15 June 2020
In this period of the year, I think about bee nutrition quite often. It’s the time of wild meadows, of flowering gardens and parks. The time of the discussions on which seed mixture to use and which flowers to plant to “save the bees”. There are initiatives focussing flowering road verges, others on connecting habitats with “stepping stones” of flowering areas and many more. I like seeing how creative all these projects are in building habitat for bees.
As always, however, everything has different aspects. A farmer once asked me what I thought about his flowering fields. He was somewhat insecure if he was doing any good after a beekeeper had told him that these areas “were no good for his bees”. Well, yes… no… it’s complex. You may have expected this. So let’s talk about honey bees first. Stick with me, I’ll come to other bee species as well. But let’s begin with the most well-known bees first.
Bee nutrition beyond honey harvest and winter feed
For many beekeepers, bee nutrition means mainly mass flowering crops, bringing them a honey harvest. They also think of the syrup or candy they give their colonies to replace the honey they harvested. That was what made the farmer insecure. The beekeeper was saying: yes, that flowering field is nice, but it doesn’t bring me any honey. That’s because the amount of nectar from all these different flowers doesn’t add up to a honey crop. For the bees, on the other hand, it does.
First, we have to differentiate between the different resources bees (all of them) get from flowers. There’s the nectar, their sugar source. They need this mainly for energy, the colonies need it to produce heat for the brood and surviving the winter. Adult bees need sugar as fuel for flying. And finally, growing is pretty energy-intense, so older larvae need it, too. Pollen, on the other hand, is the source for proteins and fats, as well as for vitamins and minerals. As you can see, the foundation of bee nutrition is quite similar to ours.
Flower diversity and bee nutrition
This isn’t a tangent, as you might think. The point is, for energy, honey bee colonies are quite happy with mass-flowering crops. This is what brings honey for the beekeeper. For pollen, however, for that stuff giving honey bee colonies most of their nutrients, they like diverse sources. Those flowering meadows, gardens with wildflowers, blooming road verges aren’t productive for the beekeeper. But they’re essential for the bees. The more diverse these structures are the better.
Honey bees are polylectic, i.e. bees that use many diverse pollen sources. This is especially important as we’re getting more and more aware of the importance of bee nutrition for bee health. If they have these diverse pollen sources,
- they’re more resistant against viruses,
- the lifespan of single workers is higher,
- they overwinter more successful etc.
Honey bees definitely need diverse pollen sources for their overall health.
Diverse flowers also support a larger bee community. For bumblebees, for instance, a recent study shows that higher floral diversity supports also a more abundant and diverse bumblebee community. Most bumblebee species, like honey bees, are polylectic. Another study shows, that bumblebees are able to detect pollen quality and they adapt their foraging behaviour according to this. Bumblebees have different flower preferences than honey bees, depending also on the species. Long-tongued bumblebees are able to use deep flowers, while short-tongued ones need flowers with more accessible flowers.
The relationship between bee nutrition and health is less studied in bumblebees. However, there are data on bumblebees using some substances in flowers (like nicotine) as a medicine against parasites. From the data we already have, I think it’s reasonable to assume that what we formerly stated for honey bees is true for bumblebees as well: they need diverse flowers for being healthy.
The opposite of diverse nutrition isn’t necessarily bad
The reason why I brought the term “polylectic” up, is because there are also bees that aren’t this way. These flower specialists are “oligolectic”, they reverse what I told you about honey bees before: they forage only a few flower species for pollen, but use many different nectar sources. Oligolectic bees are mostly solitary – when you have a colony to maintain over the season, you can’t be too picky. Solitary bees, on the other hand, are synchronized with the flowers they forage pollen on. They fly only during shorter periods and by this can “afford” to provision their nests with less diverse plants.
So, we learned that diverse nutrition is good for bee health. How does this fit with oligolectic bees? Dakota Spear and her co-authors brought up an interesting approach: they found that specializing in pollen from the daisy family helped mason bees against parasites. If this is true also for other specializations remains unclear (as far as I know).
Bee nutrition and consequences for conservation
As you can see from all the above, bee nutrition is quite complex. Which brings me back to the initial thoughts on initiatives to help bees by flowering road verges etc. Many of them already respect the complexity and offer many different flowers. However, I wonder how much the aspect of specialization is considered with these measures. Oligolectic bees are much more vulnerable than polylectic bees. If the plants they’re specialized on are missing, they won’t be able to rear their brood and persist in an area.
So, if we want to “save the bees”, we have to consider these vulnerable species as well. Of course, it always depends on the aim of single measures. In a private, urban garden the possibilities are different to larger areas like meadows or road verges. We have to see what’s already there, how we can enhance the natural structures and connect habitats. But, most importantly, we always have to keep in mind which species we want to help. I can’t repeat this enough: not all bees are the same. If the aim is to maintain bee diversity, we have to look closely what plants could attract the most vulnerable ones – the specialists, too. For the overall health of bees.