When Mom said, "Don't pick up hitchhikers," she should have listened to her own advice. Female cabbage butterflies unintentionally take parasitic wasps along for a ride after mating. The consequence is gruesome. But how do the egg-seeking wasps know which butterflies to latch onto? New research suggests it's all in the nose.
The cabbage butterfly (Pieris brassicae) mates far afield and flutters to a cabbage or brussels sprout plant, where she deposits her fertilized eggs. Parasitic wasps want to lay their own eggs inside of the butterfly eggs, allowing their offspring to feed off the butterfly embryos. In order to find these eggs, the wasps must hitch a ride on a pregnant butterfly.
Chemical ecologist Monika Hilker of the Freie Universität Berlin in Germany and colleagues wondered if the wasps took advantage of the mating system of the cabbage butterfly. Male butterflies give an extra gift with their sperm--a chemical anti-aphrodisiac that marks the mate as "done, taken, got this one." Could the wasps be smelling out the anti-aphrodisiac to find butterflies that are about to lay eggs?
To find out, Hilker and her team put one wasp at a time in a chamber with two butterflies. The wasps climbed atop mated females three times as often as virgin females. When the team doused virgin wasps with benzyl cyanide, the main component of the anti-aphrodisiac, wasps jumped on the perfumed virgins more than twice as often as unscented ones. The team then set up a tunnel with butterflies and wasps on one end and brussels sprouts on the other. Seven percent of the wasps successfully hitched a ride and parasitized the eggs, indicating the wasps likely use this tactic in the wild, the researchers report today in Nature. Hilker says a 7% success rate is enough to put some pressure on the butterflies.
"It's a fantastic way of finding eggs," says evolutionary biologist Mark Dowton of the University of Wollongong, Australia. Chemical ecologist Joseph Patt of the U.S. Department of Agriculture's Kika de la Garza Subtropical Agricultural Research Center in Weslaco, Texas, speculates that the seriousness of the parasitic wasp problem varies among populations, and butterflies can be tested to see if some produce anti-aphrodisiacs that are less attractive to the wasps. If so, that would lend support to the idea that parasites are a primary driver in sexual evolution, he says.