The decline of bees

The Economist explains

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Sep 6th 2015, 23:50 BY D.J.P.REPORTS of bee die-offs have become more frequent and more alarming in recent years. Pollen from the world's flowering plants hitches a ride most often on bees, making them an integral part of the global food-production machinery. But distribution maps of which bees are where show contractions of range, and wholesale extinctions. There are widespread reports of vast die-offs in or sudden abandonment of honeybee hives, often called colony-collapse disorder. Bees are definitely dying—but why, and what might the consequences be?

The scope of what is at stake is sometimes mis-stated: around two-thirds of the food we eat, by weight, comes from staple crops such as rice, wheat and maize that are pollinated by wind, not insects. The remaining one-third includes fruits and vegetables, nuts, many herbs and spices, coffee and chocolate (a diet free of insect-pollinated foods would therefore be short on many nutrients and altogether pretty boring). While bees are by far the biggest pollinators, one misconception is that they are one uniform group. There are, in fact, about 20,000 bee species, but only a few are "honeybees"—that is, the kind that are kept in hives and are grown, sold and traded as a commodity no less than the crops they pollinate. The rest are wild. Though the balance of honeybee and wild-bee contributions varies widely, wild bees are responsible for a majority of pollination globally, and just a few species are doing most of this work (one recent study estimated 80% of pollination was accomplished by just 2% of bee species). In contrast, intensive farming of some crops such as almonds is entirely dependent on honeybees; there are just not enough wild bees to do the job in time reliably.

This intensity of farming speaks to three distinct but interlinked challenges that are the likely causes of bee declines. One is simple: worldwide, there is less untamed land to support bees. Farmland is stripped of all but the cash-crop plants, so there is no flowery food when those crops are not in bloom. As with many displaced fauna, in many developed regions there is simply nowhere left for bees to roam. Another problem is the spread of honeybee diseases. Hives are shipped all over the world, and with them come stowaway bacteria, fungi, and parasites such as the varroa mite, which has received much attention as a possible cause of colony-collapse disorder. While associated with particular honeybee species, some of these nastiest can jump the species barrier and threaten wild bees too; all the world's bees are increasingly subject to all the world's bee pathogens. Intensive farming also dictates the use of a great many fungicides, herbicides and pesticides on crops. In recent years a relatively new family of pesticides called neonicotinoids has drawn particular fire; while studies of the chemicals have had a confusing mix of results, it seems clear that at certain doses, and in combination with other, standard plant treatments, neonicotinoids can be harmful or even deadly to bees.

In all likelihood, the threat to bees is some complex interplay between these diverse stressors. Recent work, for example, has shown that tiny doses of the neonicotinoid clothianidin turns a largely harmless viral infection of European honeybees into a deadly one. Crop chemicals' effects on bees are typically studied one at a time, while the tremendous number of combinations to which bees are ultimately exposed go unexamined. And bee populations weakened by habitat loss or food shortage will, as with any other creature, be more susceptible to additional threats. Returning some land to wild conditions is one simple fix; some schemes are already in place to reward farmers for doing so. Restricting the free passage of honeybee hives could help contain the ballooning problem of pathogens and parasites. But which combinations of problems are to blame in a particular hive or region, and whether any one cause ties together the losses worldwide, will remain a mystery until more studies try to unpick this thorny interplay—no easy task when a majority of the critters concerned are free-roaming.