Consider for a moment the contents of your fridge and pantry – it has been estimated that about a third of our diet is thanks to the industrious foraging of insect pollinators. The honey bee (Apis mellifera) is by far the most lauded. Each time a worker bee sets out to forage for pollen and nectar to feed her colony mates, she fulfills another invaluable function. She fertilises the flowers of crop plants that she visits with the pollen she carries in her pollen basket, ensuring that humans benefit from the fruits of her labour as much as her younger siblings and queen do. And then, of course, there’s the honey — the delicious sugary syrup we have been collecting from wild hives for at least the past 15 millenia, and harvesting from maintained colonies of domesticated bees since the 1700s.
Since late 2006, a mysterious affliction — known as colony collapse disorder — has threatened to end this long-standing partnership we have with bees. When a colony collapses, worker bees fly from their hives, never to return. They abandon their queen, as well as the developing brood of larvae that now lie unfed in their individual honeycomb cells. Bee disappearances like this had occurred prior to 2006, but never in as great a number, and never as wide-spread. In the USA, about a third of honey bee colonies were lost to colony collapse disorder each year since 2007, but losses in 2012-13 could be as high as 50%. Similar losses have occurred in Europe.
The reason for the calamity that has befallen bee colonies across Europe and North America is far from fully understood. The finger has been pointed at a range of potential culprits over the years — including viruses, parasitic Varroa mites, fungal infections, malnutrition, and even GMOs and electromagnetic radiation.
Varroa destrucor mites can take an especially high toll on bee colonies, and are common in areas affected by colony collapse disorder. The mites, which have a predilection for the male drones, latch onto and suck the ‘blood’ (hemolymph) of pupae emerging from their cells as well as adult bees in the hive. This feasting not only leaves the parasitised bee with an open wound, but also usually delivers other infections, particularly viruses, that the mites bring with them to the hive.
Most recently, neonicotinoid pesticides have taken center stage as honeybee enemy numero uno. Neonicotinoids, or neonics, are insecticides that are chemically related to nicotine. They target nicotinic acetylcholine receptors in the insect brain, causing paralysis and death. For farmers, they are a popular alternative to the more toxic organophosphates that they replaced in the early 1990s, and help to keep insect pests from devouring corn, soy, cotton and canola crops. But for bees, some studies suggest that the pesticides impair bee navigation, learning and memory. While some groups have called for a ban of neonics, the science is far from conclusive. Observations that bee colonies in Canada and Australia remain relatively free of the disorder, in spite of high levels of neonic use, raise the possibility that neonics may not adversely affect bees much at all.
With no single factor clearly to blame, scientists have speculated that colony collapse could be the result of a perfect storm of contributing factors that combine to cause bee deaths. A study published last week in PNAS has lent weight to the idea that the health of the bee immune system might be central to keeping bee colonies from collapsing, and also suggests that management practices could be hindering rather helping to prevent the problem.
Bees consume a diverse diet of nectar from an extraordinary range of plant species. With this diversity in nectar sources comes an equally diverse range of foreign phytochemicals that the bee must process. Wenfu Mao and colleagues from the University of Illinois at Urbana-Champaign have been studying cytochrome P450 enzymes, which play important roles in metabolic processes in everything from bacteria to humans. Often they have a critical role in detoxifying foreign chemicals, such as drugs in humans. In bees, the enzymes help to detoxify the harsh miticides (acaricides) used by beekeepers to keep mite levels under control.
Despite the potential for bees to be exposed to a broad diversity of phytochemicals in their daily pursuit of nectar, bees have a surprisingly low number of cytochrome P450 genes — just 46, compared with the usual insect complement of around 80. To compensate for their spare detoxification arsenal, and to ensure that their systems are primed to detoxify the chemical cocktails they encounter, bees use a substance in honey to crank their cytochrome P450s into action, according to previous work by the group.
In order to find out exactly which component of honey is responsible for this priming effect, the team extracted components of honey separated by liquid chromatography. Using a single P450 gene known to be induced by the honey factor, they were able to pin-point which fractions contained the most P450 priming activity using a neat bee feeding assay. They fed bees with honey extracts in a honey substitute they dubbed “bee candy,” comparing P450 gene transcript up-regulation from each fraction. Once the most active fractions were isolated, they identified the active ingredient via mass spectrometry as p-coumaric acid, a compound abundant in pollen.
Next, they looked at the entire suite of genes up-regulated by the honey factor. In addition to 12 P450 genes, some of which are known pesticide metabolisers, genes encoding antimicrobial peptides were also up-regulated. Anti-microbial peptides are fundamental components of the bee immune system.
It seems that pollen may be playing an important role in priming the bee immune system to detoxify unwanted chemicals, and to fend off microbial infection. Instead of responding to infection or the arrival of pesticides into the hive as it happens, bees ensure that their defenses are already up through their diet.
The study also suggests that a common bee management practice could be making matters worse for stressed bee colonies. Beekeepers have been struggling to meet demand for the pollinators in some areas, and some beekeepers have turned to honey substitutes, such as sucrose and high-fructose corn syrup, to bolster colony productivity. But without the natural honey factors that the real stuff contains, bees are potentially at an even greater risk of succumbing to mites and the viral infections they carry.
On the positive side, the study also suggests that p-coumaric acid could be a good immune-boosting additive that can help bees to stave off whatever factors might lead them to colony collapse.
- Mao, Schuler & Berenbaum. 2013. Honey constituents up-regulate detoxification and immunity genes in the western honey bee Apis mellifera. Proceedings of the National Academy of Sciences USA Published online before print April 29, 2013, doi:10.1073/pnas.1303884110