26 February 2009

Tricking wasps with flower porn

ResearchBlogging.orgOrchids are lovely flowers and if you think they’re just a bit sexy, you're not alone. In fact, for some wasps, they’re very sexy. As sexy as any female wasp.

It’s well known that flowers use many methods to attract insect pollinators. Many flowers have nectar. Some smell horrible, and are pollinated by flies and other insects that tend to feed off dead things. And some look like female insects, and deposit pollen on males when they come into investigate. What this paper by Anne Gaskett and colleagues show is that the insects are fooled more completely than anyone previously thought, to the point where they not only copulate, but ejaculate. This paper came out several months ago, and got some attention in the news media (e.g., here, here, here, here).

Showing that males have ejaculated is fairly simple, and actually a relatively small part of this short paper. There are some more interesting issues here.

These cases of mistaken identities are costly to the insect. Even setting aside the major finding of this paper, there’s simply the cost of the males wasting time. So you would anticipate that over time, male wasps will “push back” and try to avoid being tricked. There is an experiment that tries to show that wasps learn to avoid copulating with these orchids, but the experiment has several odd features.

The researchers first observed over 100 wasps in the field, and noted how many approached the orchids, copulated with them, and so on. They have no way of knowing the animals’ experience in the field. They then brought 30 of those wasps into the lab, presented them with a fresh orchid, and observed their response. This mix of field and lab observations raises questions: What if the effects are due to the handling or the differences between artificial and wild conditions?

More puzzling still, the researchers then took only 14 of the wasps for a third trial, giving all the remaining wasps fresh orchids again. It’s not at all clear why the sample size changed for the second exposure in the lab.

Their results showed that wasps became less likely to copulate with an orchid after they had already been exposed to one, suggesting that animals were learning to avoid the orchids. But the interpretation is problematic here, especially considering that to be pollinated, the same wasp must visit at least two orchids. The wasps in their experiment dropped from about 90% copulating with the orchid to about 30% copulating after only one (known) encounter with a flower – which seems a pretty precipitous drop-off is the goal is to get one wasp to visit twice. That said, loss may be tolerable if those males that do visit a second orchid have very high likelihood of pollinating the flower.

Interestingly, the paper also shows that most of the orchids that engage in this sort of deception are pollinated by haplodiploid insects. Without going into detail, haplodiploidy means that males in these species are laid from unfertilized eggs; females are laid from fertilized eggs. So, strangely, orchids, by “cutting in” on the female insects’ mating possibilities, the orchids may be ensuring a greater number of males in the wasp population that can serve as pollinators. If female wasps don't mate (because all the males are out satiating themselves with orchids), all the females can do is... lay eggs for more males.

It’s a hothouse of treachery and manipulation out there, I tell ya...


A. C. Gaskett, C. G. Winnick, M. E. Herberstein (2008). Orchid Sexual Deceit Provokes Ejaculation The American Naturalist, 171 (6) DOI: 10.1086/587532

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