The Green Revolution is credited with saving a billion human lives as we manicure crops with pesticides and weed killers. But a fatal contradiction lies at the heart of the innovation. The insects that are essential for pollinate one-third of the world's crops are also collateral damage, falling prey to the chemicals that are designed to protect crops; and honey bees are no exception. 'Honey bees have become very important in agroecosystems. Therefore, it is necessary to assess the effect of pesticides on pollinators', says Carolina Mengoni Goñalons from Universidad de Buenos Aires, Argentina. However, returning honey bee foragers are not the only members of the hive that are exposed to these toxins. Newly hatched worker bees brush past foragers carrying weed killers, and handle pollen and honey that bear pesticides. Knowing that the workers' abilities to respond to sweet flavours and learn are key to their nurturing role, Mengoni Goñalons and Walter Farina decided to investigate the effects of a life-long exposure to weed killers and insecticides on young bees. They discovered that worker bees exposed to insecticides and weed killers partially lost their sense of taste and find it difficult to learn, potentially placing hives in jeopardy. The team publish the discovery in Journal of Experimental Biology at http://jeb.biologists.org.
Neonicotinoids are a family of insecticides that are popular in agriculture as they are virtually harmless to mammals but potent insect neurotoxins, and glyphosate is a commonly used weed killer, so Mengoni Goñalons and Farina decided to test the effects of both on worker bees. Nurturing workers from the moment they emerged from the brood comb, Mengoni Goñalons fed the youngsters on pollen and syrup dosed with imidacloprid (a neonicotinoid insecticide), glyphosate or a combination of the two in concentrations that the insects would encounter naturally in the wild. Then, when the bees were 5, 9 and 14 days old, Mengoni Goñalons cautiously tested their sense of taste and ability to learn.
'Honey bees are small and delicate, but also dangerous', she says, recalling how she had to gently restrain each animal before touching its antennae with a series of sugar solutions -- ranging from dilute (0.1% weight/volume) to a syrupy 50% solution -- and recording when each bee began sticking out its tongue (proboscis) to show that it could taste the sugar. Sadly, both the insecticide and the weed killer diminished the bees' sense of taste dramatically -- the insects responded only to 30% sugar solutions, in contrast to the 10% sugar solutions that the untreated bees could taste. However, the combination of the insecticide and weed killer did not make the decrease in sensitivity worse.
So, how did the pesticide, weed killer and combination of the two affect the bees' ability to learn? After training the bees at the age of 5, 9 and 14 days to stick out their tongues when they smelled the sweet odour of 1-hexanol -- in much the same way that Pavlov trained dogs to salivate when they heard a bell -- Mengoni Goñalons found that the insecticide affected the bees' ability to learn to recognise the scent, even at 5 days of age. However, the bees that had consumed both agrichemicals were perfectly able to learn their new trick. And when Mengoni Goñalons trained the bees to distinguish between two odours -- sweet-smelling 1-hexanol and fruity nonanal -= the 5 and 9 day old bees that had been fed the insecticide struggled to learn to distinguish between the two.
Young worker bees can be affected by pesticides, even though they have never set foot outside the hive, and even herbicides, which are not targeted at insects, can impair their sensory and cognitive abilities. Farina warns, 'The presence of hive bees that are unable to taste the sweetness of food or discriminate between scents will worsen the efficiency of beehives as a whole, possibly reducing the efficiency of pollination and honey production'.
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REFERENCE: Mengoni Gon?alons, C. and Farina, W. M. (2018). Impaired associative learning after chronic exposure to pesticides in young adult honey bees. J. Exp. Biol. 221, doi:10.1242/jeb.176644.
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Journal of Experimental Biology