Insight into the Prospects of RNA Interference for Honeybee Pathogens and Parasite Control

1. Introduction

Honeybees are essential to the health and functioning of terrestrial ecosystems, serving as the primary pollinators of flowering plants [1]. Through pollinating a broad spectrum of wild and cultivated plants, honeybees maintain ecological balance and support biodiversity. Their pollination services are indispensable to agricultural systems, facilitating the production of approximately one-third of the food crops consumed globally, including fruits, vegetables, nuts, and oilseeds [2]. In the United States alone, the economic value of honeybee pollination is estimated at $10~20 billion annually, with some studies suggesting contributions up to $24 billion when indirect benefits to industries such as food processing, transport, and export are considered [2]. Beyond their ecological and agricultural significance, honeybees also produce a variety of economically valuable products, including honey, propolis, bee venom, royal jelly, and beeswax, which have applications in the food, pharmaceutical, and cosmetic industries.

Despite their critical importance, honeybee populations have experienced substantial declines over the past few decades. Since 2006, U.S. beekeepers have reported annual colony losses ranging from 30% to 40% [3]. This decline is multifactorial, driven by the synergistic effects of parasitic mites, pathogenic infections, pesticide exposure, habitat loss, and nutritional stress due to reduced floral diversity or excessive sugar feeding [4]. Among these, Varroa destructor mites are considered the most significant biotic threat to honeybee health. These ectoparasites not only weaken individual bees through hemolymph feeding but also serve as vectors for a range of debilitating viruses, including deformed wing virus (DWV), thereby exacerbating colony mortality [5].

Addressing honeybee health threats remains a significant challenge for apicultural researchers and practitioners. Currently, there are no approved antiviral treatments for honeybee diseases, except HoneyGuard-R^® (Eagle Vet Tech Co., Republic of Korea), making colony management, biosecurity, and vector control essential components of disease prevention strategies [6,7]. Antibiotics are commonly used to manage bacterial and fungal infections; however, their overuse has raised concerns about antibiotic residues in hive products and the development of drug-resistant pathogens [8]. Similarly, miticides used against Varroa mites are becoming less effective due to the emergence of resistant mite populations [9]. In light of these challenges, RNA interference (RNAi) has emerged as a promising and targeted biotechnological tool for managing honeybee diseases. RNAi offers a species-specific mechanism to silence the expression of critical pathogen and parasite genes, potentially enabling safer, more sustainable control methods. This review explores the potential benefits and limitations of RNAi technologies in enhancing honeybee health and mitigating colony losses.

2. Mechanism of RNAi in Honeybee

RNAi is a powerful, targeted gene-silencing mechanism with significant applications in insect research and pest management [10]. RNAi in insects begins with the introduction of exogenous double-stranded RNA (dsRNA), which can be delivered through microinjection, oral ingestion, or transgenic expression in plants for pest control [11]. Once inside the insect, dsRNA is taken up by cells via endocytosis or SID-like channels, depending on the species [12]. Inside the cell, the enzyme DICER processes the dsRNA into short fragments known as small interfering RNAs (siRNAs), typically 21~23 nucleotides in length [13]. These siRNAs are then incorporated into the RNA-Induced Silencing Complex (RISC), which guides the complex to a complementary messenger RNA (mRNA) target. Upon binding, the Argonaute (AGO) protein within RISC cleaves the mRNA, preventing its translation and thereby silencing the gene by reducing or eliminating the production of the corresponding protein [14] (Figure 1).

RNAi has emerged as a valuable tool in honeybee research and pathogen control, offering a targeted approach to enhance bee health. Researchers have used RNAi to investigate gene function in honeybees, providing insights into developmental processes, immune responses, and behavior [15]. More importantly, RNAi is being explored as a strategy to combat viral infections that pose significant threats to honeybee populations. For instance, dsRNA targeting viral genes has been successfully used to reduce the replication and pathogenicity of the Israeli acute paralysis virus (IAPV), thereby improving honeybee survival rates [16]. Additionally, RNAi has shown potential in controlling other viruses such as deformed wing virus (DWV), which is often associated with Varroa destructor mite infestations [17]. These findings highlight RNAi as a promising method not only for functional genomics in honeybees but also for developing antiviral therapeutics to mitigate colony losses.

3. Utilizing RNAi for the Treatment of Viral Infections in Honeybees

Viral pathogens pose a significant threat to honeybee health, contributing to reduced colony productivity and, in severe cases, colony collapse. Viruses such as DWV and IAPV have been linked to weakening bee colonies, leading to diminished honey yields, reduced brood production, and a decline in adult bee populations [18,19,20]. DWV, in particular, is associated with colony collapse disorder, in which entire colonies may die off or abandon their hives [21]. Additionally, viral infections increase honeybees' susceptibility to other stressors, including parasites, pesticides, and poor nutrition, further compromising colony health [19,22]. The spread of viruses is not limited to managed bee populations; many honeybee viruses can also infect wild bee species, potentially contributing to declines in global pollinator populations [23]. Compounding this issue, the detection and management of viral infections are difficult, as many viruses can persist asymptomatically, making their impact hard to detect [18,23,24]. The Varroa mite, an invasive pest, plays a crucial role in the spread of these viruses, as it weakens bee immune systems and acts as a vector for many viral diseases [25,26]. The combination of viral infections and other environmental stressors has led to high annual losses of honeybee colonies, which have averaged 30~40% in the U.S. since 2006 [19,22]. With at least 20 known viruses affecting honeybees, including DWV, IAPV, and black queen cell virus (BQCV), the impact of viral pathogens is substantial, with effects ranging from paralysis and deformities to colony collapse [17,18,23]. As honeybees play a critical role in pollinating about one-third of the world’s food crops, addressing viral threats is essential for sustaining agricultural productivity and ecosystem health [28].

Cultural management practices are fundamental components of integrated pest management (IPM) strategies for controlling viral diseases in honeybees. These practices help alleviate colony stressors that can exacerbate viral infections, as viruses typically persist at low levels within colonies and become more prominent under stressful conditions [19]. Key elements of cultural management include promoting strong colony health through practices such as effective queen management, routine equipment rotation, and control of viral vectors such as Varroa mites [23,26,28]. Varroa mites not only weaken bees but also serve as vectors for several bee viruses, including DWV, which complicates viral transmission [29,30,31,32]. Regular monitoring of colony health, with particular attention to viral symptoms and screening for other pathogens such as Nosema, is crucial for early detection and management [3,22]. Although no chemotherapies are available to treat viral diseases in honeybees, effective management of these stressors can substantially reduce the risk of viral outbreaks and enhance colony resilience [25]. Furthermore, proper management of colonies under stress can prevent asymptomatic viral infections from progressing to more severe, symptomatic stages, making cultural management an essential tool for minimizing the impact of viral pathogens on bee populations [21].

RNAi for treating viral diseases in honeybees has emerged as a promising strategy for controlling viral infections. RNAi, a key antiviral defense mechanism in honeybees, involves the recognition and degradation of viral RNA by small interfering RNAs (siRNAs) generated via the RNAi pathway. Research has shown that honeybee colonies affected by colony collapse disorder (CCD) exhibit a strong RNAi response against several major honeybee viruses, such as DWV, IAPV, and Kashmir bee virus (KBV), indicating that RNAi is an effective mechanism for virus recognition and silencing [15,33,34]. Furthermore, the application of dsRNA has shown potential to reduce viral load, particularly for controlling sacbrood virus (SBV) in honeybee colonies, suggesting that dsRNA treatments could be scaled up for broader use [36]. One advantage of RNAi is its ability to target specific viral genes, providing highly targeted therapy with minimal off-target effects.

Additionally, RNAi can be administered through various delivery methods, including feeding or injection of siRNA, or using dsRNA products like Remebee-IAPV and HoneyGuard-R^®, which have shown effectiveness in reducing virus-related mortality and improving colony health [33,35,36]. However, challenges remain, such as the potential for viruses to evolve mechanisms to suppress the RNAi pathway and the difficulty of delivering RNAi effectively to all tissues within the bee [38]. Moreover, the duration of the protective effect and the optimal frequency of dsRNA administration remain unclear [35,37]. Despite these limitations, RNAi represents a promising tool for combating honeybee viral diseases and improving colony health, with ongoing research aimed at optimizing delivery methods and addressing current challenges [38,39,40].

4. RNAi for Nosema Disease Treatment

Nosema infection, especially caused by Nosema ceranae, poses a serious threat to honeybee health and colony sustainability. Infected colonies often exhibit reduced adult bee populations, decreased brood rearing, and lower honey yields, directly impacting productivity [41,42]. At the individual level, infected worker bees may begin foraging prematurely, experience shortened lifespans, and suffer from impaired nursing abilities, which further destabilize colony dynamics [44]. In queens, Nosema infection can be fatal, leading to a cessation of egg-laying and death within a matter of weeks [45]. Additionally, Nosema increases colony vulnerability to collapse and winter mortality, compounding the risks to long-term colony survival [44,45]. The parasite invades and damages the midgut epithelial cells of honeybees, hindering digestion and nutrient absorption, while also inducing energetic stress, immune suppression, and behavioral changes [47]. Its spores are highly transmissible, allowing the disease to rapidly spread throughout a colony [45]. Given its significant implications for both managed and wild bee populations, Nosema infection is a global concern with far-reaching consequences for agricultural productivity and food security, as it reduces pollination services. Thus, vigilant monitoring and strategic management are essential for mitigating its impact.

Fumagillin has long been used as the primary chemotherapeutic agent for treating Nosema infections in honeybees, primarily by inhibiting the enzyme methionine aminopeptidase 2 (MetAP2), which disrupts critical protein modifications necessary for cellular function [48]. As the only antibiotic approved in the United States for Nosema control for over 50 years, fumagillin has played a significant role in mitigating the impact of Nosema apis and Nosema ceranae. However, its use comes with notable limitations. Fumagillin is toxic to mammals, raising concerns about contamination and accumulation in honey, leading to its ban in many countries, especially across the European Union [47,48]. Moreover, recent research indicates that Nosema ceranae can escape fumagillin treatment, with studies reporting that treated colonies exhibited post-treatment spore loads nearly 100% higher than those in untreated control groups [50]. Increasing fumagillin concentrations to enhance efficacy has been shown to alter the expression of structural and metabolic proteins in the honeybee midgut and to significantly increase worker bee mortality [51]. These findings highlight the declining effectiveness of fumagillin and underscore the urgent need for alternative, safer, and more sustainable treatments for Nosema disease.

RNAi has emerged as a promising therapeutic strategy for combating Nosema ceranae infections in honeybees, offering targeted, innovative approaches to enhance colony health and resilience. One notable approach involves silencing mitosome-related genes in N. ceranae, such as NCER_101456 and NCER_100157, using dsRNA. This method significantly reduced spore production and improved the survival of infected bees, demonstrating RNAi’s potential to suppress vital parasite functions [51,52]. Another RNAi-based technique targets the parasite's structural integrity by silencing genes encoding spore wall proteins (SWP8 and SWP12), thereby decreasing infection levels, enhancing immunity, and extending the lifespan of treated bees [54]. Beyond direct RNA delivery, researchers have also leveraged symbiont-mediated RNAi by engineering the honeybee gut bacterium Snodgrassella alvi to express dsRNA targeting essential N. ceranae genes. This approach effectively reduced Nosema proliferation and improved bee survival, even among older forager bees [55]. Notably, the engineered bacteria were shown to transmit between cohoused bees, enabling colony-level protection and making this method a scalable solution for Nosema management [55]. Similarly, Lang et al. (2023) [56] demonstrated that interfering with the Nosema redox system using engineered symbiotic bacteria could enhance host resistance to infection. Together, these studies underscore the transformative potential of RNAi-based technologies—particularly those using microbial symbionts—as sustainable, targeted alternatives to conventional chemical treatments that Nosema has increasingly become resistant to.

Despite the promising potential of RNAi in controlling Nosema ceranae infections in honeybees, several vital limitations remain that hinder its practical application. One key issue is the inconsistent efficacy of RNAi treatments. While some studies have demonstrated that dsRNA targeting specific Nosema genes can reduce spore loads and improve bee survival, the effectiveness varies significantly depending on the gene target, with some dsRNAs showing limited inhibition of spore proliferation [51,53,56]. Furthermore, combining multiple dsRNAs targeting different genes has not reliably produced synergistic effects, suggesting that further research is needed to identify optimal gene targets and combinations [52]. Delivery remains another major hurdle; although approaches such as nanocarriers have been proposed to enhance dsRNA stability and uptake, the most effective and scalable delivery method in field conditions is still under investigation [56].

Additionally, while RNAi is considered relatively specific, the potential for off-target effects—either in honeybees or in non-target environmental organisms—requires careful risk assessment. Perhaps most critically, the majority of RNAi studies have been conducted in controlled laboratory settings, and their real-world effectiveness in large-scale apiculture has yet to be thoroughly validated. As such, although RNAi remains a compelling candidate for Nosema management, further research is essential to overcome these technical and ecological challenges before widespread field application becomes viable.

5. RNAi for Varroa Mite Control

The Varroa destructor mite poses one of the most significant threats to honeybee health and global apiculture. Widely recognized as a primary driver of honeybee decline, especially in Apis mellifera populations that lack natural resistance, Varroa mites have contributed to severe colony losses worldwide [30,32,57]. These mites feed parasitically on the fat body tissue and hemolymph of both developing brood and adult bees, leading to weakened individuals with shortened lifespans, compromised immune systems, and impaired physiological functions [31]. In addition to their direct effects, Varroa mites serve as efficient vectors of devastating honeybee viruses, such as DWV, IAPV, and BQCV, which exacerbate colony morbidity and mortality [32,57]. Colonies suffering from heavy infestations display scattered brood patterns, deformed bees, impaired flight ability, and reduced worker bee weight and longevity—symptoms that often culminate in colony collapse. Alarmingly, Varroa mite populations can expand rapidly within a few years if not managed, overwhelming bee colonies' natural defenses [30,32,57]. Given its widespread prevalence and multifaceted impact, the Varroa mite is regarded as a leading cause of honeybee losses and a critical concern for beekeepers and researchers alike.

Varroa mite control relies heavily on the integrated pest management (IPM) strategies that combine cultural, mechanical, chemical, and biological approaches to limit infestation while minimizing harm to bees and the environment. Cultural methods, such as creating brood breaks and selectively breeding mite-resistant honeybee strains, aim to disrupt the mite reproductive cycle and promote long-term colony resilience [59]. Mechanical interventions, such as screened bottom boards and sugar dusting, help dislodge mites from bees, offering non-chemical control options [60]. Chemical treatments are often divided into natural compounds—including thymol, formic acid, oxalic acid, and hop beta acids—which can be effective when applied correctly but may vary in efficacy depending on environmental conditions [60,62]. Synthetic miticides, such as amitraz, tau-fluvalinate, and coumaphos, have traditionally provided strong mite suppression; however, widespread and repeated use has led to the development of mite resistance, reducing their effectiveness and raising concerns about residues in hive products and potential harm to bees [58,62]. Despite the variety of available methods, each has limitations, and overreliance on any single approach can lead to resistance or reduced efficacy, underscoring the need for rotation and combination within a comprehensive IPM framework [59,63].

RNAi has emerged as a promising approach to controlling Varroa mites (Varroa destructor), a significant threat to honeybee colonies. RNAi is a natural process in which small RNA molecules, such as small interfering RNA (siRNA), silence specific genes by preventing protein production. For Varroa control, scientists create dsRNA sequences targeting essential reproductive genes in the mites. These dsRNAs are incorporated into a sugar solution, which is then fed to honeybee larvae. When Varroa mites feed on larvae, they ingest dsRNA, triggering the RNAi pathway and impairing the mites' reproductive capacity [65]. One of the key advantages of RNAi is its non-lethal nature, as it does not harm bees or mites directly, making it a more environmentally friendly approach [66]. However, RNAi treatments require repeated applications because they are not heritable and may not eliminate the mite population, even though the genetic heritability should be further studied, given their limited duration of effectiveness [65].

Additionally, although RNAi does not alter the DNA of the species, further research is needed to understand its long-term effects on non-target species [64,65]. Studies have shown that RNAi can significantly reduce mite populations, with some research indicating reductions of over 60% [67]. Future research is focused on optimizing the dsRNA sequences to target more vital genes, improving the efficiency of delivery systems, and monitoring long-term impacts in field conditions [64,65,66].

6. RNAi for Small Hive Beetle Control

The small hive beetle (SHB), Aethina tumida, poses a serious threat to honeybee colonies, particularly in warm and humid regions where it thrives. While adult SHBs are relatively harmless, the larval stage is highly destructive—larvae tunnel through honeycomb, consuming honey, pollen, and bee brood, while contaminating the hive with excrement and secretions that promote fermentation and spoilage of honey [67,68,69]. The economic and biological impacts are considerable: SHB infestations can reduce honey yields by up to 3-fold compared to uninfested colonies [70,71], and severe infestations may lead to absconding, with one study reporting that up to 80% of untreated colonies abandoned their hives [72]. Larvae consume all stages of bee brood, significantly reducing the sealed brood area [73], while the presence of adult beetles correlates with reduced adult bee populations in susceptible European honeybee colonies. Additionally, SHB larvae can cause the loss of stored pollen and render honey unfit for consumption or sale due to fermentation and foul odor [73]. These direct and indirect damages highlight SHB as a formidable pest requiring vigilant management strategies to safeguard colony health and honey production [71].

RNAi represents a promising and species-specific strategy for managing the small hive beetle (Aethina tumida), an invasive pest that causes significant damage to honeybee colonies. This gene-silencing technology operates by introducing dsRNA designed to target essential beetle genes, ultimately disrupting vital physiological processes. In laboratory studies, injection of dsRNA targeting genes such as V-ATPase subunit A, involved in proton gradient maintenance, and laccase 2, crucial for cuticle formation, led to 100% mortality in SHB larvae, with quantitative PCR (qPCR) confirming significant gene knockdown over time [74]. Feeding larvae dsRNA targeting V-ATPase subunit A resulted in 50% mortality, although gene suppression was not detected, likely due to degradation by gut nucleases [74]. Importantly, specificity testing showed that these SHB-targeted dsRNAs had no adverse effects on honeybee survival or gene expression, demonstrating the method's safety for non-target pollinators [73,74]. Despite these encouraging results, practical challenges remain, particularly in delivering RNAi orally under field conditions. Advances in the design of nuclease-resistant dsRNA and in the optimization of delivery mechanisms are necessary before RNAi can become a viable tool for SHB management in apiculture [73,74].

7. Engineered Endosymbionts Producing RNAi Offer a Promising Method for Controlling Pathogens and Parasites in Honeybees

Engineered endosymbionts producing RNAi represent a promising and innovative strategy for preventing and treating honeybee pathogens and parasites, addressing key challenges such as delivery efficiency and cost in field applications. This approach utilizes genetically modified gut bacteria—specifically Snodgrassella alvi, a natural symbiont of honeybees—to produce dsRNA that activates the RNAi pathway. Known as Functional Genomics Using Engineered Symbionts (FUGUES), this method enables persistent, systemic knockdown of targeted genes in honeybees by up to 75% [76]. When engineered S. alvi colonizes bees, the symbionts not only improve survival after viral infections but also induce RNAi in Varroa destructor, significantly reducing mite populations [75,76]. This dual-action effect offers a sustainable alternative to traditional treatments, particularly against viral threats like DWV, which are exacerbated by mite infestations [67]. Beyond treatment, symbiont-mediated RNAi also provides a powerful tool for functional genomics in honeybees, facilitating gene-phenotype studies that were previously difficult to perform. Overall, this method holds great promise for enhancing honeybee resilience through precise, eco-friendly, and scalable biotechnological solutions.

Using probiotic Lactobacillus strains to engineer honeybee endosymbionts that carry recombinant plasmids encoding dsRNA targeting species-specific genes of viral pathogens and parasites offers a promising strategy for enhancing honeybee health and resilience. Lactobacillus species, such as Lactobacillus apis and Lactobacillus kunkeei, are naturally present in the honeybee gut and play a vital role in maintaining gut health, modulating immune responses, and protecting against bacterial pathogens, such as Paenibacillus larvae, the causative agent of American foulbrood [77,78]. These beneficial bacteria can be engineered to produce dsRNA that inhibits viral pathogens, such as DWV, and parasitic threats, such as Varroa mites, thereby directly targeting the pathogens that affect bee colonies. The expression of dsRNA can be controlled by using promoters from Lactobacillus species, such as the constitutive PldhL promoter from Lactobacillus plantarum or the inducible PnisA promoter from Lactococcus lactis [79,80,81]. Additionally, engineered Lactobacillus strains can enhance bees' immune function by supporting a balanced gut microbiome, which is critical for their ability to resist infections and environmental stressors [82,83,84]. This dual functionality of engineered Lactobacillus strains, as both a probiotic and a delivery system for RNAi-based pathogen control, provides a sustainable, targeted approach to protecting honeybees from the increasing challenges posed by pathogens and environmental changes (Figure 2).

8. Conclusions

RNAi is a promising, targeted strategy to combat major threats to honeybee health, including viral pathogens, Nosema, Varroa mites, and SHB (Table 1). RNAi silences specific genes via dsRNA, reducing pathogen virulence or parasite reproduction without harming bees. For viral diseases like DWV and IAPV, RNAi targets viral RNA to suppress replication, improving colony health and reducing mortality. RNAi treatments such as Remebee-IAPV and HoneyGauard-R^® have shown success via feeding or injection, offering specificity and minimal off-target effects. In Nosema ceranae infections, RNAi targeting genes related to spore formation or metabolism significantly reduces infection and increases bee survival—innovative approaches using engineered gut bacteria (Snodgrassella alvi) to deliver dsRNA offer scalable, colony-wide protection. Still, variability in treatment efficacy, delivery limitations, and off-target risks requires further study. For Varroa destructor, RNAi targets reproduction-related genes, disrupting mite fertility. Feeding bees dsRNA leads to ingestion by mites, reducing their populations by over 60% in some studies. While environmentally friendly, repeated applications are needed, and long-term field validation is ongoing. RNAi also shows promise against SHB by silencing vital genes, reducing larval survival. Despite technical and regulatory hurdles, RNAi's specificity, adaptability, and compatibility with integrated pest management make it a powerful tool for sustainable protection of honeybee health.