May 4, 2000 Researchers document the first case of parasitic insects cooperating to mimic the appearance and scent of their female hosts
SAN FRANCISCO, CA — The Kelso Dunes, located halfway between Barstow and Las Vegas in the Mojave National Preserve, is an unlikely setting for a soap opera. But the springtime behavior of blister beetles in the American Southwest puts the daily machinations of television soap stars to shame.
In what researchers say is a “remarkable mode of host-finding,” newborn blister beetle larvae of the species Meloe franciscanus mimic female bees as part of a three-step strategy to infiltrate and parasitize the bee’s nest. Blister beetles pull off their charade much the same way human performers create Chinese parade dragons, with individuals working cooperatively to form the illusion of a single animal. This is the first reported instance of parasitic larvae cooperating to mimic female host species, report San Francisco State University scientists in the May 4 issue of NATURE.
Upon hatching from their sandy burrow, hundreds of these dark-orange beetle larvae, called triungulins, navigate their way to the tip of the nearest plant stem, where they form wriggling masses, or aggregations, that roughly resemble—and likely smell like—female Habropoda pallida bees, says lead author Dr. John Hafernik, who with co-author Leslie Saul-Gershenz documented this behavior during the springs of 1992 and 1999 from the California State University Desert Studies Center.
According to the researchers, once the triungulin mass successfully lures a male bee into pseudocopulation, the larvae use pincher-like limbs to attach themselves to the underside of the duped bee. The male then deposits the larvae on to female bees during further mating attempts, a process called venereal transmission.
“By first attaching to a male bee, triungulins have access to multiple females and, subsequently, the multiple nests of each female,” says Saul-Gershenz.
Female bees then unwittingly transport these larvae back to their nests while provisioning them with pollen and nectar for their own eggs. “Once inside, the larvae parasitize the nest,” says Hafernik, chair of SF State’s Biology Department. “The provisions that would have produced a bee produce a beetle instead. Bee eggs already in the nest cell are likely eaten by the larvae as well.”
Hafernik adds that “while cooperative behavior is common among highly social insects, such as bees and ants, it has never been reported in blister beetles. What’s more, until now, no other insect has been known to use cooperative behavior to mimic other species.”
Blister beetles are named for their defensive mechanism of releasing a drop of bright-orange blood laced with the chemical cantharidin, which causes severe pain and blistering upon contact with the skin. This substance is also used in the dubious aphrodisiac “Spanish Fly,” which, when ingested, causes severe burning in the urinary tract.
The researchers believe the aggregations lure males into copulation through a combination of visual and olfactory cues. They noted that triungulin masses position themselves on vegetation much like female bees—perched on the top of a plant stem—and males approach and land on masses and females in the same way. To test for the possibility of olfactory cues, the researchers placed model aggregations near live aggregations that were formed or in the process of forming.
“The male bees ignored the models completely but hovered or tried to land on groups of triungulins even before they were formed into a bee-like mass,” says Saul-Gershenz, who is currently studying the chemical cues. “The triungulins are likely emitting a bee-like pheromone to attract males, and another chemical cue to form aggregations.”
During their field survey, the researchers observed the life cycle of 22 masses, noting 98 instances of bees hovering within a few centimeters of a triungulin aggregation and nine instances where bees landed on aggregations.
Researchers have long known that many blister beetle species parasitize bee nests, but never made the connection between triungulin aggregations—first reported in 1895—and nest finding. Aggressive mimicry is not new, but has only been associated with individuals of a species. The Ophrys orchid mimics the appearance and scent of female bees to attract pollinators to its flower, and female bolas spiders attract prey by mimicking the female sex pheromone of the armyworm.
Dr. Ronald McGinley of the Smithsonian Institution’s Department of Entomology says the findings raise “exciting questions” for future research. “How do newborn beetle larvae coordinate a collective pilgrimage to individual grass stems? And why is this particular bee species attracted to ‘larval clumps’? Is the attraction visual, chemical or both? We can look forward to these researchers revealing some of these answers in future work.”
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