By mimicking the look and smell of a female wasp, an Australian orchid lures lusty male wasps to pollinate it. This bit of treachery doesn't just deprive male wasps of mating opportunities--a new study shows it's downright dangerous for females, who depend on males to bring them food. But although the orchids currently have the upper hand in this evolutionary arms race, biologists are already speculating on what tricks the wasps might use to get even.
Neozeleboria cryptoides females are in some respects a helpless lot. Wingless, they depend on males to feed, copulate with, and return them to a suitable site to lay their eggs. If males cannot discriminate between chemical cues that lead them to female wasps and those emitted by orchids, a female has to use up precious energy to ratchet up her pheromone release--or worse, starve to death awaiting a male to come to her rescue.
Unfortunately for the females, the orchids have the male wasps completely fooled. In a wasp equivalent of a blind taste test, orchids and female wasps were concealed in vented opaque vessels. Monitoring the number of male wasp landings on the chambers in a 5-minute period, graduate student Bob Wong of Australian National University in Canberra, Australia, and biologist Florian Schiestl of the Geobotanical Institute in Zürich, Switzerland, found that male wasps could not discriminate between orchids and females. Although males eventually learned to avoid dense orchid patches, orchids long outlive their wasp pollinators and swindle new generations of naïve male wasps each season. As a result, Wong and Schiestl report online 3 July in the Proceedings of the Royal Society B, female wasps had difficulty attracting mates in the presence of orchids.
Because male wasps duped by the orchids are likely to leave fewer offspring, evolution would favor males who somehow learn to distinguish flowers from females. But the new study suggests that female wasps could also participate in a counterattack, says biologist John Alcock of Arizona State University in Tempe. "One would also expect females to evolve ways to reduce the costs imposed on them by the orchids." For example, if females were repelled by the smell of orchids, they might walk away from orchid patches before releasing their pheromones.
Florian Schiestl's Web site
The Pheromone Group in Lund, Sweden-- Group of 15 scientists working on chemical communication
Review of Chemical Mimicry in Pollination--contains good background plus references for further reading