Broadcast spraying of agricultural antibiotics has increased exponentially in recent decades to combat bacterial plant pathogens. Wild organisms exposed to these antibiotics can suffer altered gut microbial communities, including the loss of host-beneficial symbionts. We need to understand any unintended consequences of antibiotics, particularly on organisms that provide agricultural ecosystem services, such as bees, given the key functional roles of many symbiotic microbes on host fitness and physiology.
We hypothesized that antibiotic exposure alters bee foraging behavior by 1) decreasing foraging rate and 2) negatively affecting associative learning. We tested these predictions in managed bumble bees (Bombus impatiens) using behavioral trials with artificial flowers and with Free Proboscis Extension Reflex (FPER) assays - an automatic reflex in which bees extend their proboscis in response to stimuli associated with food rewards. We fed sub-colonies of foragers a sucrose solution with field-realistic concentrations of streptomycin (200 ppm) versus a sucrose-only control two days prior to the experiments. To examine foraging rate, we compared antibiotic-exposed and control bees foraging in an enclosure with two types of artificial flowers: low-reward (water) and high-reward, of different colors. In the FPER assays, we trained foragers to associate a color stimulus with a reward solution and subsequently assessed FPER in response to their trained color and an alternative color stimulus.
Results/Conclusions
We continue to collect and analyze data, but preliminary results from the FPER assays show that consumption of streptomycin slows bee learning (p<0.001) and leads foragers to choose stimuli with the lower rewards more frequently compared to controls (p=0.016). Longer (2 hrs) artificial flower trials show that bees fed antibiotics were as likely to choose high and low rewards (p=0.309), but that they obtained less rewards (sum of low + high rewards) than controls (p=0.028). These findings show that 1) given sufficient time, bees overcome the negative effect that field-realistic concentrations of antibiotics have on decision-making; but 2) that antibiotic consumption reduces the overall rewards acquired. A reduction in foraging activity of bees exposed to antibiotics could have consequences for bee fitness and pollination.