Dietary quercetin impacts the concentration of pesticides in honey bees

Highlights

• Honey bees orally exposed to imidacloprid exhibit reduced metabolism of quercetin.

• Dietary quercetin reduces the concentration of imidacloprid absorbed in honey bees.

• Dietary quercetin does not affect the concentration of tebuconazole or tau-fluvalinate in contact-exposed honey bees.

Abstract

Honey bees are important pollinators and are subject to numerous stressors, such as changing floral resources, parasites, and agrochemical exposure. Pesticide exposure has been linked to the decline in the global honey bee population. We have limited knowledge of the metabolic pathways and synergistic effects of xenobiotics in bees. Quercetin is one of the most abundant phytochemicals in plants and is therefore abundant in the honey bee diet. Quercetin can upregulate the detoxification system in honey bees; however, it is still unknown to what extent quercetin ingestion can reduce the content of absorbed pesticides. In this study, we investigated the effect of dietary quercetin on the contents of three pesticides in honey bees: imidacloprid (insecticide), tebuconazole (fungicide), and tau-fluvalinate (insecticide and acaricide). Bees were divided into two main groups and fed either quercetin-sucrose paste or only sucrose for 72 h. Thereafter, they were orally exposed to ∼10 ng/bee imidacloprid or contact-exposed to ∼0.9 μg/bee tau-fluvalinate or ∼5.2 μg/bee tebuconazole. After 1 h of oral exposure or 24 h of contact exposure, the bees were anaesthetised with CO₂, sacrificed by freezing, and extracted with a validated QuEChERS method. Subsequently, the concentrations of the three pesticides and quercetin in the bees were determined with a triple quadrupole tandem mass spectrometer coupled to an HPLC system. No significant effect on the concentration of tebuconazole or tau-fluvalinate was observed in bees fed quercetin. Intake of quercetin led to a reduction in the concentration of imidacloprid in honey bees. Quercetin-rich plants may be exploited in future beekeeping.

Introduction

The detrimental effect of pesticides on bee health is a topic of global concern. Bees provide a fundamental pollination service in ecosystems, and declines in their populations will have serious ecological and economic consequences (Goulson et al., 2008). Declining bee populations could lead to a pollination crisis and reduce crop yields (Holden, 2006; Goulson et al., 2015). Honey bees (Apis mellifera L.) are considered the most important pollinators due to both their efficiency and omnipresence (Winfree et al., 2011; Codling et al., 2016). Since 2007, colony collapse disorder (CCD) has impacted honey bee colonies in many parts of the world (Underwood and Vanengelsdorp, 2007; Vanengelsdorp et al., 2009). Proposed explanations for the observed cases of CCD include multiple stressors, such as reduced food availability, decreased flower diversity, parasites, modern beekeeping practices, and continuous agrochemical exposure (Goulson et al., 2015). Focus has often been on honey bee exposure to different classes of pesticides during foraging. Neonicotinoids are the most extensively used systemic insecticides worldwide due to their efficacy against a wide range of crop pests (Sánchez-Bayo, 2014). Neonicotinoids have commonly been used as seed coatings, in soil applications, and in foliar applications on agricultural crops such as oilseeds, grains, fruits, vegetables and on ornamental plants, many of which are attractive to honey bees (Codling et al., 2016; Jiang et al., 2018). Thus, honey bees consume imidacloprid during their foraging. In 2018, the EU banned the three main neonicotinoids (clothianidin, imidacloprid, and thiamethoxam) for outdoor use on plants visited by honey bees. Once exposed to imidacloprid, honey bees are quickly immobilized; however, within 24 h, they recover due to their continuous metabolism of imidacloprid (Schott et al., 2017). Imidacloprid is more toxic to honey bees when they digest it compared to when they are contact-exposed to it (EFSA, 2018). Tebuconazole, an ergosterol biosynthesis inhibitor (EBI) in the triazole fungicide group, is heavily used in Europe and may spread to flowering wild plants (EFSA, 2008; David et al., 2016). In a study in the UK, tebuconazole was found at concentrations up to 73 ng/g in all oilseed pollen samples taken (David et al., 2016). Tebuconazole was reported to be the most frequent fungicide in beeswax samples in Uruguay (Harriet et al., 2017). Tau-fluvalinate is an insecticide used as an in-hive acaricide (Mullin et al., 2010) to control the Varroa mite, which is a serious and devastating ectoparasite in honey bee colonies (Francis et al., 2013). Thus, honey bees are regularly subjected to tau-fluvalinate in their hives. Its low acute toxicity to honey bees means that acute tau-fluvalinate poisoning is rare. Tau-fluvalinate and tebuconazole have low toxicity (LD₅₀ = 12 μg/bee and >200 μg/bee, respectively) to honey bees, but investigation of the mechanisms contributing to acaricide and fungicide metabolization is essential to determine synergistic and antagonistic interactions with natural xenobiotics (EFSA, 2008, 2010).

The flavonol quercetin, an abundant phytochemical in pollen, is an intrinsic part of the honey bee diet. Daily rates of quercetin ingestion depend on the floral sources of pollen and nectar that honey bees consume. Consumption of quercetin-sucrose paste (10 mg quercetin/g) can reduce the acute toxicity of tau-fluvalinate in honey bees (Johnson et al., 2012) and increase their longevity in the presence of pyrethroids (Liao et al., 2017). Cytochrome P450s (CYPs) are a large group of enzymes that contribute to the detoxification of xenobiotics in insects (Dulbecco et al., 2018). The CYP9Q sub-family (CYP9Q1, CYP9Q2, and CYP9Q3) metabolizes imidacloprid as well as phytochemicals (Mao et al., 2011; Liao et al., 2017). Dietary quercetin upregulated P450 genes and triazole fungicide mycobutanil disturbed the metabolization pathway of quercetin (Mao et al., 2017). Thus, we hypothesized that consumption of quercetin alters the concentrations of imidacloprid and tau-fluvalinate while concentrations of tebuconazole will not be affected by dietary quercetin in honey bees (Fig. 1). The aim of this study was to quantify the effect of adding quercetin to honey bee diets on the concentration of three pesticides which honey bees are inadvertently exposed to: an insecticide (imidacloprid), a fungicide (tebuconazole), and an acaricide (tau-fluvalinate). The results of this study determine whether the exploitation of plants with high concentrations of quercetin in their pollen should be recommended as supplementary fodder in beekeeping.