; Richardson et al., 2015; Unckless et al., 2015; Tritschler et al., 2017; Kelly and Bowers, 2018; Vogel et al., 2018).

Over the last 10 years, and following the pioneering work of Simpson and Raubenheimer (2012), studies have employed geometric approaches to investigate the effects of dietary macronutrient composition on insect physiology (Box 1). Protein and carbohydrate are two of the main energy-yielding nutrients for insects, and the ratio of dietary protein to carbohydrate has important effects on insect physiology, modulating longevity and reproduction (e.g. Lee et al., 2008). There is a general trend for dietary protein being the primary regulatory macronutrient of immunity in insect larval stages. When larvae do not have access to enough protein, their immunity is usually negatively affected as well as their resistance to pathogenic bacteria, fungi or macro-parasites (Lee et al., 2006; Povey et al., 2009, 2014; Tritschler et al., 2017; Danihlik et al., 2018; Wilson et al., 2019; Wilson et al., 2020; Gomez-moracho et al., 2021). The nutritional optima of larvae and adults can, however, differ significantly. For instance, in Drosophila melanogaster, larvae require a diet high in protein to optimise growth, whereas a diet with a more balanced composition of protein and carbohydrate optimises lifetime egg production in adults (Jang and Lee, 2018). This might explain why the results of studies exploring the interactions between nutrition and immunity in adult insects differ from those in larval stages (as seen in Table 1). Different nutritional optima between adult and juvenile stages have been mainly observed in holometabolous insects (see Glossary) in which development includes a complete metamorphosis; nutritional optima might be less marked between life stages of hemimetabolous insects (see Glossary), yet there is a lack of data to fully support this conclusion.

In adult insects, the interaction between resistance and nutrition can also depend on the host species and the type of infection. When Drosophila females are infected with the bacterium Micrococcus luteus, a carbohydrate-biased diet (i.e. a diet that contains a greater concentration of carbohydrates than what naive flies usually select) promotes the expression of antimicrobials and survival after infection (Ponton et al., 2020). Similar results were observed in the Queensland fruit fly, Bactrocera tryoni. When infected with the bacterium Serratia marcescens, adult females restricted to a carbohydrate-biased diet harbour a lower pathogen load during the first few hours post-infection than those on a balanced diet (i.e. a diet with approximately similar concentrations of protein and carbohydrates; Dinh et al., 2019). In contrast, in adult honeybees, an increase in total dietary protein lowers mortality after viral infection but increases the infection load of microsporidia (see Glossary; Tritschler et al., 2017). There is no evidence of an increase in immuno-competence when the diet is rich in protein, but a decrease in immune responses is seen when the diet does not contain protein (see Brunner et al., 2014). Dietary fat (and not protein) also stimulates viral immunity in honeybees (Alshukri and Al-Esawy, 2021). Diet diversity (i.e. when individuals are offered a choice between different sources of food/nutrients) also influences immune parameters and resistance to fungal pathogens in bees (Alaux et al., 2010; Foley et al., 2012; for other insect species, see Vogelweith et al., 2016; Krams et al., 2017). Further, it has been shown that when Bombus terrestris individuals are fed a diet with a low-protein and a high-carbohydrate concentration, they harbour the greatest quantity of parasites but live the longest compared with individuals fed a high-protein diet (Gomez-Moracho et al., 2021). This result suggests that a carbohydrate-biased diet in adult bees might increase their tolerance to infection (Ayres and Schneider, 2012) but not their resistance to infection. This might illustrate that not all immune parameters respond in the same way to the nutritional composition of the diet. For instance, Cotter et al. (2011, 2019) have shown that when caterpillars of the Egyptian cotton leafworm, Spodoptera littoralis, are fed diets varying in their protein-to-carbohydrate ratio, functional immune traits – such as lytic and phenoloxidase activity (see Glossary) – increase as the protein content of the diet increases; however, immune gene expression was much less predictable in those studies. The activation of immunity can also be costly through causing self-damage or immunopathology (Sadd and Siva-Jothy, 2006), with the physiological costs carried across life stages (DeBlock and Stoks, 2008). Food availability might be an important factor in mediating these effects (Stahlschmidt et al., 2015). More research is needed, however, to produce a clearer picture of how nutrition may decrease these negative effects of immune activation.

Host nutrition not only influences immunity but also influences the resources that are available to pathogens (Smith and Holt, 1996). For example, some specific nutrients, such as iron, are required by pathogens to develop, and host supply can limit their access. Pike et al. (2019) have published a meta-analysis where they investigated how nutrition influences pathogen virulence (see Glossary) across vertebrates and invertebrates. Their analysis suggests that host nutrition does not influence pathogen virulence; however, more research targeting these specific questions is needed, as the resources required by the pathogens might be species/strain specific. It is also important to note that most studies where the dietary protein composition was manipulated solely used yeast as a main protein source. However, yeast is not only a major source of amino acids but also a source of other miscellaneous components. Therefore, protein restriction or supplementation through modulating yeast intake inadvertently causes variation in other elements in the diet (Bruce et al., 2013). Further, most studies assume that the food provided to insects is standardised; however, this is not always true. Laboratory experimental settings rarely use defined or holidic diets (see Glossary), as these might be either too challenging to prepare or too expensive to use routinely. Food deprivation is also not always well defined in protocols, and – in natural environments – insects can find themselves in diverse, nutritionally challenging situations that would affect their life history traits differently (see Zhang et al., 2019). To be able to compare the results of the different studies investigating the effects of nutritional manipulations on immunity and resistance, protocols will need to be better standardised.

The exploration of the proximate mechanisms of the interaction between metabolism and immunity has produced some fascinating findings. Nunes et al. (2021) reviewed the hormonal regulation of cellular and humoral immunity in insects. The developmental hormones 20-hydroxyecdysone (20E) and juvenile hormone (JH) have an important role in insect immunity. 20E stimulates the proliferation, differentiation and action of haemocytes (see Glossary), induces phagocytosis and nodulation (see Glossary), and increases the expression of antimicrobial genes (which code for antimicrobial peptides, AMPs), such as diptericin, cecropin and attacin, through the ecdysone receptor (Schwenke et al., 2016). 20E may also influence the duration of immune responses (Garbuzov and Tatar, 2010). In Manduca sexta and Tenebrio molitor, JH modulates the prophenoloxidase cascade and encapsulation (see Glossary) (Hiruma and Riddiford, 1988; Rantala et al., 2003; Rolff and Siva-Jothy, 2002). Overall, 20E acts as a positive regulator of immunity, whereas JH acts as an immune suppressor (Nunes et al., 2021; Schwenke et al., 2016). Nutrient-sensing pathways, JH and 20E signalling may interact. Blockage of two highly evolutionarily conserved nutrient sensors, insulin-like signalling pathway (IIS) and target of rapamycin (TOR, sensing amino acid levels), induces a decrease in JH biosynthesis (e.g. Abrisqueta et al., 2014; Leyria et al., 2023) and in ecdysone production (Kemirembe et al., 2012). Further, infections can also lead to a decrease in fat and glycogen stores through an impairment of insulin signalling decreasing JH biosynthesis (Dionne et al., 2006). The effects of JH on immunity may be highly dependent on the insect's developmental stage, and the type of host and pathogen, with contrasting effects in different species (Amaro-Sánchez et al., 2023; Remolina and Hughes, 2008). More work is therefore needed to clarify how nutrition and macronutrients modulate insect immunity through the JH and 20E pathways.

The central immunomodulators of insect immunity are molecules such as cytokines (i.e. signalling proteins), DAMPs (i.e. damage-associated molecular pattern molecules), PAMPs (i.e. pathogen-associated molecular patterns) and biogenic amines (i.e. nitrogen compounds). Metabolic signalling pathways are also involved in the activation of systemic immunity (Adamo, 2021; Lee and Lee, 2018; Fig. 1). In both insect and mammalian systems, IIS regulates the transcription factor forkhead box O (FOXO; Ahlers et al., 2019), with the insulin–FOXO pathway controlling glucose and lipid metabolism (see Fig. 1 for more details). FOXO is associated with longevity and nutritional signalling, as it induces the insulin-like receptor (InR; DiAngelo and Birnbaum, 2009). FOXO also regulates the expression of AMPs in the fat body (see Glossary; Becker et al., 2010; Fig. 1), and has another immune role through its induction of RNA interference (RNAi)-specific genes that mediate antiviral immunity (Galiana-Arnous et al., 2006; Trammell and Goodman, 2019; Wang et al., 2006).

Generally speaking, immunity against fungi and Gram-positive bacteria is mediated by the Toll-Dif/Dorsal pathway and immunity against Gram-negative bacteria is mediated by the Imd-Relish pathway in insects, leading to the expression of AMP genes (for details, see Galenza and Foley, 2019). Interestingly, Becker et al. (2010) have shown that starvation can induce AMP production through the FOXO pathway, independently of the evolutionarily conserved immune pathways Toll or Imd (Fig. 1). This might illustrate that when food is not available, some insects (at least Drosophila) show an anticipatory increase in AMP production (see also early melanisation in Pan et al., 2018). Further, Toll-like receptor activation induces the inactivation of the pathogen-induced phosphoinositide 3-kinase (PI3K)-Akt pathway (Fig. 1). Pathogen-induced Toll activation is upstream of the insulin signalling inactivation (Lee and Lee, 2018). More investigation is still needed to clarify the exact molecular relationship between the Toll and insulin signalling pathways during immune responses (Lee and Lee, 2018).

Another signalling pathway involved in immunity against viruses, JAK/STAT, is connected to IIS, which might again indicate that insects have evolved immune mechanisms that are responsive to nutritional acquisition (see discussion in Trammell and Goodman, 2019). The regulatory hormone adipokinetic hormone (AKH) – which is involved in physiological, metabolic and homeostatic processes, such as carbohydrate and lipid metabolism – also regulates innate immunity (Toprak, 2020). For instance, the principal role of AKH is to initiate lipid/carbohydrate mobilisation from the fat body, and its expression is elevated upon infection; this might induce immune responses (Ibrahim et al., 2017). There is some indication that AKH also activates the prophenoloxidase cascade, among other immune responses (Goldsworthy et al., 2003; Mullen and Goldsworthy, 2006).

Only a few studies have investigated the effects of the dietary nutrient balance on innate immunity components. Studies have generally shown that the balance of dietary protein and carbohydrate influences the expression of immune genes and other immune markers in naive and infected insects with contrasting effects in larvae and adults (e.g. Cotter et al., 2019; Ponton et al., 2020). The number of studies is, however, too limited; more investigation is needed to draw general conclusions. For instance, the two nutrient sensors TOR and AMPK might respond not only to the concentration of circulating nutrients (with TOR being stimulated and AMPK depressed either directly or indirectly by increasing concentrations of nutrients) but also to the amino acid/carbohydrate balance of the diet (Simpson and Raubenheimer, 2009). A low ratio of dietary protein to carbohydrate would decrease TOR activity, whereas a high protein to carbohydrate ratio would increase TOR activity. Whether these responses are directly linked to immune pathways needs to be confirmed. Finally, it is worth noting that although research to date has largely focused on the effects of nutrients on host immunity (i.e. ‘top-down’ regulation of pathogens), the nutrients may directly or indirectly impact the pathogen through ‘bottom-up’ non-immunological mechanisms. For example, it has recently been shown that dietary protein has a direct negative impact on the growth of bacteria in the haemolymph by altering haemolymph solute concentration, creating a significant osmotic stress for the pathogens (Wilson et al., 2020).

Insects have evolved medication behaviours that occur either in response to an active infection or to prevent an infection (prophylaxis). This is often referred to as ‘self-medication’, which is defined as animals using molecules such as secondary plant compounds or other non-nutritive substances with antiparasitic activity (de Roode et al., 2013; Raubenheimer and Simpson, 2009). Research investigating the role of macronutrients in nutritional immunology has also demonstrated that self-medication can involve the modulation of macronutrient selection (i.e. ingestion of a different ratio of macronutrients for infected individuals compared with non-infected ones) to compensate for the negative effects of the infection on fitness traits (Abbott, 2014; Bashir-Tanoli and Tinsley, 2014; Galenza et al., 2016; Ponton et al., 2011a,b; Povey et al., 2014). Self-medication can also stimulate immune responses. For example, modulation in macronutrient intake was measured in adult D. melanogaster injected with the bacterium Micrococcus luteus. Infected flies ingested a diet that was two times more concentrated in carbohydrate during the first 6 days post-infection (Ponton et al., 2020), compared with non-infected flies. However, they decreased their total food intake. This shift in nutritional behaviour increases the flies' survival after infection supposedly through an increase in the expression of immune genes (Ponton et al., 2020). Furthermore, the diet selected by infected flies is specifically beneficial during infection and is not optimal for the fitness of non-infected individuals (see discussion in de Rood and Hunter, 2019; Ponton et al., 2013, 2011a,b). Similar self-medication responses have been observed in a range of insect–pathogen interactions, including in larvae of moths from the genus Spodoptera infected with bacteria or viruses. Details of these studies are given in Table 1.

Most insects have mutualistic relationships with microbes, which provide benefits to their hosts such as immune priming (see Glossary) and stimulating immunity against pathogens (Buchner, 1965; Snyder, et al., 2012), facilitating digestion (Rio et al., 2006; Simonet et al., 2016) and supplementing host diets with micronutrients (Chomicki et al., 2020; Gil et al., 2003). Host diet is a key driver of the abundance and diversity of symbionts, and can modulate the role of mutualistic microbes in immune responses. Here, we review empirical studies on the interaction between insect nutrition, immunity and the microbiota.

The digestive tract harbours a high diversity (i.e. a high number of strains/species) and quantity (i.e. a high number of individuals) of microbes, and most of these microbes are dependent on their host's nutrition (for recent reviews, see Engl et al., 2020; Harris et al., 2019; see also Herren et al., 2014; Mason, 2020; Pernice et al., 2014; Wilkinson et al., 2007; Wong et al., 2015). In wood-feeding termites, changes in diet are accompanied by shifts in the dominance of protist species (Tarayre et al., 2015), and, in flies, the gut microbiota composition varies with plant and food type, and diet shifts (e.g. Chandler et al., 2011; Fink et al., 2013; Vacchini et al., 2017). More recently, it has been shown that black soldier fly (Hermetia illucens) larvae fed a high-fibre diet harbour a gut microbiota with greater bacterial richness (i.e. total number of bacterial strains/species) and diversity (i.e. amount of bacteria from each of the bacterial strains/species present) compared with individuals fed protein-rich and fat-rich diets (Greenwood et al., 2021). Although there is strong evidence that the nutritional conditions influence the gut microbiota composition at both larval and adult stages, the physiological significance and fitness consequences of this variation remain to be fully understood (for discussion, see Harris et al., 2019).

Microbiota diversity correlates with insect immunity and resistance to pathogens. In bumblebees, Mockler et al. (2018) found that individuals with high microbiota diversity have lower infection loads of Crithidia bombi, which is a common bumblebee gut parasite. A more diverse insect microbiota could intensify the competition experienced by pathogens when colonising their insect hosts (Engel and Moran, 2013), and this may be the case for insects that acquire most of their microbes from the environment (Boissière et al., 2012; Hammer et al., 2017). Moreover, in honeybees, the gut bacterium Snodgrassella alvi enhances survival after pathogen challenge, through an upregulation of antimicrobial genes (i.e. abaecin, apidecin and hymenoptaecin; Horak et al., 2020). The gut bacteria might also directly produce antimicrobial compounds, such as AMP and antibiotics, involved in protecting the host against infections (Daisley et al., 2020; Horak et al., 2020; Osborne et al., 2012; for review, see Jang and Kikuchi, 2020). Although the microbiome influences immunity across a large number of insect species, its influence in insects that have minimal symbiotic communities, such as lepidoptera (Hammer et al., 2017), needs to be investigated.

In addition to the gut microbiota, insects harbour specific bacterial symbionts that live intracellularly, mostly within specialised organs known as bacteriocytes (Douglas, 2011). Most intracellular symbionts have limited metabolic functions, with their growth and abundance being dependent on the host's nutrition (for review, see Whittle et al., 2021). For instance, high levels of nitrogen in the diet of the pea aphid (Acyrthosiphon pisum) increase the density of the obligate bacterial symbiont Buchnera aphidicola (Wilkinson et al., 2007), whereas in the tsetse fly, thiamine dietary supplementation results in reduced numbers of the obligate symbiont Wigglesworthia morsitans (Snyder et al., 2012). Some insects may indeed reduce the supply of nutrients to their symbionts to provide additional nutrients to host functions (Whittle et al., 2021). However, it remains to be fully explored how the host nutritional conditions regulate intracellular symbiont populations based on how obligative or facultative (obligate and facultative symbiont, respectively; see Glossary) their relationship with the host is.

Intracellular symbionts can have a substantial impact on host immunity. For example, mosquitoes are susceptible to viral infections as a result of their blood-feeding behaviour, and they rely heavily on the anti-viral function of the symbiont Wolbachia (Lu et al., 2012; Osborne et al., 2012). Mechanistically, Wolbachia induces an increase in immune gene expression (AMPs, melanisation, Toll pathway constituents, C-type lectins, serine proteases and transferrin) when mosquitoes are infected by pathogens such as dengue virus (Rancès et al., 2012). As another example, the pea aphid hosts an obligate primary symbiont, but can also host one or more facultative endosymbionts (see Glossary) that can challenge the immune system through increasing immune cell counts (i.e. granulocytes and plasmatocytes; Laughton et al., 2016). The results above suggest that intracellular symbionts are generally, but not always (e.g. Graham et al., 2012; Sinotte et al., 2018) beneficial to the immunity of their insect hosts. Indeed, the presence of symbionts at varying densities may involve a trade-off between their impact on host immunity and metabolic resources (Whittle et al., 2021).

Insects generally also have fungal symbionts, which may reside extracellularly or intracellularly (Gibson and Hunter, 2010). Despite being neglected in many earlier studies on insect symbioses, there has been an emerging interest in the role of insect–fungal symbioses in immunity (Brentassi et al., 2017; Cappelli et al., 2014; Gibson and Hunter, 2010; Meriggi et al., 2019; Tauber et al., 2019). Symbiotic fungi provide some developmental and survival benefits to their hosts (Brentassi et al., 2017), and can produce antimicrobial molecules (Cappelli et al., 2014). In the paper wasp (Polistes dominula) infected by Escherichia coli bacteria, yeast strains reduced the E. coli load in foundresses but not in workers (Meriggi et al., 2019). Associated with their enhanced immunity, wasp foundresses with prior yeast treatment show increased diversity and changes in the composition of their gut bacterial communities, but without changes in their fungal communities (Meriggi et al., 2019). Further, when honeybees were exposed to the pathogenic fungus Nosema ceranae, the growth of the bacterial symbiont Lactobacillus Firm-5 was supported when young individuals were fed Wickerhamomyces anomalus yeast, which contributes to the honeybee's immune health (Tauber et al., 2019). Fungi might have caste- and developmental stage-specific direct involvement in immunity, or may be involved indirectly by supporting the growth of other important symbionts.

Understanding the direct effect of symbionts on immunity and resistance to infection is challenging. The first challenge is to define a healthy composition of the symbiont communities inhabiting the host. Studies usually compare the symbiont composition of treated animals with those of control ones, and, in parallel, measure some physiological and life history traits to inform on the health status of the individuals. The composition of the symbiont communities also changes with physiological parameters such as age and developmental stage (Wang et al., 2020), implying that parameters need to be well controlled during experiments. Diet can influence the composition of the symbiont communities. 16S rRNA sequencing of Hermetia illucens gut microbiota showed, for instance, strong differences between soybean and insect diet groups in both type and relative abundance of microbial species (Zhineng et al., 2021). Some recent studies have attempted to explore the impact of insect diet on their immunity in relation to their microbiota. In greater wax moth (Galleria mellonella) larvae, a more diverse diet is associated with a higher number of Enterococci in the microbiota, which may be linked to a greater expression of AMP genes (Krams et al., 2017). A greater expression level of AMPs is also induced when soldier fly larvae have a high inhibition of Gram-negative bacteria linked to high-protein diets (Vogel et al., 2018). In another study, Maes et al. (2016) demonstrated that aged pollen/substitute (i.e. when hive frames were housed with 500 adult bees at 50% humidity and 35°C until bees reached an average of 14 and 21 days old) leads to a dysbiosis (see Glossary) of the gut microbiota, involving a reduction in numbers of the core symbiont S. alvi, but an increase in the amount of the pathogenic bacteria Frischella perrara and Parasacharibacter apium (Cariveau et al., 2014; Engel et al., 2015). Gut dysbiosis in honeybees results in decreased survival and colonisation by another pathogenic fungus, Nosema; this can be prevented by the core symbiont S. alvi taking up residence in the gut in early development (Maes et al., 2016). Castelli et al. (2020) examined whether nutritional stress simultaneously affects honeybee gut microbiota, immunity and infection by Nosema. Honeybees fed a nutritionally poor diet (i.e. Eucalyptus grandis pollen) showed a lower abundance of Lactobacillus Firm-4, Firm-5 and Bifidobacterium spp. in their gut, and a higher abundance of Bartonella apis compared with bees fed a polyfloral pollen. They also observed a decrease in the expression levels of vitellogenin and genes associated with immunity (glucose oxidase, hymenoptaecin and lysozyme).

Work on the African armyworm, Spodoptera exempta, also shows the importance of the microbiota in immunity. When larvae of this species were brought into the laboratory and fed a semi-synthetic diet (i.e. a wheatgerm-based diet including a broad-spectrum antibiotic), their gut microbial diversity was reduced and this led to a reduction in their capacity to resist infection with the baculovirus Spodoptera exempta nucleopolyhedrovirus (Donkersley et al., 2023). The capacity to resist the virus was greatly improved when the gut microbiota was restored following a shift onto a diet supplemented with field-collected armyworm faeces.

Overall, studies on the interplay between nutrition, symbionts and infection in insects convincingly show that a healthy microbiota corresponds to effective immune responses, which in turn depends on host dietary conditions. Immune responses of insects are affected by their nutrition, with the microbiota modulating this interaction. Although high microbiota diversity seems to have immunological benefits generally, this is not always the case (Yoon et al., 2019). Within the symbionts inhabiting the insect body, multiple taxa may interact to either compromise insect host immunity (Fast et al., 2020) or synergistically enhance the capability of individuals to fight infections (Consuegra et al., 2020; Oliver et al., 2014; Xie et al., 2014), making the combined transient and resident symbionts important. Apart from bacterial communities and yeasts, other kingdoms such as viruses, archaea and protozoa might also be important (Gurung et al., 2019) and may be influenced by nutrition. Only a small number of studies in insects have investigated the combined effects of food quality on symbiont communities, immunity and resistance. Hence, the functional relationships between symbionts, diet and infection remain to be fully explored.

There is extensive evidence across taxa that diet quality and access to specific nutrients, both during early development and in the parental generation, affects the development and performance of the immune system. Nutrition in early development plays a key role in the maturation of the immune system (Calder et al., 2006). If nutritional needs are not met during the critical period of early development, the immune system may be ineffective upon infection or even over-sensitised against innocuous and beneficial antigens (Calder et al., 2006; Nwaru et al., 2010; Vassallo and Camargo, 2010).

We know from studies on mammalian species that nutrition may also have effects on the immune system across generations. Individuals may be more susceptible to diseases if relatives from previous generations have suffered from malnutrition (Barker, 2004; Bateson et al., 2004; De Boo and Harding, 2006). In general, the effects of diet and nutrition during development or across generations can be considered under two alternative (but not necessarily exclusive) hypotheses. On the one hand, these could be seen as ‘silver spoon effects’, whereby high-quality diet enhances the immune response of the adult (in terms of developmental effects) or offspring (for transgenerational effects; Monaghan, 2008). On the other hand, such effects can also be seen as a form of ‘predictive adaptive response’ or ‘immune priming’, whereby low-quality diet is indicative of a more pathogenic environment; thus, individuals exposed to a low-quality diet during development or in the parental generation show a heightened immune response (Gluckman et al., 2005; Little et al., 2003). Whether empirical findings fit one hypothesis or another depends on the type of dietary manipulation, the immune assay conducted and the life history of the species in question. Here, we highlight how studies in insects provide unique insights into the evolution and mechanisms by which nutrition affects immunity through development and across generations. This section also proposes methods to further test alternative hypotheses explaining the immunological impact of insect nutrition.

In insects, nutrition during juvenile stages has a significant impact on the development of adults (Koyama et al., 2013; Mirth and Shingleton, 2012; Nijhout, 2003; Nijhout et al., 2014). For example, when larvae of the greater wax moth are experimentally provided with a low-nutrition diet (i.e. diluted diet), they develop into adults with lower body mass and require longer to develop (Kangassalo et al., 2018). The immune development of insects is negatively affected by poor nutrition during larval development with affected adults showing a lower encapsulation strength (Kangassalo et al., 2018; Wilson et al., 2019).

Priming effects have also been observed, whereby larvae which experience low nutritional conditions, as a result, develop into adults with enhanced immune responses in anticipation of suboptimal conditions (Wang et al., 2016). Priming effects in response to non-nutritional cues have been observed; for example, mosquito larvae exposed to the insecticide permethrin show heightened immune responses (i.e. melanisation and antibacterial responses) as adults (Hauser and Koella, 2020). In this case, however, nutritional limitation weakened such priming as larvae that competed for food during development were less affected by insecticide exposure.

Such priming effects may be more prevalent in holometabolous insects in comparison to hemimetabolous species, given that the larval stage requires greater adaptive plasticity compared with the later adult stage, as larvae tend to show a reduced ecological range relative to the adults. This means that they have less opportunity to alter their nutritional state through their diet and modulate their immune system in this manner (English and Barreaux, 2020). Further, predicting the effect of larval diet in adults is likely to be dependent on reproductive strategies (e.g. some adults do not feed, live for only a few days and then die); further investigation is needed to explore this.

The experimental studies cited above highlight how nutrition in development is important for later immunity at adulthood. However, these effects can also occur across generations in the form of a carry-over or priming effect (Uller et al., 2013). Parental nutrition in insects affects offspring immunity: poor nutrition has been shown to have negative effects across generations. For example, in a study on Indian meal moths (Plodia interpunctella), offspring immune reactivity was reduced when either parent received poor nutrition, with further reduction in immunity when both parents received poor nutrition (Triggs and Knell, 2012). By contrast, parental nutrition effects can also influence offspring responses in potentially adaptive ways. In the greater wax moth, a maternal low-nutrition diet increased the survival of offspring when exposed to the pathogenic fungus Beauveria bassiana (Kangassalo et al., 2015). The results from Kangassalo et al. (2015) support the hypothesis that poor parental nutrition is being used as an indication of the resources that are likely to be available to the offspring. If the offspring have fewer resources to fuel their immune systems, then selection may favour parents that give their offspring an immunological ‘helping hand’. This mechanism may also provide the offspring with a predictive cue to allow them to adapt and maximise their fitness in a poor environment (Uller et al., 2013).

When the parental diet also includes pathogenic agents, this can affect offspring immunity through a phenomenon known as ‘transgenerational immune priming’ (TGIP), which is likely to be an adaptive effect whereby parents prepare their offspring's immune system for encounters with common pathogens (e.g. Futo et al., 2017; Little et al., 2003; Moret, 2006). For example, when female larvae of the greater wax moth were provided with diets containing pathogenic bacteria, the eggs deposited by the developed females showed an induced expression of various immunity-related genes (Freitak et al., 2014). The transmission of dietary bacteria occurred from the gut of mothers to their haemocoel, followed by the ovary and lastly in the outer membrane of the laid eggs (Freitak et al., 2014).

In the context of TGIP, microbes that are consumed may be considered as nutritional supplements, akin to probiotics in foods. These ‘probiotics’ are known to enhance immunity across a wide range of insect orders (Grau et al., 2017; Maruščáková et al., 2020; Msaad Guerfali et al., 2021; Rossoni et al., 2017). Ideally, ‘dietary pathogens’ and nutrition should be studied separately: for example, in a study of cabbage looper moths (Trichoplusia ni), Shikano et al. (2015) separated the factors ‘dietary pathogens’ and ‘nutritional quality’ in parental diets. They found that when moth offspring are fed the same bacterial pathogen as their parents, the offspring's resistance to the pathogen and antibacterial activity increase tremendously in comparison to those of offspring whose parents are not exposed to the pathogen; this enhanced immunity is not observed when offspring are exposed to a pathogen not previously encountered by their parents (Shikano et al., 2015). By contrast, when parents consume diets of lower nutritional quality, the nutritional stress tolerance of their offspring is increased, as is their resistance to both pathogens (Shikano et al., 2015). Moreover, when both pathogenic and nutritional stressors are applied, the cross-generation transfer of pathogen resistance takes precedence over nutritional stress tolerance (Shikano et al., 2015). Taken together, these results emphasise the potential trade-offs between TGIP and the transfer of nutritional stress tolerance. This trade-off may be environment dependent as, for example, TGIP would confer greater fitness advantages in an environment where disease risk is high compared with a resource-depleted environment.

The effects of parental diet and nutrition can be sex specific, although the results from some studies can be complex. In some cases, the diet of one parent but not the other has an effect. In the greater wax moth example discussed above (Kangassalo et al., 2015), the effects on offspring survival during pathogen exposure were evident only when the maternal diet is manipulated, but there was no effect of paternal diet. In a study by Zirbel and Alto (2018) on mosquitoes, both maternal and paternal effects were shown to interact and significantly enhance the survival of their larval offspring when infected by the dengue virus. Specifically, there was a greater positive effect on offspring immunity when diets differed between fathers and mothers (which received a low and high quantity of food, respectively), compared with when the parents were both fed a high quantity of food (Zirbel and Alto, 2018). In the cabbage looper moth, TGIP has been shown to be provided not just by mothers but also by fathers (Freitak et al., 2009). When both parents are reared on diets containing bacteria, offspring upregulate their immune enzymatic activities, immune protein expression and immune gene transcription when immune challenged by a bacterial injection (Freitak et al., 2009). Most insects provide relatively little paternal care and resources to their offspring; fathers may compensate for this by priming their sperm following bacterial exposure, to transmit epigenetic factors that enhance the offspring's resistance to diseases and parasitism (Ashe and Whitelaw, 2007).

Although studies have shown both positive and negative effects of parental diet on offspring immunity, there are also several studies showing no effect. For example, in the Queensland fruit fly, there was no significant effect of parental diets on offspring immunity when the offspring were infected by the pathogenic bacterium Serratia marcescens (Dinh et al., 2021). Given the mixed results and interpretations, the transgenerational effects of insect nutrition on immunity can indeed be complex and depend on factors determining the mechanistic link between nutrition and immunity (e.g. the type of diet manipulation and infection assay) and the potential adaptive nature of such effects (e.g. whether a diet cue in the parental generation indicates a likely pathogen threat). Hence, future studies may further explore the effects of parental diet on insect immunity across several generations, focusing on a broad range of immune-related measurements.

In summary, insect nutrition impacts immunity both developmentally and across generations, although the effects are complex. To elucidate the mechanisms and test potential adaptive explanations, further experimental studies could incorporate diverse experimental systems exhibiting a high likelihood of adaptive effects, such as insects with short generation times and those that display a clear link between infection with a pathogen and the subsequent measurement of pathogen load. As a result, comparative work could be carried out to identify general patterns of insect nutrition on immunity, while highlighting the life history of different insects and types of nutritional manipulation and immune assay across empirical studies. As also discussed above, dietary microbes seem to closely interact with the insect host microbiota to affect immunity (Ponton et al., 2011a,b; Tetreau et al., 2019). Given the crucial role of the microbiota, future studies may investigate whether developmental or parental nutrition effects act directly on the immune system, or whether the immune system operates indirectly through nutritional effects on the microbiota.

Animals do not exist in isolation, and how they interact with other individuals can impact: (i) their nutrition, through how and what they eat; (ii) their immunity, through exposure to various types and abundances of parasites, and through the direct or environmental social transfer of immune molecules (Masri and Cremer, 2014); and (iii) their microbiome, which plays a role in both nutrition and immunity (see above), through direct or indirect transfer of symbionts.

Insects cover all levels of sociality, from solitary, where adults interact only during mating, through various forms of semi-social and social interactions, including species with biparental care and cooperative breeders, through to fully eusocial, where there is a strict division of labour, sterile castes and overlapping generations (West et al., 2006). In addition, many species, although not social in the evolutionary sense, occur at high densities, in aggregates or gregarious groups, where many individuals gather to access resources or increase survival.

Parasite transmission is typically density dependent (McCallum et al., 2001); therefore, sociality can influence the level of individual exposure to parasites. In solitary individuals, the risk of parasite exposure is usually stochastic, but it is likely to be lower than that experienced by social individuals as a result of less frequent interactions with conspecifics (except for ‘solitary’ species that occur at very high densities, as commonly occurs in many lepidoptera; e.g. viruses in African armyworm; Rose et al., 2000). Beyond parasites, an organism's sociality can also impact the microbiota they are exposed to at various life stages. The exchange and maintenance of symbionts through sociality facilitates the sharing of characteristic and beneficial microbiota, including those required for nutrition and pathogen defence (Engel et al., 2012). Although solitary individuals still have some methods and opportunities to transfer microbial symbionts (discussed in Engel and Moran, 2013; Voulgari-Kokota et al., 2019), they are comparatively limited.

The social environment in which an individual lives and grows will also influence how it forages, and the type of food it consumes. In solitary species, individuals may be entirely independent in their choice of food (e.g. Auerbach and Simberloff, 1989), or constrained in their dietary choices (Austin and Gilbert, 2021). Social species will always control offspring diet to some degree but offspring can signal their needs (Hendriksma et al., 2019; Lihoreau et al., 2015; Royle et al., 2012; Dussutour and Simpson, 2008; Kraus et al., 2019; Pernal and Currie, 2002). These differing levels of nutritional control mean that the ability of an individual to adjust its intake in response to infection may be dependent on changing the foraging behaviour of others (but see Simpson and Raubenheimer, 2012, for ingestional mechanisms to modify the nutrients digested, absorbed and assimilated).

Sociality can also play a role in an individual's immunological investment. First, in ‘outbreaking’ species, where population density fluctuates across time and space, many species have evolved plastic investment responses, whereby immunity is ramped up in the presence of conspecifics to combat the increased risk of parasite transmission; this is known as density-dependent prophylaxis (Wilson and Cotter, 2009, 2013; Wilson and Reeson, 1998). We might therefore expect that group-living species would invest more heavily in their immune systems than those that typically live alone (but see Wilson et al., 2003).

As discussed above, solitary species have been shown to alter their intake of nutrients to fight infections, but how would such a response be achievable on a collective scale? We know that within social insect systems there is often some form of feedback between the larvae and the adults in terms of nutritional requirements (especially in progressively provisioning systems where adults feed larvae directly after hatching until complete development). Honeybee individuals, for example, show the ability to forage differentially to make up for nutrient deficiencies (Hendriksma and Shafir, 2016). Adult social bees also show different foraging behaviours when infected by certain parasites (e.g. Ferguson et al., 2018; Table 1), and both honeybees and bumblebees can adapt at the colony level to changes in nutrient requirements (Hendriksma et al., 2019; and also ants: Dussutour and Simpson, 2009). It stands to reason, then, that social insects could also alter their colony-level foraging behaviour in response to infection, because an individual's nutrition has a direct impact on its ability to respond immunologically (Danihlik et al., 2018). Furthermore, nutrition can affect group-level defences (Kay et al., 2014). Thus, there is scope for colonies to self-medicate, altering not simply individual-level immune responses but social immune responses too. More research is needed to understand how sociality may interact with the interplay between immunity and nutrition in invertebrates.

For a long time, the immune system of insects has been considered as non-complex and non-specific. Yet, studies in recent decades have shown that insect immunity can be trained and primed (for recent review, see Lanz-Mendoza and Contreras-Garduno, 2022), and that it is influenced by a variety of biotic and abiotic factors. In this Review, we have shown that the nutritional environment of insects is an essential factor influencing their immunity and resistance/tolerance to infections. It is interesting to note that although the metabolic pathways and innate immunity are highly conserved across invertebrates, their interactions can be highly specific to individual groups/species. This is certainly due to the large diversity of social forms, life cycles and environments in which insects live, leading to specific responses that may or may not be evolutionarily adaptive. Investigating the factors that interact with the interplay between nutrition and resistance gives a more comprehensive and integrative understanding of the complex field of nutritional immunology. In the future, pathogen exposure, nutrition, immune function, sociality and the gut microbiome should be considered individually and in combination when attempting to understand an individual's nutritional immunology. The next research exploring the depths, intricacies and mechanisms of these components and their impacts in more detail will advance our understanding of nutritional immunology.

In terms of the bigger picture, understanding how changes in the environment influence the fitness costs associated with infections is essential when deploying conservation measures for insect populations that have been steeply declining in the last few years (Seibold et al., 2019). For instance, the recent development of natural probiotic treatments (i.e. using native microbiome strains) is a promising route for protecting insect health and, more particularly, pollinators' health (Motta et al., 2022). The development of this field of research directly results from knowledge regarding the interactions between metabolism, gut microbiota, immunity, resistance and the food sources that insects forage on. Other fields that may benefit from a greater understanding of nutritional immunological interactions include novel approaches to insect pest management (e.g. Qadri et al., 2020; Xu et al., 2020, 2022), disease vector control (Caragata et al., 2013; Kho et al., 2016; Hoffmann et al., 2011), insect conservation (van Rijn and Wackers, 2016; Vaudo et al., 2015), and insects as food and feed (Qian et al., 2022; Rumpold and Schluter, 2013). In this Review, we have focused our discussion on a few factors influencing the interaction between nutrition and resistance; however, other environmental conditions – such as temperature and pesticides – might also be of importance to nutrition and resistance, and further discussion is needed. The field of eco-immunology may, in the future, offer solutions to help insect populations to survive our changing world.