Guest post by Alun Salt
If you’re a plant, then having a pollinator you can rely on seems like a good idea. Yucca plants have a partnership with Yucca moths. Yucca plants have flowers that are specialised for Yucca moths. The payback is that Yucca moths can’t find food so easily anywhere else, because they’re now adapted to access Yucca flowers. A Yucca plant can provide food, knowing its pollen won’t be wasted by a pollinator that visits other species of plant. It sounds like a good deal, but it’s also a dead end.
When plants and pollinators become dependent, a threat to one can wipe out the other. For example, if a new blight hit Yucca plants, the Yucca moth would find its food source had disappeared, and it would starve to extinction itself. Once a species has specialised it’s very hard to dismantle adaptations and build new ones.
However, sometimes Nature can provide a species with help reversing out of a dead end. Rentsch and Leebens-Mack have an Open Access paper in AmJBot, Yucca aloifolia (Asparagaceae) opts out of an obligate pollination mutualism, that finds a change in the ecosystem could be help one species of Yucca become a generalist.
Yucca aloifolia (public domain/Flickr)
The paper is based on two species of Yucca plant, Y. filamentosa (Adam’s Needle) and Y. aloifolia (Spanish Bayonet). The species are sympatric, meaning that they live in the same areas, so they’re a good match if you want to see if one species is doing something that the other isn’t.
What Rentsch and Leebens-Mack wanted to find out was, do Yucca plants need Yucca moths to be pollinated? Can they use another insect? There’s an obvious problem with this experiment: How do you exclude just Yucca moths, while letting other insects through? Do you need hundreds of dedicated volunteers standing over the flowers, with a keen eye and a swatter?
What they did was bag the flowers in a mesh, and block access to flowers between an hour before dusk till dawn – the period when the Yucca moth would be active. They also ran some other tests. On other flowers there was no blocking, so they could be sure Yucca moths were around and feeding. On other flowers there was a complete block of pollinators, to see if the flowers were setting fruit themselves. Finally, they also looked at blocking daytime pollinators, to be sure it was the Yucca moths doing the pollinating.
What they found was that Y. filamentosa was reliant on the Yucca moth to set fruit, but that Y. aloifolia was getting pollinated during the day. So who was the unexpected visitor?
The answer was found using some fluorescent dye added to the stamens of target flowers. Apis mellifera, the European Honey Bee was caught red-handed – or rather orange-legged where the dye had been collected. Like the European bit of the name suggests, A. mellifera is not native to the USA, but was introduced by European settlers. It’s spread, and its generalist foraging can mean it pushes into settled relationships elsewhere. In the case of the Yucca plants, Y. aloifolia has a larger opening on its stigmas, and this was enough for the bees to exploit.
Rentsch and Leebens-Mack suggest that while bees explain fruit set in Y. aloifolia, other species may use other species, for example they note Lapping flies visit Y. glauca (Soapweed Yucca). So, when there’s a lot of competition among pollinators, plants might be able to adapt to take advantage of a wide range of pollinators after all – despite specialisms.
I have simplified the paper, so I recommend reading it for yourself if you’re looking for more detail, but the experiments are themselves actually simple ideas executed elegantly. The core question is does a Yucca plant need a Yucca moth, and the bagging is a simple and effective way of excluding one set of pollinators in favour of another.
It also neatly questions something that “everybody knows”. Everybody knows that Yucca plants are obligate mutualists with Yucca moths. Rentsch and Leebens-Mack show that’s not actually so.
J. D. Rentsch, J. Leebens-Mack, 2014, ‘Yucca aloifolia (Asparagaceae) opts out of an obligate pollination mutualism’, American Journal of Botany, vol. 101, no. 12, pp. 2062-2067 http://dx.doi.org/10.3732/ajb.1400351
About the author: When he’s not the web developer for AoB Blog, Alun Salt researches something that could be mistaken for the archaeology of science. His current research is about whether there’s such a thing as scientific heritage and if there is, how would you recognise it?
Plant Scientist 
Yuccas are widely cultivated in gardens, although I’m not sure which species or whether they set seed. Why didn’t they just look at Yuccas growing outside their native range and therefore presumably outside of the range of the moth?
Hi Susan, thats a good point. The researchers were interested in what’s going on in the Yucca’s natural environment around its natural pollinators, but now that they know that one of the two species (Y. aloifolia) isn’t so reliant on the Yucca moth, it would be interesting to study it in different places where the Yucca moth is absent.