The quiet drone of busy bees in our garden flower patch is as much a fixture of summer as the barbecue and beach. With their fabled industry, bees move between flowers, collecting pollen that nourishes the young in their hive and pollinates each new flower. Most of our cultivated crops are pollinated in this way, as are a large fraction of the wildflowers that decorate the landscape. Unfortunately, this crucial ecosystem service that we take for granted is in peril. Bees are in precipitous decline across the world, suffering from the mysterious blight called colony collapse disorder (CCD). Two controversial papers published recently in the journal Science provide compelling evidence that pesticides are to blame.
Neonicotinoids are neurotoxic pesticides that can paralyse and kill insect herbivores by acting as neurotransmitter receptor agonists and are used to protect more than 100 crop species worldwide. Because they spread to all parts of the plant, including pollen and nectar, bees are exposed to low pesticide doses while foraging. Until now, the effects of such sublethal exposure on bees have been unclear.
Using a miniaturized electronic tagging system to barcode more than 600 bees, Mickael Henry and colleagues from the INRA in France examined the consequences of neonicotinoid exposure on honey bee foraging behaviour (Henry et al., 2012). The team individually monitored bees as they left or entered the experimental hive. The bees in this experimental set-up had two fates: they either returned to the hive or died trying. The researchers found that risk of death in pesticide-exposed bees increased roughly twofold when compared with control bees. This increased risk was evident if the bees were naive or experienced foragers, and with hives that were far away or nearby. Essentially, the pesticide-exposed bees got lost on their way home.
In a second study, Penelope Whitehorn and colleagues from the University of Stirling, UK, found equally striking effects of pesticides on bumblebees. The researchers fed bees a diet of pesticide similar to what they would experience in the field and then monitored colony growth rates (Whitehorn et al., 2012). Compared with unexposed controls, the treated bee colonies were nearly 10% smaller. This may seem a small difference, but the consequences of this reduction were dramatic. The treated colonies only produced one or two queens, compared with nearly 15 for the untreated bee colonies. Without queens, a colony of bees will leave no descendants.
Neonicotinoid pesticides may not kill the two examined bee species immediately, but their demise comes all the same through more subtle and difficult to detect means. Does this mean that neonicotinoid pesticides are the cause of CCD? Although the evidence of these studies is damning, it is perhaps too early to say. In addition to pesticides, other studies have implicated mites, viruses, general habitat loss or some complex interaction between these as causes of CCD. However, given the irreplaceable role of bees in crop pollination and honey production – not to mention their staggering economic value – it would seem most prudent to ban neonicotinoid pesticides while the jury is out. As of June 2012, France has taken this cautious step, much to the displeasure of pesticide manufacturers. UK authorities, however, are inexplicably yet to do so.