Why honey bees cannot sense metal pollutants in food

Whether animals can actively avoid food contaminated with harmful compounds through taste is key to assess their ecotoxicological risks

by Lorenzo Ciotti
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Why honey bees cannot sense metal pollutants in food

"Whether animals can actively avoid food contaminated with harmful compounds through taste is key to assess their ecotoxicological risks. Here, we investigated the ability of honey bees to perceive and avoid food resources contaminated with common metal pollutants known to impair behaviour at low concentrations.

In laboratory assays, bees did not discriminate food contaminated with arsenic, lead or zinc and ingested it readily, up to estimated doses of 929.1 μg g-1 As, 6.45 mg g-1 Pb and 72.46 mg g-1 Zn. A decrease of intake and appetitive responses indicating metal detection was only observed at the highest concentrations of lead and zinc through contact with the antennae and the proboscis.

Electrophysiological analyses confirmed that only high concentrations of the three metals in a sucrose solution induced a consistently reduced neural response to sucrose in antennal taste receptors. Overall, cellular and behavioural responses did not provide evidence for specific mechanisms that would support selective detection of toxic metals (arsenic, lead), as compared to zinc, which has important biological functions.

Our results thus show that honey bees can avoid metal pollutants in their food only at high concentrations unlikely to be encountered in the environment. By contrast, they appear to be unable to detect low, yet harmful, concentrations found in flowers.

Metal pollution at trace levels is therefore a major threat for pollinators." This is what was explained in the study: Honey bees cannot sense harmful concentrations of metal pollutants in food, published on the Chemosphere.

About honey

Honey is produced by the bee on the basis of sugary substances that it collects in nature. The main sources of supply are nectar, which is produced by flowering plants, and honeydew, which is a derivative of tree sap, produced by some sucking insects such as metcalfa, which transform the sap of plants by retaining its nitrogen and expelling the excess liquid rich in sugars.

For plants, nectar is used to attract various pollinating insects, in order to ensure the fertilization of the flowers. Depending on their anatomy, and in particular on the length of the proboscis, honey bees can collect nectar only from some flowers, which are called honey bees.

The composition of nectars varies according to the plants that produce them. However, they are all composed mainly of carbohydrates, such as sucrose, glucose, fructose and water. Their water content can be important, and can reach up to 90%.

Honey production begins in the goiter of the worker during her return flight to the hive. In goiter invertase, an enzyme which has the property of splitting sucrose into glucose and fructose, is added to the nectar, producing a chemical reaction, hydrolysis, which gives glucose and fructose.

Once in the hive, the bee regurgitates the nectar, rich in water, which must then be dehydrated to ensure its conservation. For this purpose, the foragers lay it in thin layers on the cell wall. The worker ventilators maintain a current of air in the hive which causes the water to evaporate.

When this is reduced to 17-18%, the honey is ripe. It is then stored in other small cells which, once full, will be sealed with a thin layer of wax.