Bumble Bee Nibbles Stimulate Plants To Bloom Early

When pollen is scarce, bumble bees will nibble on the leaves of plants that lack flowers thereby damaging them, which compels the plants to bloom two to four weeks earlier

Plants and their pollinators rely on each other for survival. Just as pollinators depend upon flowers for important foods, plants need pollinators to ensure their reproduction. But this relationship is threatened by climate change, which drives warming temperatures that awaken hibernating pollinators before springtime blooming has begun.

Pollen is the only food provided to bumble bee larvae and it is an important protein source for workers (ref), so the lack of adequate pollen resources has serious consequences for bumble bee colonies. For example, a recent study found that flowers located within one kilometer of a newly founded bumble bee nest strongly influences the colony’s survival and performance (ref), whilst another study showed that a shortage in flower abundance during a critical period of early colony establishment has profound effects upon that colony’s later reproductive success (ref).

Considering how vitally important it is that bumble bees and their flowers synchronize their activities, you might suspect that bumble bees have evolved a method for accelerating flowering times.

If you suspect this, then you’d be correct.

Hungry bumble bee workers nibble on plant leaves

A recent study by an international team of scientists originally observed that worker buff-tailed bumble bees, Bombus terrestris, damage plants by nibbling distinctively-shaped holes in plant leaves (video).

The researchers did not see the bumble bees carrying bits of leaves back to the hive, nor were the bumble bees snacking on the leaf morsels, so why were they biting holes in the leaves? The scientists hypothesized that the resulting leaf damage might stress the plants, thereby accelerating flower production (ref).

Do bumble bee leaf bites influence flowering times?

To test their hypothesis, the researchers designed some laboratory experiments (Figure 3) using two distantly related plants, tomatoes, Solanum lycopersicum, and black mustard, Brassica nigra, and compared the flowering time of bee-damaged plants (Figure 1) to that of undamaged plants and to plants that had been mechanically damaged by people using metal forceps and a razor to approximate bee-damage.

To create bee-damaged plants, microcolonies of bumble bees were either given abundant pollen inside their hives, or they were put on a diet of no pollen at all. After several days of this treatment, the bumble bee colonies were given flowerless black mustard in mesh enclosures (Figure 3A). The bee-damaged plants were daily replaced with undamaged plants and the proportion of damaged leaves were counted.

This experiment established that, as had been suspected, leaf damaging behavior was strikingly higher when bumble bee workers were pollen-deprived compared to those in pollen-satiated colonies (Figure 3B). Further, the researchers found that pollen-satiated colonies only inflict minor leaf damage, whereas a period of pollen deprivation of just three days is sufficient to inspire bumble bee workers to nibble on plant leaves.

To investigate the effects of bumble bee damage on flowering times, each bee-damaged plant was paired with a mechanically damaged plant and flowering times compared (Figure 2).

As you can see, these experiments document that bumble bee-damage to plant leaves was highly significant for accelerating flowering in both tomatoes and black mustard (Figure 2). Bee-damaged tomatoes flowered 30 days earlier than undamaged plants and 25 days earlier than mechanically damaged plants, whilst black mustard flowered 16 days earlier than undamaged plants, and 8 days earlier than mechanically damaged plants.

This study points to important differences between mechanical damage and bumble bee-damage: is there some sort of bee-derived cue that may influencing flowering plants? Additionally, because pollinators strongly influence plant evolution, is leaf damage adaptive for some sorts of flowering plants, especially as the climate warms?

Flower scarcity poses serious challenges for wild pollinators, and bumble bees have a particularly pressing need for floral resources during early spring, when hibernating queens are awakening and must establish their colonies. Damaging plant leaves when floral resources are scarce may be an adaptive strategy for accelerating flower production and to help synchronize the abundance of both flowers and pollinators.

Source:

Foteini G. Pashalidou, Harriet Lambert, Thomas Peybernes, Mark C. Mescher, Consuelo M. De Moraes (2020). Bumble bees damage plant leaves and accelerate flower production when pollen is scarce, Science, 368(6493):881-884 | doi:10.1126/science.aay0496