'Pollen Thief' Bees Could Threaten Plants

It's no secret that the world desperately needs bees. With worrying declines around the globe, their importance in agriculture and forest management is as obvious as ever. However, new research has found that, worryingly, some bees can cheat the system -- stealing pollen without pollinating plants in return.

Traditionally, bees do two things when they visit a flower. They hunker down and suck up as much sweet nectar (used for energy and honey production) as they can, and they also harvest pollen.

You average grade-schooler might tell you that this second part is accidental. Bees kind-of just brush against a flower's anther while diving for nectar and then unknowingly deliver that pollen to a stigma, which eventually leads to seed fertilization.

However, while this is partially what occurs, bees' bodies are also adapted to pick up large amounts of pollen, which are eventually brought back to the hive. There, the pollen is collected and processed into a substance called bee-bread. This bread, packed full of proteins and energy, is then fed to larvae destined to become strong workers (sterile females).

In this way, the more pollen collected, the better. Researchers have found that if bees had their way, plants would be stripped of all their nutritious pollen, leaving nothing for the plants to reproduce. So how do flowers keep their 'helpers' in check? (Scroll to read on...)

"It's a co-evolutionary arms race between plants and bees," Mario Vallejo-Marin, an evolutionary biologist at the University of Stirling, explained in a statement. "Some plants have fortified structures called anthers, where pollen is locked up behind a thick wall. The only way to open these 'pollen vaults' is through small pores at the tips."

"However," he added, "some species of bees, such as bumblebees, have adapted to produce high-frequency vibrations to counteract this and get at the pollen, a process known as buzz-pollination."

Looking at a buzz-pollinated plant called buffalo-bur, Vallejo-Marin and his colleagues found that "more than 80 percent of bees visiting its flowers collected pollen but failed to contact the female floral parts, therefore contributing little, if anything, to seed production."

In this way, the buffalo-burs' bees were cheating in a mutual relationship, taking pollen and nectar while leaving their plant partners stripped of reproduction means. It remains unclear how negatively this cheating actually impacts the plants, but Vallejo-Marin worries it's a worsening trend. (Scroll to read on...)

As described in a study recently published in the journal Arthropod-Plant Interactions, the researcher and colleagues from the University of Puebla in Mexico did find that most cheating bee species are smaller, stay longer at each flower, and visit few flowers in a run. Legitimate pollinators tend to be larger, buzzing bees which visit many flowers, spreading genetic diversity over long distances.

"The bee's size is the key determinant of whether it will be a pollen thief," Lislie Solis-Montero, who was involved in the study, said. "The flowers of buffalo-bur work as a 'lock-and-key' mechanism to ensure that the bee collects and deposits pollen with the right body parts. Bees which are too small fail to contact the female organs, while still taking away pollen grains to feed their larvae." (Scroll to read on...)

Intriguingly, this means that the European honeybee - a major player in the EU battle against pesticides - is actually a prevalent thief. This, the researchers warn, could mean there will be major repercussions if these bees are introduced into areas where they can out-compete native pollinators.

Vallejo-Marin  added that the "decline of natural populations of bees around the world, combined with the expansion of non-native pollinators, could have important repercussions for the evolutionary future of specialized, buzz-pollinated plants."

It's important food for thought as experts struggle to save pollinators across the world. Many want to save bees because they are useful, but if some species are cheating their way out of this use, should they be ignored?

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