The life of a bee isn’t all nectar and pollen. Bees have to watch out for trickery, because some plants entice them without offering any reward. Certain orchid flowers, for example, look and smell like female bees, prompting males to pollinate them when they try to mate. “Deception is everywhere,” says May Berenbaum, an entomologist at the University of Illinois Urbana–Champaign.
A newly discovered form of deception is more convoluted yet: The larvae of blister beetles emit a mix of volatile scents that resemble a flower, attracting bees. The beetles then latch onto the bee, hitch a ride to its nest, and eat the eggs. This luring strategy, described last week in a preprint on bioRxiv, is the first known example of an animal mimicking the smell of a flower. “It’s a beautiful study,” says Consuelo De Moraes, a chemical ecologist at ETH Zürich who was not involved. “It’s amazing to me how sophisticated these interactions are.”
Ryan Alam, an organic chemist at the Max Planck Institute for Chemical Ecology, never imagined he’d apply his training to studying insects—until he became fascinated by blister beetles. For self-defense, this family of flightless beetles oozes the toxin cantharidin, which was used in antiquity as medicine and also as an aphrodisiac in medieval Europe. One North American species attracts pollinators by concocting and emitting imitation sex pheromones.
Alam was intrigued to learn that this species and others in Europe climb en masse to the tops of grass and plant stems. To better understand this behavior, he decided to investigate whether any smells emanate from the larvae of the 3-centimer-long European black oil beetle (Meloe proscarabaeus).
On walks through meadows near the institute in Jena, Germany, Alam collected about 40 of the black and blue–tinted adult beetles and brought them to a greenhouse cage with sandy soil planted with wheatgrass and other species they like to eat. The beetles mated, burrowed into the soil, and laid thousands of eggs. After they hatched, the larvae climbed the grass stems. Working with gloves to avoid blisters from cantharidin, Alam collected larvae and ground them up. Then he put the puree into an instrument called a gas chromatography-mass spectrometer that can reveal the chemicals in a complex mixture by plotting them on a graph called a chromatogram.
The analysis showed a mix of relatively lightweight molecules called monoterpenoids, rare in insects but common in plants. Several of the eight most abundant molecules, such as linalool oxide and lilac aldehyde, are often found in flowers and are known to attract pollinators. “This was totally unexpected for us,” says the study’s senior author, Max Planck biochemist Tobias Köllner. “We saw the chromatogram and thought, ‘This is a flower, not an insect.’”
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The next step was to test the effect on bees. Alam set up a Y-shaped passageway where they could choose a scent from either beetle larvae or wheatgrass, used as a control. Red mason bees (Osmia bicornis) and bare-saddled cellophane bees (Colletes similis) both preferred the scent of linalool oxide to the control. The same was true of other ingredients in the larvae puree, and of the mix as a whole.
Unlike deceptive imitations of sexual pheromones, the larvae’s flowerlike scent attracts both male and female bees. That could benefit the beetles because only female solitary bees return to their nests; hitchhiking onto a female would avoid the need to transfer from a male to female. “In effect, they cut out the middleman,” Alam says.
The beetles have also cut out the need for flowers, Köllner notes. The larvae aggregate—perhaps following the scent of their peers—on the top of grass stems in early spring, before real flowers have bloomed.
Alam and his colleagues identified the biochemical pathways larvae use to make the flowerlike chemicals. The process seems to rely on the same kinds of enzymes that plants employ to make their flower scents, Köllner notes—an example of convergent evolution.
The results are “both surprising and conceptually exciting,” says Ko Mochizuki, an ecologist at the University of Tokyo. The new work “raises the intriguing question of how widespread chemical mimicry of this kind may be, and how often it may have gone unnoticed.”
Chemical forms of deception are more rarely uncovered than visual mimicry, in part because humans are better at seeing than smelling, Berenbaum says. But she’s confident more examples will show up with the right effort. “It takes really careful observation and intuition to figure out some of these very strange relationships.”