Sexually Deceptive Chemistry: Beetle larvae fake the scent of female bees

Researchers can now explain how a male bee looking for love out in the desert can be misguided enough to embrace a writhing clump of beetle larvae instead of a female bee.

SMELLS RIGHT. Blister beetle larvae cluster (left) on a plant stem and release a seductive odor. Drawn by the pheromone, a male bee suddenly finds himself covered with beetle larvae. Saul-Gershenz/PNAS

Those larvae, which grow into blister beetles, release compounds similar to the sex pheromone of the female bee, says ecologist Leslie S. Saul-Gershenz of the Center for Ecosystem Survival in San Francisco. When the deluded male touches the beetle clump, larvae rush onto his body. They use him as an air taxi to reach a female and then raid her underground nursery.

This finding of the larvae’s lure puts the immature blister beetles among the few animals known to practice chemical mimicry for hunting, say Saul-Gershenz and her coauthor Jocelyn G. Millar of the University of California, Riverside.

“All around, it’s a pretty remarkable system,” comments chemical ecologist Kenneth F. Haynes of the University of Kentucky in Lexington.

The blister beetle species Meloe franciscanus and the host bees meet in the dunes of southwestern U.S. deserts. Biologists have long known that the orange-brown larvae climb plant stems.

In 2000, Saul-Gershenz and a colleague described male bees, Habropoda pallida, visiting clumps of beetle larvae and picking up larvae (SN: 5/6/00, p. 295: Ah, my pretty, you’re…#&! a beetle pile!). The researchers proposed that the tiny, flightless larvae hitch a ride on the male bee and then transfer to a female that he encounters. Carrying the beetle larvae, the female burrows several meters deep into the sand and then lays a single egg and stores some pollen. The egg and pollen can sustain tagalong beetle larvae.

A new paper, published in the Sept. 19 Proceedings of the National Academy of Sciences, describes how the larvae fool a bee. Researchers designed a larvae-cluster model from crumpled aluminum foil dabbed with water-based paint. To human eyes, the model looked good.

When Saul-Gershenz set out models in the desert, though, unscented ones didn’t attract male bees. However, male bees paid visits to models scented with extracts from blister beetle larvae as often as they did to real beetle clumps.

Millar, a chemical ecologist, found that female-bee allure depends on blends of hydrocarbons, but the beetle larvae produce only a few of those components. The mimicry is “good enough,” says Saul-Gershenz.

Diluting the larval extract before applying it to a model reduced its seductive power. Therefore, it takes many larvae to release an effective dose, says Saul-Gershenz.

She points out that the match isn’t as close as it is in the two other well-studied examples of what scientists call aggressive chemical mimicry. Orchids emit accurate knockoffs of female-bee pheromones, thereby fooling futilely romantic male bees into transferring orchid pollen.

Adult female bolas spiders of North America also mimic insect sex pheromones, says Haynes. He and his colleagues have shown that adult female spiders of one bolas species produce the appropriate ratio of two components of a moth’s sexual attractant. Male moths that respond make up most of those spiders’ diet.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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