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Mimicry based on attractive odors -- or scary ones

Bolas spiders eat male moths, luring them within range of their bolas (a blob of glue on a short throwing thread) by releasing odors similar to the sex pheromones made by female moths.(1) Some orchids are pollinated mainly by male insects, again luring them with pheromones. You might think the males would learn from their first unsatisfying encounter and switch to a different flower species, rather than taking the pollen from the deceptive orchid to another flower of the same species, but the female-mimic orchids haven't gone extinct, so apparently not.

Some insects (usually those that often have close relatives nearby) make volatile chemical alarm signals. Some wild potato plants scare aphids away by releasing odors that mimic the aphid alarm signal. To cut costs, they package the key chemicals in glandular hairs (trichomes) on their leaves, which break and release the odors only when an insect lands on the leaf.(2)

Some of the odors plants make under insect attack can attract rather than repel insects, particularly wasps that attack plant-eating insects, eating them or laying eggs in their bodies.(3) I've often wondered how this apparent cooperation between plant and wasp evolved. There wouldn't be any selective advantage to the plant from producing these odors until the wasps had evolved an interest in them. So I'm guessing that the first evolutionary step was more like eavesdropping than signaling: wasps that were attracted to the smell of wounded plants found more prey and had more offspring. Then, perhaps, plants evolved to produce more of those odors, luring wasp "bodyguards" to themselves rather than their faint-smelling competitors. Plant alarm signals that attract beneficial wasps can also attract plant pests, however, such as the Colorado potato beetle.(4)

A paper published last year shows that some orchids pollinated by wasps lure them with similar plant alarm signals, falsely promising prey insects rather than females.(5) This week, the same group reported in Current Biology that an "Orchid Mimics Honey Bee Alarm Pheromone in Order to Attract Hornets for Pollination."(6) A wounded bee is an easy meal for a hornet.

The survival of this orchid species, like that of other orchids relying on false signals, depends on their pollinators being fooled at least twice, once by the pollen donor and one by the recipient. Popular insect game show: Are You Smarter than an Orchid? Actually, that's a bit unfair. Individual orchids are presumably less intelligent than individual insects, but orchids are using DNA-programmed strategies that have been thoroughly tested by natural selection. The game should really be called: How Do Your Limited Intelligence and Limited Life Experience Stack Up Against Evolution's Time-Tested Tricks? We humans could ask ourselves the same question.

References

1. Stowe, M. K., Turlings, T. C. J., Loughrin, J. H., Lewis, W. J. & Tumlinson, J. H. The chemistry of eavesdropping, alarm, and deceit. Proc. Natl. Acad. Sci. USA 92, 23-28 (1995).
2. Gibson, R. W. & Pickett, J. A. Wild potato repels aphids by release of aphid alarm pheromone. Nature 302, 608-609 (1983).
3. Turlings, T. C. J. et al. How caterpillar-damaged plants protect themselves by attracting parasitic wasps. Proc. Natl. Acad. Sci. USA 92, 4169-4174 (1995).
4. Bolter, C. J., Dicke, M., van Loon, J. J. A., Visser, J. H. & Posthumus, M. A. Attraction of Colorado potato beetle to herbivore-damaged plants during herbivory and after its termination. J. Chem. Ecol. 23, 1003-1023 (1997).
5. Brodmann, J. et al. Orchids mimic green-leaf volatiles to attract prey-hunting wasps for pollination. Current Biology 18, 740-744 (2008).
6. Brodmann, J. et al. Orchid Mimics Honey Bee Alarm Pheromone in Order to Attract Hornets for Pollination. Current Biology 19, 1-5 (2009).

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