Due to global warming, climate change has occurred, leading to a decrease in the habitat of honeybees and a subsequent decrease in the number of honeybee populations (Le Conte and Navajas, 2008). The gradual and consistent reduction in the capacity of honeybee populations to survive through winter in different parts of the world, such as North America and Europe, is a concerning occurrence that has been reported in various studies (vanEngelsdorp et al., 2015; Gray et al., 2019). In the last few decades, an increase in mortality rates during overwintering has been observed (vanEngelsdorp et al., 2009), which is likely attributable to a combination of biotic and abiotic stressors. These stressors include the Varroa destructor mite (van Dooremalen et al., 2012), Nosema sp. Microsporidia (Higes et al., 2008), viral infections (Highfield et al., 2009), exposure to pesticides (Simon-Delso et al., 2014), dietary and nutritional deficiencies (Requier et al., 2017), unfavorable winter conditions (Steinhauer et al., 2015), and climate change (Le Conte and Navajas, 2008). Depending on the temperature change, the honeybee population can be divided into two groups: summer bees with a short lifespan and winter honeybees with a long lifespan. Summer bees mainly engage in brood rearing, nectar and pollen collection, and have a lifespan of about 15-38 days. On the other hand, as brood rearing begins to decline in the fall, winter bees begin to emerge, and their main task is to keep the colony warm throughout the winter and to raise the first new generation in late winter or early spring, with a lifespan of about 140-320 days (Page and Peng, 2001).

Although many studies have revealed the biochemical parameters of the difference in lifespan between summer honeybees and winter honeybees, relatively few studies have been conducted on metabolic changes within honeybees. Recently, differences in metabolic profiling between summer honeybees and winter honeybees were revealed through ¹H NMR analysis (Lee et al., 2022). According to the results, there were 28 metabolites that showed statistically significant differences between the two groups, which can be divided into carbohydrates, amino acids, choline-containing compounds, and unknown compounds based on the metabolite groups. Compared to summer bees, winter bees had higher concentrations of fructose, sucrose, and trehalose, while their levels of amino acids, except for proline and alanine, were lower than those of summer bees. Carbohydrates such as sucrose and trehalose are essential energy sources for bees and are stored as glycogen in the fat body (Blatt and Roces, 2001). Therefore, high levels of carbohydrates can be converted to high levels of glycogen, which can be used as a survival energy source during the winter. Additionally, trehalose is known to protect bees from various stresses such as dryness, low temperature, and hibernation (Watanabe et al., 2002; Kostal et al., 2007; Overgaard et al., 2007). Therefore, the high trehalose levels in winter bees are considered an adaptive response to withstand cold temperatures and other environmental stresses.

Proline is the primary amino acid found in the hemolymph of honeybees (Hrassnigg et al., 2003) and is recognized as an insect cryoprotectant, similar to trehalose (Kostal et al., 2012). Studies on freeze tolerance in fruit flies (Drosophila melanogaster) have shown that proline and trehalose are the most abundant metabolites accumulated during cold adaptation (Kostal et al., 2012; Williams et al., 2016). Therefore, the elevated levels of trehalose and proline in winter bees function as cryoprotectants, safeguarding the integrity of cellular membranes during the cold winter period, maintaining optimal water levels, and serving as an energy reservoir.

Meanwhile, Lee et al. (2022) showed results that adenosine monophosphate (AMP) and oxidized nicotinamide adenine dinucleotide (NAD⁺), which are nucleotide metabolites, were also produced more highly in winter bees than in summer bees. The changes in the concentrations of AMP and NAD⁺ are closely related to the cellular energy metabolism, and it is known that they regulate the expression of AMP-activated protein kinase (AMPK) and sirtuin genes (Hardie, 2007; Bonkowski and Sinclair, 2016), thereby regulating lifespan. In other words, activated AMP and NAD⁺ have been shown in studies using C. elegans and D. melanogaster to activate the expression of AMPK and sirtuin, thereby extending lifespan. Therefore, the increased expression of AMP and NAD⁺ in winter bees can be considered as one of the switches that promote the transition from a short-lived generation (summer bees) to a long-lived generation (winter bees).

When nutrient sources are limited, the productivity of honeybees is negatively impacted. Inadequate nutrition causes a decrease in the supply of replacement bees, leading to a decline in the number and vitality of bees within colonies, as well as a reduction in their resistance to prevalent diseases (Paray et al., 2021). Therefore, managed honeybee colonies are commonly provided with artificial diets known as “pollen substitutes” due to insufficient pollen availability in the environment and the consequent risk of malnutrition. Protein-rich components such as soy, corn gluten are typically used in various diet formulations to supply essential amino acids (Brodschneider and Crailsheim, 2010). However, despite the widespread use of these diets, their effectiveness compared to natural pollen remains uncertain. Recent studies have explored the potential of microalgae as a dietary supplement for managed honeybees (Madeira et al., 2017). Notably, Chlorella, a eukaryotic microalga, and Arthrospira, a prokaryotic cyanobacterium commonly known as spirulina, have been identified as rich sources of protein, fatty acids, sterols, and other bioactive compounds with nutraceutical properties. These microalgae are easily digestible and have been found to mimic the growth characteristics of a natural pollen diet. However, despite promising results, the underlying metabolic mechanisms responsible for the positive effects of microalgae on bee health remain poorly understood.

Ricigliano et al. (2022) have found that microalgae can be used as a feed additive, which can be utilized as a natural product to improve honeybee health. They added Chlorella vulgaris and Arthrospira platensis to the feed and provided it to the honeybees to study the changes in feed consumption and metabolite profiles. Untargeted metabolite analysis was performed using LC-MS and GC-MS to measure the metabolite profiles of honeybees that consumed the natural feed and those that consumed the feed containing microalgae. Two groups of honeybees that consumed different diets shared 248 metabolites in LC-MS and 87 metabolites in GC-MS. Twenty-five specific metabolites were identified according to the diet, which were found to contain lipids, essential fatty acids, vitamins, and plant compounds. The honeybees that consumed pollen diet with algae showed higher levels of linoleic acid, zeaxanthin, lutein, α-tocopherol, and β-carotene. These metabolite analysis results confirmed that algae such as Chlorella included in artificial pollen diet can regulate honeybee growth, antioxidant activity, and heat shock protein gene expression. Therefore, algae like Chlorella can be used as sustainable artificial supplementary and can be utilized as a natural product to improve honeybee health. This metabolite analysis provides important information for the development of precise nutritional management of honeybee diet.

Pesticides have a prevalent application in agriculture, albeit posing significant threats to the well-being of numerous non-target organisms (Desneux et al., 2007; Schafer et al., 2012). In recent times, there has been a significant focus on the impact of neonicotinoid pesticides and their potential to harm the health of honeybee colonies worldwide (Blacquiere et al., 2012). Neonicotinoid insecticides act as systemic insecticides, causing abnormal excitement and convulsive paralysis in insects, and are widely used in agriculture to control various types of pests (Brown et al., 2006; Simon-Delso et al., 2015). The exposure to insecticides is not limited to direct contact; rather, it can also occur through the ingestion of contaminated food sources, such as nectar, pollen, and water. Extensive research has reported the presence of neonicotinoid insecticides such as imidacloprid, thiamethoxam, and thiacloprid residues in various bee-derived products, including honey, beeswax, and pollen, as documented in several scientific studies (Codling et al., 2016; Sanchez-Hernandez et al., 2016; Tosi et al., 2018). Most neonicotinoid insecticides are highly toxic to bees and can induce excessive toxicity reactions, but it has been found that outdoor insecticide doses do not cause acute toxicity (Blacquiere et al., 2012). However, research reports have been published indicating that exposure to sublethal doses of neonicotinoid insecticides can affect brain and thorax development (Oliveira et al., 2014), impair learning and memory (Aliouane et al., 2009), and destroy the immune system in bees (Brandt et al., 2016). Furthermore, many outdoor experiments have shown that neonicotinoid insecticides can reduce the foraging and cognitive abilities of bees returning to their hives (Fischer et al., 2014). In a recent study, an investigation was conducted using LC-MS and GC-MS-based metabolomic techniques to explore the metabolic profile variations between honeybees treated with thiacloprid and those in the control group (Shi et al., 2018). The aim of this investigation was to gain a better understanding of the intricate interactions between neonicotinoids and honeybees at the molecular level. Researchers investigated the metabolite profiles of bee heads exposed to 2 mg/I of thiacloprid continuously for three days using both LC-MS and GC-MS. The analysis showed that 115 metabolites showed differences in expression compared to the control group, with 80 metabolites appearing in the LC-MS analysis and 35 metabolites appearing in the GC-MS analysis. The 115 metabolites included various amino acids, fatty acids, phospholipids, and other compounds, with over 70% of these metabolites showing higher expression than in the control group. Interestingly, among the metabolites that showed differences in expression, thiacloprid itself or its metabolites were not detected. This suggests that thiacloprid is rapidly metabolized in the thorax after being absorbed by bees, so only minimal amounts of thiacloprid can reach the bee head. According to previously published research results, when detoxification of alkaloid nicotine occurs in bees, metabolic activity, antioxidant and heat shock responses are increased (Rand et al., 2015). Genes induced by the neonicotinoid thiamethoxam that show differences in expression are primarily abundant in biosynthetic and metabolic pathways related to ribosomes, tyrosine metabolism, and drug metabolism pathways.

Nosema ceranae, a gut parasite prevalent in A. mellifera, is widely distributed worldwide and poses a significant threat to the Western honeybee, both at the individual and colony levels (Goulson et al., 2015). As with other microsporidian species, N. ceranae is capable of manipulating bee physiology and behavior to create a more favorable environment for its reproduction. N. ceranae infection has been shown to weaken the integrity of the midgut tissue (Dussaubat et al., 2012), alter energy demand (Alaux et al., 2009) and hemolymph sugar levels (Mayack and Naug, 2010), and suppress the immune response of honeybees (Alaux et al., 2009). Infected honeybees have been found to have shorter lifespans than uninfected honeybees in some studies, but the presence of N. ceranae is not always associated with honeybee mortality, indicating variations in the parasite’s virulence (Vidau et al., 2011). These variations may be influenced by host or parasite genetics, climate, nutrition, or interactions with other stressors such as environmental contaminants or other parasites.

Jousse et al. (2002) have developed a novel approach for conducting metabolomics analysis on hemolymph samples through a combination of LC/MS and NMR techniques. Their objective was to investigate the metabolic changes occurring in honeybees after experimental infection with N. ceranae. Hemolymph samples were collected at two time points, 2 and 10 day post-infection, from both uninfected and infected bees. LC-MS was used to analyze individual hemolymph samples, while NMR spectra were obtained from pooled samples. The statistical analysis revealed that the age of bees had the most significant effect on the metabolite profiles. The study identified 15 biomarkers that could distinguish between infected and uninfected bees, as well as the combined effect of age and infection. These biomarkers included carbohydrates (α/β glucose, α/β fructose, and hexosamine), amino acids (histidine and proline), dipeptides (Glu-Thr, Cys-Cys, and γ-Glu-Leu/Ile), metabolites involved in lipid metabolism (choline, glycerophosphocholine, and O-phosphorylethanolamine), and a polyamine compound (spermidine) (Fig. 2). Overall, the untargeted metabolomics-based approach used in this study provides valuable insights into the pathophysiological mechanisms underlying honeybee infection by N. ceranae.

Fig. 2. 
Fifteen biomarkers were successfully detected and categorized as potential biomarkers that indicate infection alone or the combined effect of age and infection. The identified biomarkers include various carbohydrates (α/β glucose, α/β fructose, and hexosamine), amino acids (histidine and proline), dipeptides (Glu-Thr, Cys-Cys, and γ-Glu-Leu/Ile), metabolites related to lipid metabolism (choline, lycerophosphocholine, and O-phosphorylethanolamine), as well as a polyamine compound (spermidine) (Jousse et al., 2020).