Honey bees cannot sense harmful concentrations of metal pollutants in food

Acute toxicity and bioaccumulation of common urban metals in Bombus impatiens life stages

Metal contamination is ubiquitous in urban areas and represents a risk to arthropod species. Bees are exposed to metals while foraging within contaminated landscapes from multiple sources. Eliminating the risk of bee exposure to metals is complex, and requires an understanding of how bees become contaminated, how metals accumulate within bee bodies, and how this exposure influences their health. We selected Bombus impatiens, the common eastern bumble bee, as our focal species because it is the most frequently encountered bumble bee species in the eastern United States and common within urban greenspaces. The aims of this study were to quantify the lethal concentration exposure limit (LC50) for B. impatiens foragers, assess the bioaccumulation ability of environmentally relevant concentrations of common urban metals in adults, larvae, and pupae, and compare the LC50 values against field relevant concentrations collected by foraging bumble bees within a legacy city. Bumble bees were orally exposed to arsenic oxide, cadmium chloride, or chromium oxide in sucrose solution to encourage consumption. The LC50 for arsenic (As2O3 36.4 mg/L), cadmium (CdCl2 10.3 mg/L), and chromium (CrO3 189.6 mg/L) are 202×, 79×, and 1459× greater than concentrations found within urban bumble bee collected provisions, respectively. Adult bumble bees fed field realistic concentrations of metals accumulate significant amounts of cadmium and lead within their bodies, but do not accumulate chromium and arsenic. Additionally, adults accumulate significantly higher concentrations of metals than brood. While bumble bee foragers are unlikely to encounter lethal metal concentrations while foraging in contaminated landscapes, it is crucial to consider and understand how sublethal concentrations impact overall colony functioning. The results from this study highlight the need to identify hazards and bioaccumulation ability of common metals as bees respond differently to each metal species, as well as the impacts of metal mixtures on bioaccumulation and toxicity.

Environmental exposure to metallic pollution impairs honey bee brain development and cognition

Laboratory studies show detrimental effects of metallic pollutants on invertebrate behaviour and cognition, even at low levels. Here we report a field study on Western honey bees exposed to metal and metalloid pollution through dusts, food and water at a historic mining site. We analysed more than 1000 bees from five apiaries along a gradient of contamination within 11 km of a former gold mine in Southern France. Bees collected close to the mine exhibited olfactory learning performances lower by 36% and heads smaller by 4%. Three-dimensional scans of bee brains showed that the olfactory centres of insects sampled close to the mine were also 4% smaller, indicating neurodevelopmental issues. Our study raises serious concerns about the health of honey bee populations in areas polluted with potentially harmful elements, particularly with arsenic, and illustrates how standard cognitive tests can be used for risk assessment.

Impact of Brood Cell Cocoons on Metal Accumulation and CYP450 Detoxification Gene Expression in Apis cerana cerana

Honey bees play a critical role as pollinators. However, their reproduction success and survival face severe threats due to the deterioration of their living environment. Notably, environmental conditions during their preimaginal stage inside brood cells can influence their immune capabilities and overall health after emergence. During the in-cell developmental stage, workers are in close contact with cocoons, which can become a source of stress due to accumulated metals. To investigate this potential threat, experiments were conducted to examine the impact of cocoons in brood cells used to rear different generations on the metal content and detoxification gene expression levels in Apis cerana cerana. Our findings indicated significant differences in the layers, weight, base thickness, and metal contents like Cr, Cd, Pb, Mn, Ni, and As of cocoons in multi-generation brood cells compared to single-generation brood cells. These increases led to significant elevations in metal levels and upregulations of the four CYP450 detoxification genes in both six-day-old larvae and newly emerged workers. In conclusion, this study highlights the negative impact of cocoons in multi-generation brood cells on bee health and provides evidence supporting the development of rational apiculture management strategies for ecosystem stability.

Trace metals with heavy consequences on bees: A comprehensive review

The pervasiveness of human imprint on Earth is alarming and most animal species, including bees (Hymenoptera: Apoidea: Anthophila), must cope with several stressors. Recently, exposure to trace metals and metalloids (TMM) has drawn attention and has been suggested as a threat for bee populations. In this review, we aimed at bringing together all the studies (n = 59), both in laboratories and in natura, that assessed the effects of TMM on bees. After a brief comment on semantics, we listed the potential routes of exposure to soluble and insoluble (i.e. nanoparticle) TMM, and the threat posed by metallophyte plants. Then, we reviewed the studies that addressed whether bees could detect and avoid TMM in their environment, as well as the ways bee detoxify these xenobiotics. Afterwards, we listed the impacts TMM have on bees at the community, individual, physiological, histological and microbial levels. We discussed around the interspecific variations among bees, as well as around the simultaneous exposure to TMM. Finally, we highlighted that bees are likely exposed to TMM in combination or with other stressors, such as pesticides and parasites. Overall, we showed that most studies focussed on the domesticated western honey bee and mainly addressed lethal effects. Because TMM are widespread in the environment and have been shown to result in detrimental consequences, evaluating their lethal and sublethal effects on bees, including non-Apis species, warrants further investigations.

The role of AccCDK20 and AccCDKN1 from Apis cerana cerana in development and response to pesticide and heavy metal toxicity

Apis cerana cerana is a native bee species in China and plays a key role in agricultural production and ecological balance. However, the growth and development of Apis cerana cerana has not been smooth, and pesticide and heavy metal stress are key factors that have forced a dramatic decline in population size. This study was performed with the objective of investigating the role of AccCDK20 and AccCDKN1 in honey bee resistance to pesticide and heavy metal stress. RT-qPCR analysis revealed that AccCDK20 transcript levels were highest in brown-eyed pupae and AccCDKN1 transcript levels were highest in 1-day-old worker bees. In different tissues and body parts of adult bees, AccCDK20 transcript levels were highest in the head, and AccCDKN1 transcript levels were highest in the thorax. It was further observed that environmental stress can affect the transcript levels of the AccCDK20 and AccCDKN1 genes. Silencing of the AccCDK20 and AccCDKN1 genes resulted in altered activities of antioxidant-related genes and antioxidant-related enzymes. AccCDK20 and AccCDKN1 transcript levels were upregulated under glyphosate stress, and silencing of the genes resulted in reduced resistance to glyphosate and greatly increased mortality in Apis cerana cerana. In addition, gene function was verified by in vitro repression assays. Overexpression of the AccCDK20 and AccCDKN1 proteins in E. coli cells increased the resistance to ROS damage induced by CHP. In conclusion, AccCDK20 and AccCDKN1 play an indispensable role in honey bee resistance to pesticide and heavy metal stress.