1 Introduction

Pollinators are inextricably linked to the natural environment and modern society; these pollinators maintain a healthy, genetically diverse ecosystem of wild plants and are critical for the pollination of many food crops and, thus, food security (Potts et al. 2006). The European honey bee (Apis mellifera) is a particularly important pollinator species on a global scale, both environmentally and economically (Van Engelsdorp et al. 2010; Al Naggar et al. 2018). However, the recent decline in bee populations has prompted a flurry of research investigating the factors influencing honey bee health (Potts et al. 2010; Vanbergen et al. 2013; Rand et al. 2015). Pesticides, starvation, habitat loss, parasites, and pathogens, among other things, have been cited as important drivers of honey bee decline (Goulson et al. 2015; Al Naggar et al. 2015; Samuelson et al. 2018; Al Naggar and Baer 2019).

Neonicotinoids are among the most studied pesticides; there is evidence of their negative impact on certain bee species (Woodcock et al. 2016, 2017; Arce et al. 2018; Main et al. 2018; Siviter et al. 2018). Many of these studies revealed the honey bee’s extraordinary susceptibility to this class of insecticides, even though some other studies indicated that honey bees are less sensitive to neonicotinoids than other bee species (Rundlöf et al. 2015; Wintermantel et al. 2018; Osterman et al. 2019). Bee immunity, as well as behavioral characteristics including communication (Di Prisco et al. 2013) and foraging (Henry et al. 2012), has been shown to be harmed by neonicotinoids. All these harmful effects of neonicotinoids resulted in the global debate and, in certain circumstances, legislative re-evaluation of their usage (Siviter et al. 2019). As a result, three neonicotinoids have been banned in all outdoor crops in the European Union and Egypt, leaving a gap that sulfoximine-based pesticides may be able to fill in part (Brown et al. 2016).

Sulfoxaflor is a nicotinic acetylcholine receptor (nAChR) agonist, similar to neonicotinoids. It was recently introduced to global markets as a viable alternative to neonicotinoids for managing sucking pests that cause significant economic loss, such as aphids (Aphis gossypii) and whiteflies (Bemisia tabaci) (Barrania et al. 2019; Jiang et al. 2019) with lower residual levels in pollen (< 14.2 µg kg−1) and in nectar (< 0.68 µg kg−1) (Jiang et al. 2020). Several studies, however, have found that field-realistic sulfoxaflor concentrations have a negative impact on honey bee survival (Cheng et al. 2018) and bumblebee egg-laying and reproductive success (Siviter et al. 2018, 2020a, b), and adversely impact bumblebees and pollination services under semi-field conditions (Tamburini et al. 2021a). On the contrary, Siviter et al. (2019) have found that acute exposure to the field-realistic sulfoxaflor concentrations did not affect either honey bee and bumblebee olfactory conditioning or working memory of bumblebees. Furthermore, recent research found that sulfoxaflor has no effect on honey bee colony development or activity in a semi-field experiment with spray application because Closer® was applied before crop flowering, resulting in very low residue levels. However, because application to flowers is permitted in some countries, this design does not represent the worst-case scenario (Tamburini et al. 2021b). These contradicting findings highlight the urgent need for more studies particularly at field conditions to determine if the adverse effects of sulfoxaflor exposure that have been reported in some laboratory studies would be observed under the field conditions.

In the agroecosystems, bees are exposed to pesticide products that also include co-formulants, which are often more hazardous to bees than active compounds (Mesnage and Antoniou 2018; Azpiazu et al. 2021; Straw and Brown 2021; Straw et al. 2021). However, agrochemical risk assessments focus on the active ingredients of pesticides and ignore the spray adjuvants that are widely utilized in their application which would miss important toxicity effects that are harmful to bees. As a result, the acute oral toxicity (LC50) of the sulfoxaflor-containing pesticide Closer® 24% SC on honey bee foragers was investigated. Then, in an in-hive semi-field experiment, we assessed the effects of repeated exposure of honey bees to a very low concentration of Closer® to see if it can have chronic effects at a level where it has negligible acute effects. Therefore, we chronically fed honey bee colonies a sublethal concentration (LC5) of Closer® for 21 days. In the next step, we evaluated the colony development and activity as well as the weight and foraging activity of individual foraging bees. We hypothesized that chronic exposure to a sublethal concentration of Closer® will deteriorate honey bee colony development and foraging activity. To our knowledge, this is the first study to investigate the long-term impacts of honey bee colonies being exposed to a field-realistic concentration of Closer® in a semi-field in-hive experiment.

2 Materials and methods

2.1 Honey bees

Colonies of A. mellifera carnica headed by mated sisters’ queens were maintained in the apiary yard of the agricultural experimental station, Faculty of Agriculture, Cairo University, Egypt, which was surrounded by arable land with minor floral recourses, and were used from June to August 2021. Colonies had no visible honey bee diseases, had adequate bee bread, honey stores, and a large number of young honey bee workers, and each colony had 8 combs (containing 2–3 combs of brood) covered with bees. All colonies were treated with Bayvarol® (flumethrin 3.4%) 1 plastic strip/month/colony to control Varroa mites. Varroa levels have been checked before the experiment and were equivalent between all the colonies.

2.2 Pesticide

Closer® SC (24% sulfoxaflor) was purchased from Dow AgroSciences LLC. The formulation was kept at room temperature in its original packaging. Stock solutions were freshly prepared with tap water and diluted immediately at room temperature.

2.3 Determination of LC50

The acute oral toxicity (LC50) of Closer® was measured under controlled laboratory conditions using a total of 300 honey bee foragers (older than 3 weeks) collected from the hive entrance of three selected colonies. We chose forager bees over in-hive bees (OECD Test No. 213 1998) because they are more likely to be directly exposed to contaminated nectar. Furthermore, foragers are more sensitive to pesticides than in-hive bees, according to a recent study (Tosi and Nieh 2019). Based on the limit range test, four serially diluted concentrations of Closer® (0.0006, 0.006, 0.06, 0.6 mg/L (ppm)) were prepared in 50% (w/v) sugar solution and used to determine the LC50 of Closer®. Batches of 20 honey bees were kept each in a wooden cage (40 cm height × 30 cm diameter) with the front face of these cages designed as an openable door made of metal wire with wooden edges. The foragers were then allowed to feed ad libitum for 24 h on sugar water containing the tested concentrations or sugar water free of pesticide (control). All treatments were kept at room temperature (31–33 °C, 60 ± 10% Rh.), and exposures to each concentration were performed in triplicate (three cages with 20 foragers each). Mortality was recorded after 24 h, and LC50 for Closer® was then calculated using the LdP LineR program using the log-probit model (Ehabsoft, http://www.ehabsoft.com/ldpline).

2.4 Sublethal effects of Closer® 24% SC

To test for potential long-term impacts on colony development and activity, as well as on the weight and foraging performance of individual honey bee foragers, we used a worst-case exposure scenario for 21 days with a sublethal and field-realistic concentration of sulfoxaflor to simulate natural insecticide exposure. A total of six colonies were assigned randomly to two groups: exposure to Closer® or control (3 colonies per group). A 0.0003 ppm (0.3 ppb) (represents the calculated LC5 value) of Closer® was incorporated into 0.5 L of sugar solution 50% (w/v) and introduced to the treatment group, and the sugar solution containing the pesticide was replaced with fresh preparations every 2 days for 21 days. The control group received the same preparations without the insecticide, and both solutions were placed in feeders inside the hives.

Sulfoxaflor residues in nectar and pollen of treated crops appear to degrade more quickly than neonicotinoids, but they last at least 11 days (the longest duration examined) (U.S. EPA 2016, 2019). Pollen collected by honey bees foraging on a cotton field treated with sulfoxaflor at label recommendations had up to 510 ppb (U.S. EPA 2016); other studies have indicated that concentrations in forager-collected pollen can be substantially higher (e.g., strawberry pollen = 12,700 ppm to 110 ppb, pumpkin pollen = 162 ppb to 9 ppb) (U.S. EPA 2019). Our exposure level (0.3 ppb) was therefore an order of magnitude lower than available estimates for sulfoxaflor residues in forager-collected nectar (5 ppb) (U.S. EPA 2016) and consistent with residue levels (0.3 ppb) from the Pest Management Regulatory Agency of Canada (PMRA Canada 2016). Even though the sulfoxaflor residues found in pollen and nectar suggest that bees will be exposed to it at rather high concentrations, we were conservative and only exposed bees to the lowest residue found in nectar (0.3 ppb) which corresponded to the LC5 calculated in our study.

2.4.1 Effects on colony growth and activity

The areas of brood (eggs, larvae, and pupae), honey (sealed and unsealed), and bee bread combs were measured twice, once before and once after treatment. Initially, colonies were outed, and each comb was shacked down to remove the attached bees, after which both sides of the comb were monitored. The length and width of each patch type (brood, honey, bee bread) were measured and recorded, and the area of each patch type was calculated. The total area of each type in all combs of each colony was added up and treated as a replicate. The area of each patch was evaluated to the nearest shape of a rectangle; then, the remaining cells–out of shape–were counted and multiplied by cell area then added to the area of a rectangle to get the total areas in cm2, like using transparent grid paper over the frame (Delaplane et al. 2013).

2.4.2 Effects on the body mass of foraging bees

Twenty honey bee foragers were collected at hive entrance of both treated and non-treated (control) colonies, and weighed periodically at 10, 12, 14, 16, 18, and 21 days of chronic exposure. The average body mass (mg) of foraging bees on various days was then pooled and reported. We started our assessments after the standard ICPPR 10-day chronic exposure test (OECD 2017) because our primary goal was to see if repeated exposure to sulfoxaflor at a level where it had negligible acute effects will impact the health of bees over time.

2.4.3 Effects on foraging activity

The number of foraging bees (outgoing and incoming) for both treated and non-treated colonies was monitored at 14, 16, 18, and 21 days of chronic exposure. The number of foraging bees was counted five times over the day at 8 and 10 am and at 12, 3, and 6 pm; each observation lasted for 3 min. The total number of foraging bees leaving or returning at various days and times was then pooled and reported.

2.5 Statistical analysis

All statistical analyses were performed using R studio version 4.1.2 (Team RC 2021), while data visualization was carried out by use of GraphPad Prism version 8.00 for Windows (GraphPad Software, La Jolla, CA, USA, www.graphpad.com). To compare the change in the areas of brood, honey, and bee bread between treated and non-treated colonies and before and after treatment, we used ANOVA (type II) tests in a linear mixed model (LMM). Pesticide exposure and time of assessment were used as independent, fixed factors (predictors) while the colony was included as a random factor. To test for significant interactive effects of exposure to pesticide exposure, time of assessment of areas of brood, honey, and bee bread, we inspected the pesticide × time interaction terms in all models.

We used ANOVA (type II) tests in LMM to test the effect of the treatment on body mass and the number of foraging bees, with pesticide exposure as an independent, fixed factor and the colony and day of assessment as random factors. For all analyses, the level of type I error was set as p < 0.05.

3 Results

3.1 Acute oral toxicity of sulfoxaflor (LC5)

The average mortality of controls and honey bee foragers exposed to different concentrations of Closer® is reported in Figure S1 (Supplementary information). The lethal concentrations (ppb) of Closer® required killing 5, 50, and 90% (LC5, LC50, and LC90) of honey bee foragers after 24-h acute exposure is reported (Table I). The calculated LC50 value of Closer® to honey bee foragers was 0.012 mg L−1 (12 ppb) (see supplementary information Figure S1). The mortality percentage in the control group was 1.7% (see Table S1 in Supplementary information).

Table I Acute toxicity of Closer® 24% C (sulfoxaflor) to honey bee foragers after 24 h of exposure
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3.2 Sublethal effects of Closer®

3.2.1 Effects on colony growth and activity

The areas of honey, brood, and bee bread were measured before and after the 21-day exposure phase to sublethal (LC5) concentration of Closer®, as shown in Figure 1. When we compared the changes in areas of brood, honey, and bee bread between treated and non-treated colonies before and after treatment, we found a significant treatment × time interaction term (LMM, brood comb area: Wald χ2 = 1826.72, df = 1, p > 0.0001, honeycomb area: Wald χ2 = 3811.81, df = 1, p > 0.0001, bee bread comb area: Wald χ2 = 370.90, df = 1, p > 0.0001), indicating that the change in the brood, honey, and bee bread areas with time was dependent on whether colonies were exposed to the pesticide or not.

Figure 1.
figure 1

Effects of chronic exposure of honeybee colonies (n = 3) for 21 days to a sublethal concentration (LC5: 3 ppb) of Closer® (active ingredient sulfoxaflor) on areas (cm2) of brood (a), honey (b), and bee bread (c) before and after treatment. Column bars represent (mean ± SEM). There was a significant treatment × time interaction term (LMM, p > 0.0001).

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3.2.2 Effect on body mass and foraging activity of foragers

The body mass and foraging activity (number of outgoing and incoming bees) of individual honey bee foragers were measured at various intervals after 10 days of chronic exposure to a sublethal (LC5) concentration of Closer®, as shown in Figures 2 and 3, respectively. The body mass of bee foragers collected from treated colonies decreased significantly (LMM, Wald χ2 = 27.24, df = 1, p > 0.0001) compared to the body mass of individual bee foragers collected from non-treated control colonies (Figure 2). We also observed a significant decrease in numbers of both outgoing and incoming bee foragers in treated colonies (LMM, outgoing foragers: Wald χ2 = 29.21, df = 1, p > 0.0001, incoming foragers: Wald χ2 = 48.83, df = 1, p > 0.0001) compared to the numbers of foragers in control bee colonies (Figure 3).

Figure 2.
figure 2

Effect of chronic exposure of honey bee colonies (n = 3) for 21 days to a sublethal concentration (LC5: 3 ppb) of Closer® (active ingredient sulfoxaflor) on the body mass of foraging bees. Symbols on the box plot represent maximum and minimum values (n = 18 bees) (whiskers: ┬ ┴), mean values (-). Different lowercase letters denote significant differences from the control (LMM, p > 0.0001).

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Figure 3.
figure 3

Effects of chronic exposure of honey bee colonies (n = 3) for 21 days to a sublethal concentration (LC5: 3 ppb) of Closer® (active ingredient sulfoxaflor) on the number of both outgoing and incoming bee foragers. Symbols on the box plot represent maximum and minimum numbers of bees (n = 12 trials) (whiskers: ┬ ┴), mean values (-). Different lowercase letters denote significant differences from the control (LMM, p > 0.0001).

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4 Discussion

We found that sublethal exposures to Closer® SC (24% sulfoxaflor) indeed impacted the honey bee colony growth and amount of brood, honey, and pollen. Body mass and individual foraging activity of bees (number of outgoing and incoming foragers) were also impacted when colonies were kept under field conditions.

Sulfoxaflor is a sulfonamide-based pesticide that has proven to be efficient in plant protection against pest insects (Bacci et al. 2018). It affects insect acetylcholine receptors (nAChRs) in the same way that neonicotinoids do, but it has a different mechanism of action (Babcock et al. 2011; Watson et al. 2011). The LC50 value of Closer® SC (24% sulfoxaflor) in honey bee foragers determined in our study was 12 ppb. Given that honey bee workers consume an average of 27 µL of sugar syrup per day in controlled lab experiments (Al Naggar and Baer 2019), we might calculate and predict the LD50 by multiplying the LC50 value by 27 µL of Closer®-sucrose solution consumed in 24 h by each worker bee. The estimated LD50 of Closer® was then 0.32 ng/bee which was an order of magnitude lower than the value given in the dossier for registration of this compound (146 ng/bee) in which sulfoxaflor (the active ingredient of Closer®) was tested against in-hive bees (U.S. EPA 2019; Abdourahime et al. 2019) and 645-fold lower than the LD50 (206.4 ng/bee) of Isoclast™ active (50% sulfoxaflor WDG) using newly emerged bees (Li et al. 2021). These differences could be explained by differences in the age of the bees used (foragers vs. in-hive bees or newly emerged bees) and the type of insecticide used (active ingredient vs. formulated product). It has been shown that foragers are more vulnerable to insecticides than in-hive bees (Tosi and Nieh 2019), and honey bees are sensitive to pesticide co-formulants (inert substances), making formulations more toxic to bees than active ingredients (Mullin et al. 2015; Mullin 2015; Mesnage and Antoniou 2018; Azpiazu et al. 2021; Straw and Brown 2021; Straw et al. 2021).

There is a lot of debate over the impact of sulfoxaflor on various pollinators, as some research did not find negative effects on pollinators (Siviter et al. 2019; Al Naggar and Paxton 2021; Tamburini et al. 2021b), while other studies suggested it had negative effects on honey bees, especially if it was used during the flowering stage (Zhu et al. 2017a; Cheng et al. 2018; Chakrabarti et al. 2020; Li et al. 2021) and bumblebees (Siviter et al. 2018, 2020a, b). Here, chronic exposure to a sublethal concentration of Closer® reduced colony growth and activity by decreasing pollen deposition as well as honey and brood production. This could be explained by the observed decrease in the individual foraging performance of bees at various intervals reported in the current study, since colony strength and brood-rearing activity have been also shown to influence foraging activity (Amdam et al. 2009; Abou-Shaara et al. 2013).

Shortage periods of nectar or pollen have a negative impact on honey bee colony activities (Sihag and Gupta 2011; Pande and Karnatak 2014; Manning 2016), as pollen contains nearly all of the protein and vitamins required by bees (Mattila and Otis 2006; Brodschneider and Crailsheim 2010). Sufficient protein and carbohydrate storage helps bees resist or tolerate unfavorable conditions (Hrassnigg and Crailsheim 2005; Naug and Gibbs 2009; Brodschneider and Crailsheim 2010). Less brood-rearing however results in fewer adult bees, which affects pollination services and honey production (Duff and Furgala 1986; Nelson 1987; Fewell and Winston 1992; Herbert 1999).

Foraging is essential not only for the colony but also for plant pollination (Potts et al. 2006). There are multiple factors that can influence foraging activity (Abou-Shaara 2014). Exposure to agrochemicals represents one of the environmental factors that have been shown to have a negative impact on foraging activity. For example, more interval periods between visits after imidacloprid exposure (Yang et al. 2008), as well as a reduction in foraging activity and longer foraging flights after treatment with either clothianidin or imidacloprid neonicotinoids, have been reported (Schneider et al. 2012). Thiamethoxam exposure has been shown to reduce pollination services provided by bumblebee colonies to apple trees at the beginning of their foraging career (Stanley et al. 2016). Furthermore, Stanley and Raine (2016) also demonstrated that pesticide-exposed bees brought back less pollen throughout their foraging career, implying that impacts on pollination services may become exacerbated over time. A reduction in bumblebee colony development and size, as well as individual foraging performance, following spray application of the pesticide sulfoxaflor-containing product Closer® under semi-field conditions, has recently been reported (Tamburini et al. 2021a), which is comparable to what we found in the current study. The consequences of long-term exposure, on the other hand, have yet to be investigated in terms of pollination services.

Given the various exposure paths to sulfoxaflor (e.g., wax, water puddles, pollen pellets, bee bread, or royal jelly) and residues in pollen and nectar (Sanchez-Bayo and Goka 2016; U.S. EPA 2019), it is worth looking at the long-term effects of chronic exposure at the colony level. Although we were very conservative and only exposed bees to the lowest residue found in nectar (0.3 ppb), our results were comparable to the findings reported by Tamburini et al. (2021a) on bumblebees upon spray application of Closer® according to standard application practices under semi-field conditions. Furthermore, our findings concur with the long-term adverse effects of sulfoxaflor exposure on bee mortality that have been reported in laboratory research due to exposure to the highest residual levels of sulfoxaflor (3 ppm a.i.) (Zhu et al. 2017a, b).

5 Conclusions

At realistic quantities, the potential danger of sulfoxaflor as a new insecticide to pollinators merits further investigation. Our findings could add to the body of knowledge about sulfoxaflor’s impact on honey bees. To properly understand the long-term impacts on pollination services, more research is needed.