I’m always fascinated by coevolution, and especially by coevolved mutualisms, but in the plant-pollinator world, the tight, specific interactions that have typically characterized coevolution are the exception rather than the rule. Indeed, most plant-pollinator interactions are generalized to a great extent.
That’s why those few exceptions, the tight specialized interactions, are so interesting. I’ve written before about Darwin’s moth and the Rediviva bees in Africa; today I thought I’d talk about another classic mutualistic interaction, the Yucca and its moths.
Yucca is a genus of plants in the asparagus family (Asparagaceae) with tough, sharp leaves and distinctive stalks of white flowers. The Yucca is only pollinated by Yucca moths.
The Yucca moth is a common name for many species in three genera in the family Prodoxidae. Two of the three genera have an obligate mutualism with the Yucca. And to me they are fascinating because they perform what is known as “active pollination”. Instead of passively distributing pollen grains carried loosely on their bodies, they have specialized mouthparts (I almost wrote “mothparts”, which would not have been so bad in retrospect), to pick up pollen and move it to the stigma.
But as usual in nature, the story is a bit more complicated than a simple mutualism. In fact, the interaction between these moths and plants varies from predation to mutualism.
The larvae of the moths feed exclusively on the seeds of the Yucca, but there is a constraint on the number of seeds they can eat because if they eat too many, the Yucca population will decline and, inherently, so will the population of the Yucca moth. Naturally, there are cheaters on both sides of the game, and it is balanced by selection over time.