Parasites are renowned for their deceptive dealings with their hosts, but the blister beetle takes its subterfuge to a carnal extreme. According to a new study, the beetle's larvae attract the males of a solitary desert bee using chemicals that mimic a female's pheromones. The ruse allows the parasites to hitch a ride on the males and ultimately reach a treasure-trove of pollen and nectar that surrounds the female bee's egg.
The blister beetle (Meloe franciscanus) lays its egg at the base of the Borrego milkvetch plant. Once the larvae hatch, thousands clamber to the tips of branches. The reasons behind these swarms were unknown until 2000, when ecologists John Hafernik and Leslie Saul-Gershenz of the Center for Ecosystem Survival in San Francisco, California, noticed that the aggregation of larvae resembled a female bee. When male bees approached, the tiny larvae grabbed onto their bellies. Hafernik and Saul-Gershenz hypothesized that chemical cues from the larvae could play a role in the males' initial attraction.
Indeed, these chemicals do the trick. In the current study, Saul-Gershenz and Jocelyn Millar, a chemist at the University of California, Riverside, show that mimicking a female bee's appearance is not enough to lure the males: Models of female bees, made of painted aluminum foil, failed to get the guys' attention unless they were smothered with extracts from either the beetle larvae or female bee heads. A comparison of the extracts revealed that both contain similar chemical signatures. In addition, the higher the concentration of the extract, the more the males were attracted. This suggests that the larvae "cooperate with each other to increase the dose" of the signal, says Saul-Gershenz, whose team reports its findings online this week in the Proceedings of the National Academy of Sciences.
Once they latch onto the male bees, the larvae hang on till he mates, at which point they quickly move on to the female. After mating, the female goes to her underground nest to lay a single egg, which--along with nearby stores of pollen and nectar--becomes a feast for the larvae.
The authors "have good evidence that they are really on to the chemistry of the sex attractant," says Thomas Eisner of Cornell University. Jack Shultz, a chemical ecologist at Pennsylvania State University in State College, says the evidence is compelling, and he hopes future studies would look into the impact of the beetles on the bee population.