Flowers produce chemicals to keep greedy bees at bay: Study

Wednesday, February 2, 2011

WASHINGTON - It is no secret that many flowering plants have evolved structures that prevent pollinators such as bees from taking too much pollen.

Now, Swiss researchers have produced experimental evidence that flowering plants might also use chemical defences to protect their pollen from some bees.

Claudio Sedivy and colleagues from ETH Zurich collected pollen from four plant species - buttercup, viper’s bugloss, wild mustard and tansy - using an ingenious method.

Instead of collecting pollen from plants, the researchers let bees do the legwork, harvesting pollen from the nests of specialist bees that only feed on one type of plant.

They then fed the pollen to larvae of two closely related generalist species of mason bee (Osmia bicornis and Osmia cornuta) to see how well the larvae developed.

They found major differences in the ability of these generalist mason bees to develop on pollen from the same plant species.

“While the larvae of Osmia cornuta were able to develop on viper’s bugloss pollen, more than 90 % died within days on buttercup pollen,” said Sedivy.

“Amazingly, the situation was exactly the opposite with the larvae of Osmia bicornis. And both bee species performed well on wild mustard pollen, while neither managed to develop on tansy pollen,” he said.

“As far as we know, this is the first clear experimental evidence that bees need physiological adaptations to cope with the unfavourable chemical properties of certain pollen,” he added.

According to the researchers, plants would have good reason to protect themselves. Bees need enormous amounts of pollen to feed their young, pollen that could otherwise be used by the plants for pollination.

The pollen of up to several hundred flowers is needed to rear one single larva, and bees are very efficient gatherers of pollen, often taking 70-90% of a flower’s pollen in one visit, they said.

“Bees and plants have conflicting interests when it comes to pollen. While most plants offer nectar to visiting insects as bait for insects to transport the pollen from flower to flower, bees are very efficient pollen collectors. Therefore, plants have evolved a great variety of morphological adaptations to impede bees from depleting all their pollen,” said Sedivy.

This study provides strong evidence that pollen chemistry might be at least as important as flower morphology to constrain pollen loss to bees,” he added.

The results will be published next week in the British Ecological Society’s journal Functional Ecology. (ANI)

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