No evidence for neonicotinoid preferences in the bumblebee Bombus impatiens

Abstract

Neonicotinoid pesticides can have a multitude of negative sublethal effects on bees. Understanding their impact on wild populations requires accurately estimating the dosages bees encounter under natural conditions. This is complicated by the possibility that bees might influence their own exposure: two recent studies found that bumblebees (Bombus terrestris) preferentially consumed neonicotinoid-contaminated nectar, even though these chemicals are thought to be tasteless and odourless. Here, we used Bombus impatiens to explore two elements of these reported preferences, with the aim of understanding their ecological implication and underlying mechanism. First, we asked whether preferences persisted across a range of realistic nectar sugar concentrations, when measured at a series of time points up until 24 h. Second, we tested whether bees' neonicotinoid preferences were driven by an ability to associate their post-ingestive consequences with floral stimuli such as colour, location or scent. We found no evidence that foragers preferred to consume neonicotinoid-containing solutions, despite finding effects on feeding motivation and locomotor activity in line with previous work. Bees also did not preferentially visit floral stimuli previously paired with a neonicotinoid-containing solution. These results highlight the need for further research into the mechanisms underlying bees' responses to these pesticides, critical for determining how neonicotinoid-driven foraging preferences might operate in the real world for different bee species.

Keywords: activity; bee; behaviour; imidacloprid; learning; locomotor.

Figure 1.

Summary of all experiments.

Figure 2.

The relative preference for the neonicotinoid-containing solution relative to control solution (shown as the difference in the amount consumed of these two solutions) over time for (a) water, (b) 5% sucrose, (c) 15% sucrose and (d) 30% sucrose. Within each graph, data are shown for the three different imidacloprid (IMD) concentrations; these are offset on the x-axis to make data easier to visualize but were all measured at the same time points. Data shown are means ± s.e.m.

Figure 3.

Experimental design of experiment 2. The specific location, scent and colour of stimuli shown here are a single example; in the experiment, eight possible combinations were used. Similarly, the order that the reward type was presented (sucrose or sucrose + neonicotinoid first) was represented equally across treatments.

Figure 4.

The amount of time (mean ± s.e.m.) during the 5 min test phase that bees spent on either the stimulus that had previously been paired with a neonicotinoid-containing solution or the stimulus that had previously been paired with a control (sucrose only) solution.