Bumble Bees Control When Flowers Bloom, Cushioning Climate Uncertainty

Bumble bees have developed a remarkable solution to the dangers of coming out of hibernation before food is available. They take advantage of the fact plants with damaged leaves flower earlier to make food suppliers hurry up. The discovery could be used to mitigate one of the consequences of climate change.

Plants and pollinators need each other, but like all relationships timing is key. Outside tropical zones, pollinating insects hibernate but if they emerge too soon before widespread flowering, their prospects are grim, taking down whole bumble bee colonies. When flowers do arrive, there may be no one around to fertilize them. The evolutionary pressure to synchronize can be met by plants and animals using the same cues or by one triggering the other. Dr Foteini Pashalidou of ETH Zurich has found bumblebees and some of their favorite plants have adopted the second approach.

In Science, Pashalidou and co-authors report that bumblebee workers have been seen cutting distinctively shaped holes in the leaves of four species, including Brassica oleracea, whose cultivars include broccoli, cabbage, and cauliflower. The activity only took place when flowers, and therefore pollen, was scarce. However, it appears starving bees are not finding an alternative food source, since the cutters neither consume bits of leaf on site or take them back to the hive.

Pashalidou noted that stress can induce flowering, by giving plants the message time may be short to pass their genes on before they die. However, as the paper notes, studies of this phenomenon have focused on lack of nutrients, not insect damage.
 

To test whether the bees were influencing the plants' flowering times, Pashalidou let bees loose on plants from two species they had previously been seen cutting. Matching plants were kept unharmed for comparison, while the damage to leaves was replicated mechanically in a third set. Bee-cut tomato plants flowered 30 days earlier than the undamaged ones and 25 days before those cut with razors. Black mustard flowered 16 days earlier when cut by bees than left uncut.

When bees were provided with abundant pollen, their leaf-cutting behavior fell dramatically. Leaf damage changes flowering times, but probably doesn't increase abundance, and may harm it. While the experiment was done on Bombus terrestris bees, two species of wild bumble bees were observed entering the researcher's rooftop garden and engaging in the same leaf-cutting behavior.

"An encouraging interpretation of the new findings is that behavioral adaptations of flower-visitors can provide pollination systems with more plasticity and resilience to cope with climate change than hitherto suspected," Queen Mary University's Professor Lars Chittka writes in an accompanying column.

Chittka also ponders how the behavior evolved and comes up puzzled. Although mechanical cutting did not fully replicate the bee-induced early flowering, something the authors admit they cannot explain, it may still be useful to encourage plants to flower at optimum times.