A typical aggregation of parasitic blister beetle larvae, Meloe franciscanus, on a grass blade.

Credit: National Academy of Sciences

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SYDNEY, 12 September 2006: Ever plucked up the courage to approach a mysterious girl in a bar, only to discover she isn't a girl at all, but an aggregation of 1,000 parasitic larvae? Probably not, but such is the fate of many an amorous bee in the deserts of the south-western United States.

In a remarkable act of cooperation, the tiny larvae of the blister beetle Meloe franciscanus, known as triungulins, impersonate female bees as part of a devious campaign to parasitise their nests. While cooperative behaviors are common among social insects, they are previously unknown in insect species that use aggressive mimicry to exploit prey or hosts, according to a paper published today in the U.S. journal, Proceedings of the National Academy of Sciences.

"[The triungulins] emerge from eggs laid at the base of plants and immediately aggregate on vegetation in tight, dark-coloured masses of 120 to 2,000 individuals that visually mimic [a single female] of the host bee species in size, colour, and perching location," wrote Leslie Saul-Gershenz of the Centre for Ecosystem Survival in San Francisco, California, and Jocelyn Millar of the University of California in Riverside, co-authors of the paper.

According to the researchers, the triungulins cooperate to remain aggregated, moving as a unit up and down the stem and to different branches. They also produce a scent that mimics the sex pheromone of the female bee.

"Male bees are lured to larval aggregations, and upon contact [pseudocopulation] the beetle larvae attach to the male bees," wrote Saul-Gershenz and Millar.

They do so with extraordinary speed, the entire mass of larvae transferring to the male bee in less than two seconds, the researchers said.

The infested and no doubt disappointed bee then goes in search of a real female bee, to which the triungulins transfer during mating. Finally, the female bee transports the triungulins to her nest, where they dismount to feed on the nest's pollen and nectar - as well as the bee's egg - until fully developed.

The phenomenon in which one organism is transported by another is known as phoresy, and is common among insects. "Phoresy provides an effective means of dispersal for organisms with limited mobility, particularly in extreme environments," the authors wrote.

"The sand dune habitat represents a formidable barrier to dispersal and host location [for] the small and flightless triungulins, and, to circumvent this barrier, the triungulins have commandeered their hosts' sexual communication system."

But the triungulins don't just stand around looking pretty, waiting passively for an unsuspecting male bee to approach. According to Saul-Gershenz and Millar, slow-motion video footage shows the triungulins rapidly rearranging themselves to reach out to male bees. This way, their target finds himself suddenly closer to the 'female' than expected, and is quickly ensnared by the larvae.

Infested male bees also tend to attract other males, presumably because they smell like females, the authors said. "These male-to-male contacts, lasting one to 15 seconds, may serve to enhance triungulin dispersal."