Current honey bee stressor investigations and mitigation methods in the United States and Canada

Abstract

Introduction

Humans have a long association with the western honey bee (Apis mellifera L., Hymenoptera: Apidae) due to hive products such as honey and beeswax (Prendergast et al. 2021), but have more recently become dependent on the pollination services bees provide to agricultural crops (Papa et al. 2022). Honey bees are the most extensively managed pollinator globally and are suffering a health decline, thereby jeopardizing food security and agricultural economics into jeopardy as the human population grows and pollination needs exceed pollinator populations (Hristov et al. 2020, Neov et al. 2021). Honey bees directly pollinate as much as 34% of agricultural crops, although estimates vary depending on location (Khalifa et al. 2021).

In addition to their agricultural importance, these eusocial insects serve as model research organisms due to their ability to investigate applied and basic scientific questions in light of their geographically widespread range, amenability to human management, and ubiquitous presence throughout various landscapes. As such, honey bee research has been generated on a large scale worldwide, which has in turn led to additional research avenues. Members of the American Association of Professional Apiculturalists (AAPA) and the Canadian Association of Professional Apiculturists (CAPA) have prominently contributed to research efforts related to honey bee biology on both basic and applied scales. AAPA and CAPA have cooperatively organized a special issue composed of the recent work that research members of both organizations have conducted. Specifically, we have compiled Canadian and American studies that address honey bee health in the context of: (i) abiotic and biotic stressors such as Varroa destructor Anderson and Trueman (Mesostigmata: Varroidae), the leading cause of colony mortality in North America; and (ii) potential stress mitigation methods that can be applied in scientific and stakeholder settings. Here, we introduce these studies and further identify areas where future investigation is needed.

Abiotic and Biotic Honey Bee Stressors

Abiotic stressors refer to non-living, physical, or chemical components of an ecosystem and common examples include the atmosphere or climate. Within apidology, commonly discussed abiotic stressors may include pesticides, macro- or micro-nutrients, and potentially viruses due to their inability to independently replicate. Biotic stressors refer to living or once-living components of an ecosystem. In apidology, this category is commonly used to describe pests or parasites, but can also encompass honey bee inter- and intracolony statuses that contribute stress, such as robbing behaviors or inadequate genetic diversity within the colony.

In this special issue, there are several studies that center around abiotic stressors. Given that the beekeeping industry relies on providing bees supplemental nutrition to get colonies through periods of dearth, encourage colony growth to achieve pollination or colony size metrics, and for successful overwintering, nutrition-based research is a continual need (DeGrandi-Hoffman et al. 2020, Tsuruda et al. 2021, Hoover et al. 2022). Quinlan and colleagues (2023) found that different carbohydrate-based artificial diets had significantly influenced individual physiological markers and tended to increase survival and population size at a colony level (Quinlan et al. 2023). Similarly, as some agroecosystems, especially those with large monocultures, continue to rely on chemical controls of pests and pathogens, especially those with large monocultures, research investigating bee health in relation to pesticides is always at the forefront of the apicultural industry needs (Malaj and Morrissey 2022). Several abiotic studies within this special issue center around pesticide exposure. Tokach and colleagues (2024) found that colony behaviors and aging were significantly impacted after exposure to an agro-chemically contaminated environment, where exposed workers aged faster and queens lowered egg-laying rates. Work by Couvillon et al. (2023) found that pesticides associated with mosquito control did not impact honey bee foraging or recruitment behaviors. A review of spray adjuvants by Shannon et al. (2023) pointed out that spray adjuvants in the United States are widely used and yet their risk assessments are not as rigorous as those for active ingredients. Given that honey bees are commonly exposed to tank mixes containing spray adjuvants, their review synthesized current knowledge and highlighted pesticide applicator best practice recommendations to preserve pollinator safety.

Pesticides are one environmental exposure that honey bees contend with, but in the real-world, they are continually facing a multitude of threats simultaneously. Studies looking at interactions between multiple stressors are often quite complex, attempting to capture a bit of what honey bees and beekeepers contend with during their daily lives. Two studies looked at the interactions between pesticide exposure and Varroa parasitism on colony health (Bartlett et al. 2024, Rinkevich et al. 2024). Rinkevich and colleagues (2024) performed a 2-year longitudinal study on colonies near corn and soybean fields. They found that high Varroa populations were associated with smaller honey bee populations, decreased honey production, and lower colony survival while pesticide exposure from agricultural fields and beekeeper-applied miticides did not consistently correlate with health metrics. Bartlett and colleagues (2024) took an experimental approach to determine if enhanced genetic diversity could be a tool to combat these combinatorial stressors. Although increased colony diversity is associated with increased survivorship in general (Mattilla and Seeley 2007), it alone did not mitigate the negative effects of both neonicotinoid exposure and the resulting increased Varroa populations (Bartlett et al. 2024).

Of all the biotic stressors that honey bees encounter in North America and globally, Varroa destructor is still currently the single strongest driver of colony mortality (Jack and Ellis 2021), a reality acknowledged by beekeepers (Steinhauer et al. 2018, CAPA 2024). A case study published in this special issue by Bartlett et al. (2024) showed that Varroa population growth varies within colonies throughout the year and that led to increased parasite populations across entire apiaries and associated mortality in colony neighbors. At the individual Varroa and bee level, Reams and colleagues (2024) have found that there may be a relationship between larval starvation and Varroa populations, although the role nurse bees may play in this interaction is not yet defined. Continuing to understand the dynamics of parasitization at the pupal, colony, apiary, and broader regional level will ultimately lead to more holistic strategies to combat this deadly parasite.

Other work surveyed biological threats across regions, including a comparison of viral levels in Africanized feral colonies and colonies in a nearby managed apiary that found titers of Black queen cell virus and Deformed wing virus varied temporally, but not colony-type (Dickey et al. 2024). Viral infections were also investigated by Fowler and colleagues (2024), as part of their investigation into European foulbrood (EFB; causative agent Melissococcus plutonius). They found that viral coinfections and hygienic behavior did not correlate with the presence of EFB infection, suggesting other drivers may be responsible for observed outbreaks, particularly those noted during blueberry pollination (Grant et al. 2021, Thebeau et al. 2023).

Potential Stress Mitigation Methods

While documenting, examining, and investigating how and why particular stressors influence bee health is critical work, the end goal is to use that information to develop strategies that can be applied by relevant stakeholders. As such, a substantial portion of papers in this special issue focus on stress mitigation measures, including investigations into Varroa treatments, other pest management, nutritional supplementation, and research methods. Three papers in this special issue investigated the use of oxalic acid as a Varroa control method in relation to application method, treatment periods, and seasonality (Prouty et al. 2023, Bartlett et al. 2023, Berry et al. 2023). Prouty and colleagues (2023) investigated varying oxalic acid applications and treatment intervals and found that vaporization at 5- to 7-day treatment intervals was the most effective. Bartlett and colleagues (2023) found that, contrary to an anecdotally recommended use within the beekeeping industry, applying glycerol-oxalic acid to colonies via shop towels was not an effective method for Varroa control in the Southeastern United States. Berry and colleagues (2023) found that oxalic acid vaporization efficacy was increased by approximately 5× if beekeepers induced a brood break in their colonies during application. Cook and colleagues (2024) investigated the use of a novel compound for Varroa treatment, 1-allyloxy-4-propoxybenezene, and found it had 78.5% efficacy against Varroa with minimal negative effects on honey bees, thus showing promise for continued development.

Another pesticide-based Varroa control method was investigated by Aurell and colleagues (2024), who found that combining amitraz-based treatments with thymol-based treatments was particularly effective for Varroa control in the Southeastern United States. These findings may allow beekeepers to decrease the selection pressure on Varroa that has led to resistance (Rinkevich 2020), although the resulting drop in bee population after combined treatments calls for further study. Jack and colleagues (2024) evaluated various Varroa control methods in Florida, finding seasonal variations of efficacy for the 8 methods tested. Interestingly, treatments that delayed Varroa resurgence for 2–6 months in the winter and spring were not all effective when applied in the summer and fall. Plamondon and colleagues (2024) found that utilizing natural chemical-based treatments throughout the summer in Eastern Canada could appreciably lower Varroa levels, but not Varroa-associated viruses. Micholson and Currie (2024) investigated a nonchemical control measure for Varroa, evaluating if high-grooming bees discriminate between items that trigger grooming behavior. They found that high-grooming bees responded strongly to Varroa and less so to chalk dust, suggesting nuances to grooming behavior yet to be discovered that could potentially be selectively bred for by beekeepers.

One study in this special issue examined aspects of small hive beetle control or biology (Hackmeyer et al. 2023). Hackmeyer and colleagues (2023) found that an anthranilic diamides, specifically chlortraniliprole, was effective in preventing small hive beetle infestations in laboratory trials and a small field trial in the Southeastern United States, which may allow a new chemical treatment for small hive beetles.

Bawden and colleagues (2024) also established a new research protocol, in this case, to utilize an indoor management method for honey bee colonies that allows research labs to continue conducting experiments during the winter in temperate regions. This management method is both introduced and discussed in their paper, which also shares data on the impact it has on the learning and thermoregulatory behaviors of bees.

Three studies in this special issue investigated how nutritional supplements impact individual bee or colony health. Ewert and colleagues (2023) demonstrated that various essential oils and propolis extracts impacted individual bee lifespan and gut microbiota abundance and that some oils and extracts affected genes associated with nutrient assimilation and detoxification. Ultimately, their work suggested that essential oils such as lemongrass and spearmint, and propolis benefit bee health, with the caveat that thymol and potentially others not investigated are toxic rather than beneficial at field-relevant levels. Bleau and colleagues (2023) evaluated 2 probiotic formulas, Bactocell and Levucell, and found that they increased brood levels of colonies in spring, although they did not lower gut parasites such as Vairimorpha (formerly Nosema) spp. spore loads. Rodriguez and colleagues (2023) reviewed recent literature on probiotic use in improving disease resistance in honey bees, and they concluded that individual and in vitro studies were promising but that colony or field evidence is thus far lacking to form the best management decisions.

Management decisions by beekeepers have a large impact on the survival and profitability of their colonies. Gray and Goslee (2024) analyzed beekeeper responses from 2017 to 2022 in Pennsylvania and identified that the strongest predictor of colony survival was Varroa control. Beekeepers who used Varroa treatments had higher colony survival than those who did not (this was particularly true for beekeepers who used multiple forms of treatment), and Varroa control had a greater impact on survivorship than weather events. Holmes and colleagues (2023) found that beekeepers who attempted to requeen healthy colonies by placing queen cells in honey supers only had a 6% success rate, although the requeening method may still be useful if colonies were already failing. Bixby and colleagues (2023) conducted profit modeling on data from British Columbia beekeepers and discovered that replacing colony losses with packages generates less profit than splitting colonies and that beekeeping operations that had diversified income from nuc or package sales or pollination contracts were generally more profitable than operations that did not.

Conclusions and Future Research Needs

The aim of many honey bee researchers is to gain maximal insight into this insect which is used as the workhorse for pollination services while showcasing the importance of pollinators to the general public. Along the way, we hopefully improve our understanding of everything from the evolution of sociality to epidemiological modeling to the critically important task of enhancing the best management practices of honey bees. While we continue to make progress in our understanding of this beneficial insect, the new questions never stop. Additional research to improve the sustainability of the beekeeping industry must advance for both apidologists and beekeeping stakeholders of our applied research to truly succeed.

Currently, research into Varroa destructor biology, its susceptibility to chemical control measures, and its genetics continues to progress, which is vital to improving honey bee survival in light of the disproportionate role Varroa infestation plays on short-term colony survival (Steinhauer et al. 2018, Jack and Ellis 2021). However, we have made relatively less progress in discerning how nonchemical disease, pest, and pathogen control techniques can effectively be used or paired with chemical control techniques to form sustainable best management practices that will be embraced by beekeepers. This work has begun for Varroa (Berry et al. 2023, Aurell et al. 2024, Plamondon et al. 2024), but continues to be needed for other pests and diseases such as EFB (Grant et al. 2021), American foulbrood (Zabrodski et al. 2022), chalkbrood (Aronstein and Murray 2010), small hive beetles (Mustafa et al. 2014), Vairimorpha (formerly Nosema) spp. (Goblirsch 2018), viruses (Beaurepaire et al. 2020), and emerging parasites of concern such as Lotmaria passim (Gomez-Moracho et al. 2020) and Tropilaelaps spp. (Ling et al. 2023). Research into the biology and management of these pests is vital to achieve a stable beekeeping industry. The stability of the beekeeping industry can only occur if scientifically supported best management practices are effectively communicated to stakeholders and are focused on safeguarding long-term colony productivity and survival.

In the long term, emerging threats like the potential spread of other parasites (e.g., Tropilaelaps and L. passim) and the continual stresses brought on by widespread pesticide usage, global climate change, and extreme weather events maintain the need for rapid responses and long-term outlooks to improve bee health and resiliency. In the largest study of honey bee stressor networks so far, French et al. (2024) found that honey bees near crop agriculture experience complex stressor networks composed of various pesticide exposure, viruses, parasites, and nutritional deficiencies that tend to interact with each other. Richardson et al. (2023) clearly showed that across Canada, herbaceous land covers were associated with factors that further positively correlated with overwintering survival such as fall colony weight and other nutritional factors. It is reasonable to expect a similar trend with land cover and colony metrics in the United States, and smaller-scale studies conducted in the United States have indicated that this is the case (Meikle et al. 2020, Calovi et al. 2021). As the Anthropocene continues to progress and honey bees encounter rapidly changing land use in conjunction with climate change and other threats, future research into mitigating the impact of these factors is vital to boost the ability of honey bees to fight for themselves and achieve stability for an insect that we have built our agroecosystem around.

Author Contributions

Elizabeth Walsh (Conceptualization [supporting], Investigation [lead], Project administration [lead], Writing—original draft [lead], Writing—review & editing [equal]) and Michael Simone-Finstrom (Conceptualization [lead], Investigation [supporting], Writing—original draft [supporting], Writing—review & editing [equal])

References