Can bumble bees be good gardeners?

Sometimes bumble bee queens emerge from hibernation too early for the flowering plants which provide them with vital pollen and nectar. But researchers in Switzerland have noticed a curious phenomenom. The hungry early bees cut holes in the plants' leaves. This caused the plant to flower almost twice as quickly as those without holes.  Could bees be using gardening techniques to get their vital food supply?

At first, the researchers thought the insects might be feeding on fluid from the leaves, but the bees didn’t stay long enough for this. Nor did they appear to be taking any part of the leaves back to their colonies. Another key observation was that bumble bees from colonies with less food were more avidly damaging the leaves.

So the researchers set up a greenhouse experiment with black mustard (Brassica nigra). Ten plants were put in mesh bags with bumble bees that hadn’t eaten any pollen for 3 days. These bees proceeded to nibble five to 10 holes in each plant. On average, those plants flowered after 17 days; undamaged plants that had not been exposed to bumble bees took an average of 33 days. In a similar experiment, tomato plants sped up their flowering by 30 days.

Hunger seems to be driving the bees: Another lab experiment showed that pollen-deprived bumble bees cut about four times as many holes as bumble bees that were fully fed. When the researchers put potted plants and a colony of bees on a roof at the ETH Zürich campus in early spring, before flowers were blooming, the bumble bees cut holes in the leaves. But as spring progressed, making more pollen available, the bees made fewer new holes.

To find out whether the leaf damage alone triggers the plants to flower sooner, the researchers cut similar-size holes in leaves. Those plants flowered earlier than controls, but not as early as the ones bitten by bumble bees. An intriguing possibility is that bee saliva might contain chemicals that prompt flowering. These would be similar to chemicals in the saliva of plant-eating insects that prompt plant defense responses.
The researchers hope to investigate further, and to study chemical signaling inside the plants after they are bitten. “It would be a horticulturist’s dream if you could find some relatively low-cost biochemistry that would actually speed up flowering,” says Mark Mescher, an evolutionary biologist at ETH Zürich who co-led the research.

As human gardeners, it is important we support our pollinating bees. You can make sure your growing space includes flowering plants which will feed them throughout the seasons - from early Spring to late autumn.  See here for a full list. 

Tuesday, 15 September 2020