Bees Bite Leaves to Induce Flowering

Photo by Ivar Leidus licensed under CC BY-ND 2.0.

Imagine spending all winter sleeping underground, living off of the energy reserves you accumulated the previous year. By the time spring arrived and you started waking up, your need to eat would be paramount to all other drives. Such is the case for emerging queen bumblebees. Food in the form of nectar and pollen is their top priority if they are to survive long enough to start building their own colony, but flowers can be hard to come by during those first few weeks of spring.

Spring can be very unpredictable. If bees emerge from their slumber too early or too late, they can miss the flowering period of the plants they rely on for food. By the same token, the plants themselves then miss out on important pollination services. Mismatches like this are becoming more common as climate change continues to accelerate. However, not all bees are helpless if they emerge onto a landscape devoid of flowers. It turns out that, with a little nibble, some bees are able to coax certain plants into flowering.

Over a series of experiments, scientists were able to demonstrate that at least three species of bumblebee (Bombus terrestris, B. lapidarius, and B. lucorum) were able to induce early flowering in tomatoes (Solanum lycopersicum) and mustards (Brassica nigra) simply by nibbling on their leaves. The queens would land on the leaf and make a series of small holes with their mandibles before flying off. The bees did not appear to be feeding on any of the sap, nor were they carrying chunks of leaf when they flew away. Amazingly, the act of nibbling on the leaves in each experiment resulted in earlier flowering times across both species of plant.

(A) Sequential images of a worker penetrating a leaf with its proboscis. (B) A worker cutting into a leaf with its mandibles. (C) Characteristic bee-inflicted damage. [SOURCE]

The results were not minor either. Flowers on bee-nibbled plants were produced an average of 30 days earlier than non-nibbled plants. Amazingly, when scientists tried to simulate bee nibbles using tweezers and knives, they were only able to coax flowering an average of 8 days earlier than non-damaged plants. What this means is that there is something about the bite of a bee that sends a signal to the plant to start flowering. Perhaps there’s a chemical cue in the bee’s saliva. Indeed, this is not unheard of in the plant kingdom. Some trees have shown to respond to the detection of deer saliva, ramping up defense compounds in their leaves only once they have detected deer. More work is needed before we can say for sure.

Through a complex series of experimental trials, scientists were also able to demonstrate that this behavior was the result of pollen limitation rather than nectar. As pollen availability increased both artificially (by adding already flowering plants) or naturally (as time wore on, more plants came into bloom), the leaf biting behavior declined. Such was not the case when only nectar was available. Pollen is a protein-rich food source for bees and is especially important for their developing larvae. By inducing plants to flower early, the bees are ensuring that there will be a ready supply of pollen when they and their developing larvae need it the most.

Considering the role bees play in pollination of plants like tomatoes and mustards, it is likely that this interaction benefits both players to some degree; bees are able to coax floral resources much sooner than they would normally become available while the plants are flowering when effective pollinators are present in the area. These exciting results open yet another window into the multitude of ways in which plants and their pollinators interact. Given that plants have been known to skew the caste systems in eusocial bees, it should come as no surprise to learn that some bees have a few tricks up their sleeves as well.

Photo Credits: [1] [2]

Further Reading: [1]

Do Yeasts Aid Pollination For the Stinking Hellebore?

Photo by Mark Gurney licensed under CC BY-NC-SA 2.0

Whether they are growing in their native habitat or in some far away garden, Hellebores are some of the earliest plants to bloom in the spring. Hellebore flowers can often be seen blooming long before the snow has melted away. All early blooming plant species are faced with the challenge of attracting pollinators. Though the competition for insect attention is minimal among these early bloomers, only the hardiest insects are out and about on cold, dreary days. It stands to reason then that anything that can entice a potential pollinator would be of great benefit for a plant.

That is why the presence of yeast in the nectar of at least one species of Hellebore has attracted the attention of scientists. The species in question is known scientifically as Helleborus foetidus. The lack of appeal in its binomial is nothing compared to its various common names. One can often find H. foetidus for sale under names like the "stinking hellebore" or worse, "dungwort." All of these have to do with the unpleasant aroma given off by its flowers and bruised foliage. Surprisingly, that is not the topic of this post.

Photo by Bernd Haynold licensed under CC BY-SA 3.0

What is more intriguing about the flowers of H. foetidus is that the nectar produced by its smelly green flowers harbors dense colonies of yeast. Yeasts are everywhere on this planet and despite their economic importance, little is known about how they function in nature. For instance, what the heck are these yeast colonies doing in the nectar of this odd Hellebore?

To test this, two researchers from the Spanish National Research Council manipulated yeast colonies within the flowers to see what might be happening. It turns out, yeast in the nectar of H. foetidus actually warms the flowers. As the yeast feed on the sugars within the nectar, their metabolic activity can raise the temperature of the flowers upwards of 2 °C above the ambient. As far as we know, the only other ways in which floral heating has been achieved is either via specific metabolic processes within the floral tissues or by direct heating from the sun. 

In heating the flowers, these yeast colonies may be having serious impacts on the reproductive success of H. foetidus. For starters, these plants are most at home under the forest canopies of central and western Europe. What's more, many populations find themselves growing in the dense shade of evergreens. This completely rules out the ability to utilize solar energy to heat blooms. Additionally, floral heat can mean more visits by potential pollinators. Experiments have shown that bees preferentially visit flowers that are slightly warmer than ambient temperatures. Even the flowers themselves can benefit from that heat. Warmer flowers have higher pollination rates and better seed set.

Bombus terrestris was one of the most common floral visitors of Helleborus foetidus. Photo by Vera Buhl licensed under CC BY-SA 3.0

Yeast colonies also have their downsides. The heat generated by the yeast comes from the digestion of sugars. Indeed, nectar housing yeast colonies had drastically reduced sugar loads than nectar without yeast. This has the potential to offset many of the benefits of floral warming in large part because bees are good at discriminating. Bees are visiting these blooms as a food source and by diminishing the sugar content of the nectar, the yeast may be turning bees off to this potential source. The question then becomes, do bees prefer heat over sugar-rich food? The authors think there might be a trade-off, with bees preferring heated flowers on colder days and sugar-rich flowers on warmer days.

Helleborus foetidus flowering before the snow has had a chance to melt!

Though the authors found evidence for heating, they did not test for pollinator preference. All we know at this point is that yeast in the nectar significantly warms H. foetidus flowers. Since this piece was originally published, more attention has been paid to the benefits of the heat generated from yeast. Interestingly, researchers found that pollen tube formation was higher for H. foetidus flowers that experienced heat earlier in the season but not for those that experienced heat later on. This response, however, was not due to the warming directly. Instead, it had more to do with bee preference.

As it turns out, bumblebees do in fact prefer to visit heated flowers but their preference is limited to the early periods of flowering when ambient temperatures are still quite low. More bumblebees visiting heated flowers in the early spring equated to more pollen being deposited on the stigma, which in turn led to an increase in pollen tube formation and higher seed set. Later on in the season, when ambient temperatures increased a bit, this positive effect dropped off as bees apparently spent more time foraging elsewhere.

Photo Credits: [1] [2] [3] [4]

Further Reading: [1] [2] [3]