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]