Pesticide causes bumblebee flight to fall short

Blog written by Daniel Kenna & Richard Gill. Read the full paper here.

For social bees, such as bumblebees, colonies rely on the foraging behaviour of worker individuals to collect nectar and pollen. Efficient foraging is underpinned by flight performance, hence any environmental stressor affecting the dynamics of flight could translate to reduced colony success. Studies revealing what factors affect bee flight, therefore, can help us to understand why bee populations across the world are in decline and mitigate the threat to the pollination service these vital insects provide.

The growing challenge of feeding the human population across the globe has seen conventional farming practices becoming heavily reliant on the application of chemical pesticides for crop protection. The use of these pesticides in agricultural landscapes has been implicated as a contributing driver of bee declines, but knowledge gaps remain as to the mechanism by which pesticides pose a risk. While acute exposure to pesticide residue is unlikely to kill a bee, we are becoming increasingly aware that sublethal effects on bee behaviour can have detrimental knock-on effects to colony functioning and health.

Previous studies on bumblebees exposed to the neonicotinoid class of insecticide have reported that workers not only take longer to forage, but also bring back less food to the colony, which can lead to reduced colony growth. One potential explanation for this reported impairment to foraging efficiency is that certain aspects of flight capability, such as flight endurance and speed, are affected by neonicotinoid exposure. We set out to test this by feeding workers of Bombus terrestris audax (the Buff-tailed bumblebee) a field-realistic dose of neonicotinoid (imidacloprid), and investigated the possible effects on flight performance using a tethered flight mill setup (Figure 1).

Figure 1: Left panel: one of the flight mills used in our experiment; right panel: bumblebee worker attached to the arm of the flight mill via a metal tag glued to the thorax. Images by Danny Kenna

Modifying a previous flight mill design used on moths and honeybees, we were able to let bumblebees fly inside a lab while attached by a magnet to a revolving arm of a flight mill. This led to the tested bees effectively flying around in circles, in which we could record the time it took for every full circuit to be completed. We were able to calculate the length of time each bee flew, and by considering the circumference of each circuit could also measure the speed of each circuit and total distance flown, allowing us to investigate the dynamics of flight for neonicotinoid-exposed and unexposed workers.

Our experiment revealed that exposure to the neonicotinoid had a striking negative effect on flight endurance, with exposed workers only managing to fly around a third of the distance and duration of non-exposed workers on average. This equated to a 1 km decrease in flight distance, and if taken in the strictest sense, exposure to neonicotinoids at a field-realistic concentration therefore has the potential to reduce the total foraging area of a bumblebee colony by over 80%. Consequently, neurotoxic pesticide exposure will place increased stress on bumblebee colonies. For example, exposed foraging bees may find themselves unable to reach previously accessible resources, or incapable of returning to the nest following exposure to contaminated flowers. Not only would this reduce the abundance, diversity, and nutritional quality of food available to a colony, but could also limit the pollination service a colony is able to provide.

Interestingly, analysis of flight velocity revealed a response by the exposed workers that possibly explains, at least in part, why their flight was terminated prematurely. We found that exposed workers flew significantly faster in the initial part of the flight test, with average velocity only converging with unexposed workers once most of the exposed bees had dropped out of the flight trial (Figure 2). Our findings suggest that neonicotinoid exposure resulted in a hyperactive-like state causing exposed workers to fly faster than expected. As neonicotinoids are similar to nicotine and act by stimulating neurons, this ‘rush’ or hyperactive burst of activity does thus make sense. However, our results suggest there may be a cost to this initial rapid flight, potentially through increased energy expenditure or a lack of motivation, in the form of reduced flight endurance.

Figure 5 - flight velocity over time - subset
Average velocity (m/s) flown by each treatment group (control = solid red, pesticide = dashed blue) plotted for each consecutive circuit for the first 2,500 circuits. Numbers at the bottom of the graph refer to the number of bees still flying on the corresponding circuit.

Our findings take on an interesting parallel to the story of the ‘Tortoise and the Hare’. As the famous fable states, ‘slow and steady wins the race’. Little did Aesop know that this motto may be true for bumblebees in agricultural landscapes. Just like the Hare, being speedier does not always mean you reach your goal quicker, and in the case of bumblebees, exposure to neonicotinoids may provide a hyperactive ‘buzz’ but ultimately impairs individual endurance.



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