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Science Insider

Hole-punching Bumble Bees Urge Plants to Flower Faster

- August 1, 2020 - Alison McAfee - (excerpt)

bumble bee hole-punching

And this could buffer against the effects of climate change

Bumble bees are smart. They can master complex tasks like learning to roll a ball onto a target to get a sugar reward — a task similar, in principle, to using tools.1 They can even recognize objects in the dark, which they have previously only seen through a window, just by feeling their shape.2 But new research shows that bumble bees aren’t just intelligent, they’re also manipulative.

Researchers at the Swiss Federal Institute of Technology Zurich (ETH Zurich) have found that worker buff-tailed bumble bees (Bombus terrestris, one of the species managed for commercial pollination) manipulate plants to flower earlier than they normally would, particularly when the bees are starved for pollen.3 The bees do this by biting holes in the plants’ leaves, which stimulates the plants to bloom early and gives the desperate bees their pollen.

“It always surprises me how, even in systems we think we know so much about, there are still so many things we don’t know,” says Dr. Consuelo De Moraes, the study’s senior author. “That’s one of the most fascinating things about science, there is still so much to discover.”

Initially, the researchers were working on a project on butterfly egg laying, but while some students were working in the greenhouse, which was also home to a bumble bee colony, they noticed this bizarre behavior. They saw bumble bees landing on leaves and perforating them, but why? They called in Harriet Lambert, a PhD student and one of the lead authors on the study, to take a look.

“Initially, I think we all assumed that something was wrong with the colony,” Lambert says. They purchase bumble bee colonies from commercial suppliers, and the colonies could have varying degrees of health. But over time, they observed this behavior in colonies they obtained from many different sources, so it wasn’t just an aberration.

“It soon became clear that under the right conditions, every hive we worked with, both inside and outside, would damage the leaves of plants,” says Lambert. Either the bees were acquiring some kind of resource from the leaves, or something else was going on.

But unlike some other bee species, bumble bees do not use pieces of leaves in their nests. And while some plants secrete nectar from their leaves or stems through extra-floral nectaries, these particular plants (tomato and mustard varieties) do not. When Lambert combed through the literature to learn more about this curious leaf-perforating habit, though, she found nothing — the behavior had not yet been described. They had accidentally stumbled on something new.

And when the researchers followed what happened to bee-bitten and unbitten plants over time, they found that the tomato plants flowered a full month earlier than their unassaulted counterparts, corresponding to 38 days after the bees perforated their leaves. The mustard plants responded similarly, flowering about two weeks early, or 17 days after biting.

“There are a lot of studies on pathogens and mostly abiotic stresses, like drought, inducing flowering,” says De Moraes. “That’s where the hypothesis came from.” Not very much is known about insect herbivores changing the timing of flowering, she explains, but that was the first thing that came to her mind: Maybe the bees were manipulating the plants to flower early.

Many plants have evolved to accelerate their flowering time when they become stressed. This is because when growing conditions are poor, their future is uncertain, often making it more advantageous for the plant to invest in reproduction sooner rather than later. “It’s possible that the bees are tapping into those internal signalling mechanisms,” says Dr. Mark Mescher, one of the study’s coauthors, making the plants flower earlier than they normally would.

And it looks like there is something uniquely special about the bite of a bee — simply punching holes in the leaves with tweezers had very little effect. When tomato plants were mechanically damaged by humans, they flowered only 5 days earlier than normal, compared to 30 days when bitten by bees. But it’s still not clear why.

“We tried to replicate the bee’s damage as closely as possible,” says De Moraes, including doing the mechanical damage at the same time the bees were damaging other plants, and matching the sizes and shapes of the holes. “We tried our best, but it’s possible that we’re not replicating the damage in the same way.”

The other, more interesting possibility is that there is something in the bees’ saliva — a cue — that the plant perceives, and it’s this cue that induces flowering. Indeed, the bees not only bite the leaf, creating a hole — they also can be seen sticking their proboscis through the hole, apparently licking it. What that cue is, if it does exist, remains unknown.

The initial observations of leaf biting were made in a greenhouse, and subsequently confirmed to be linked to pollen deprivation in laboratory experiments. But in these kinds of artificial conditions, there was still the possibility that this leaf-punching was just a maladaptive behavior induced by living in a more stressful environment. The next step, therefore, was to take the experiments outside.

Over the course of two years, the researchers conducted rooftop experiments to see if the bees would still do the behavior when they were under more natural conditions. In the first experiment, they showed that bumble bees do indeed perform the behavior even when they have broader foraging options, and that their propensity to punch leaf holes on experimental plants declines as the surrounding forage sources increase during the spring.

In another experiment, bumble bee colonies were placed on two different rooftops — at both sites, the bees had patches of flowerless plants nearby, but on one roof, there was also a wildflower garden. As predicted, the bees without the wildflower garden punched more holes than the bees with access to flowers. And when the garden was cut down, hole punching ensued on that rooftop, too.

These outdoor observations also revealed that leaf-biting behavior isn’t restricted to buff-tailed bumble bees. During the course of their studies, the researchers also saw wild red-tailed and white-tailed bumble bees (B. lapidarius and B. lucorum, respectively) punching leaf holes. Honey bees also visited the study sites, but never even landed on a flowerless plant — they only visited flowers. This hole-punching behavior may have evolved in bumble bees, specifically, because they are more vulnerable to resource scarcity.

Unlike honey bees, who pack away all the pollen and nectar they can for hard times ahead, bumble bees don’t store much food. They keep some surplus nectar and pollen in wax pots within their nests, but it is  ….