Date: 21st February 2020
Why do bees visit some flowers and not others?
Do bees prefer certain flowers to others?
Why are flowers visited by some bee species but not others?
The reason bees visit flowers is because they offer what bees need (primarily food) and they prefer some flowers to others because they offer greater food reward, usually nectar and pollen.
In other words, bees prefer to visit flowers where they can gather as much food as possible For this reason, bees may focus their attention on particular flowers, whilst ignoring others close by.
A few bees collect other things, such as floral oils and also nesting materials, in which
case, they visit flowers that offer what they need. In addition, flowers have developed other ways to entice bees and other pollinators,
(the technical term is ‘Pollination Syndromes’) including scent and UV markings
on the petals. However, for the sake of
this article, I’m going to focus on food rewards: nectar and pollen.
So, the need for food is the primary driver for bees visiting flowers.
it’s worth noting that this topic gets a little more complicated. It takes the clever little bees to work out
which flowers are worth visiting, and which ones are not worth bothering with.
It is the case that some flowers genuinely do offer higher nectar or pollen rewards than others (and of course, some flowers may offer both).
Not all pollens and nectars are the same! The nutritional content of flower pollen and nectar (carbohydrates, micronutrients, lipids, protein) varies widely among plants, and scientists have found that this influences foraging activity(1).
However, although some flowers offer more nectar, others may have a faster nectar refill rate. This means that the same flower may be visited by bees multiple times to extract the nectar. For some time, the flower will continue to replenish the nectar reward offered, but the rate at which this occurs varies between species. For example, borage flowers refill with nectar about every two minutes, comfrey flowers provide lots of nectar and refill in around 40 minutes. Bird’s foot trefoil, on the other hand takes about 24 hours to replenish with nectar(2).
Does this mean bees will prefer borage flowers to bird’s foot trefoil? The answer is “it depends”. This brings me to
my next point about why some bees may prefer certain flowers to others…
Different bees are more easily able to extract the food rewards from particular flowers than others. For example, short tongued bees may struggle with flowers that have deep florets (although bees may adapt to this challenge by engaging in nectar robbing). Some bee species may also struggle to release pollen from anthers. These factors may result in flowers being more commonly visited than some types of bees and not others.
It also depends on the time of year when the flower is at its peak for producing nectar and pollen, and the species of bees that are active at that time, and of course, the needs and preferences of those bees.
For example, bird’s foot trefoil is an important flower for the Silvery
leafcutter bee (Megachile leachella). This flower not only provides food, but sometimes the petals
may also be used in nest construction. Indeed, flowers and plants have a number of uses for bees beyond food, although food is the most important.
Even flowers that are favourites for one week may suddenly be abandoned by bees in favour of different flowers the next week.
For example, I have observed bees visiting some of the wild dog roses growing in my area, and have taken photographs of many bee species foraging on them. Eventually, the roses were abandoned (probably because of depleting food rewards) in favour of bramble flowers which also offer a great food reward for bees. Yet research has shown that even the bramble flowers will ultimately become less appealing to bees as the food offering inside the flowers eventually reduces.
In view of all these factors above, which plants should you include
in your garden to feed the bees?
I have so many resources on this website, that it seems unnecessary to repeat the various lists of flowers here – instead, I include links below and at the bottom of this page.
However, some general guidelines:
(1) Vaudo, A. D., Tooker, J. F., Grozinger, C. M., & Patch, H. M. (2015). Bee nutrition and floral resource restoration. Current Opinion in Insect Science, 10, 133-141. . https://doi.org/10.1016/j.cois.2015.05.008
(2) A Sting in the Tale - by Dave Goulson
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