Short CommunicationDelayed effects of a single dose of a neurotoxic pesticide (sulfoxaflor) on honeybee foraging activity
Graphical abstract
Introduction
Within the framework of the pesticide risk assessment procedure, test guidelines require toxicological data on honeybees (Apis mellifera), a major pollinator of crops (Garibaldi et al., 2013) and wild plants (Hung et al., 2018). The first tier assessment relies on acute and chronic toxicity tests measuring the survival rate of bees over the duration of the tests, i.e. 48 h and 10 days for acute and chronic exposure, respectively (OECD, 1998a, OECD, 1998b, OECD, 2013; OECD, 2017). Thus, long-term mortality risks are often neglected in pesticide risk assessment, especially when effects on bee mortality occur not during but after chronic exposure to pesticides (i.e. after 10 days) (Dechaume Moncharmont et al., 2003; Rondeau et al., 2015). In addition, there is growing evidence for long-term sublethal effects upon chronic exposure to pesticides, such as the impairment of bee behavior, physiology or colony performance (Faucon et al., 2005; Sandrock et al., 2014; Prado et al., 2019; Colin et al., 2019; Al Naggar and Baer, 2019; Hesselbach et al., 2020; Traynor et al., 2021). These data therefore show that time-to-effect experiments rather than fixed-duration tests are required for evaluating chronic toxicity (Rondeau et al., 2015; Simon-Delso et al., 2018; Bommuraj et al., 2021).
Similarly, long-lasting or delayed effects induced by an acute exposure have often been neglected. For instance, in the acute toxicity tests, mortality rate is measured over 48 h and to a maximum of 96 h if the mortality increases by more than 10% after the first 24 h (OECD, 1998a, OECD, 1998b). Moreover, sublethal effects have been generally assayed immediately or over the days following acute exposure to pesticides (Barascou et al., 2021a). In fact, relatively little is known about the occurrence of long-lasting or delayed effects upon acute exposure (Schneider et al., 2012; Shi et al., 2020). After exposure to a single dose, a rapid reduction in pesticide concentration is generally observed in honeybees, which contributes to shortening the exposure to the toxic compound (Ardalani, 2021; Barascou et al., 2021b). Relatively short-term effects can thus be expected; however, some pesticides may cause in-depth changes in bee physiology. For instance, neurotoxic insecticides, by targeting the neurotransmission pathways of insects, impact the neural plasticity, and brain function and structure of bees (Cabirol and Haase, 2019). Thus, single pesticide exposure may lead to life-long impacts if they inhibit or modify a physiological and/or developmental process in the bee brain.
In honeybees, foraging activity is preceded by orientation flights, during which individuals develop highly complex cognitive capacities essential to navigation and homing (Capaldi and Dyer, 1999; Degen et al., 2016). The experience accumulated during this pre-foraging stage then positively influences their foraging capacities and lifespan (Prado et al., 2020). We therefore hypothesized that a perturbation of this neurocognitive process by a neurotoxin could trigger chronic effects. To test this hypothesis we exposed bees at 7-days old, the median age of first orientation flights (Requier et al., 2020; Prado et al., 2020), to two sublethal doses of sulfoxaflor and tracked their flight activity and lifespan with automated life-long monitoring devices (optic bee counters) (Prado et al., 2019, Prado et al., 2020). We expected a modification of bee behavioral performance, given that sulfoxaflor is a new sulfoximine-based insecticide that shares a mode of action with neonicotinoids as selective agonists of nicotinic acetyl choline receptors (nAChRs) (Zhu et al., 2011; Sparks et al., 2013), and nAChRs play a central role in honeybee cognition (Gauthier and Grünewald, 2012).
Section snippets
Experimental setup
Experiments were performed in a peri-urban area (Avignon, France, 43°540N-4°-520E) with honeybees (Apis mellifera) from INRAE livestock. Newly emerged bees were collected from 3 colonies and marked with a data-matrix barcode (3 mm of diameter) glued on the thorax (Sader®). They were then released into a colony equipped with a bee counter, which consists of a camera that monitors the hive entrance and image analysis software that detects and registers the activity of bees (direction: in or out
Bee survival
Bee survival did not significantly differ between experimental groups, demonstrating that the tested doses were non-lethal (Kaplan–Meier: p = 0.1; log Rank test, control vs. dose 16 ng: p = 0.56; control vs. dose 60 ng: p = 0.19, and dose 16 ng vs. dose 60 ng: p = 0.12; Fig. S1).
Total flight activity
At day 7 (day of exposure to sulfoxaflor), the percentage of bees that had already performed their first flight did not differ between experimental groups (19.10 ± 0.01% control bees, 18.74 ± 0.02% and 14.81 ± 0.001%
Discussion
In the present study, we showed that acute exposure in honeybees does not necessarily cause short-term effects but can generate delayed effects, as previously found in others insects (Beketov and Liess, 2008; Wolz et al., 2021). Long-term effects following exposure to a single dose of pesticide have been previously described in honeybees. However, contrary to our study, latency effects were not investigated and effects were only reported at doses that significantly increased honeybee mortality (
CRediT authorship contribution statement
L.B., Y.L.C and C.A. conceived the study. L.B., D.S., D.C. and C.A. conducted the experiments. L.B., C.A. and F.R. analyzed the data, Y.L.C. and C.A. contributed to reagents. L.B, C.A and F.R. wrote the manuscript. All authors read and reviewed the manuscript.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This project received funding from the European Horizon 2020 research and innovation program under grant agreement no. 773921 (LB, YLC and CA).
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