I’m just back from a great week in the Nebraska Sandhills. I saw an amazing array of wildlife, invertebrates, plants and landscapes. Of the many wildflowers in bloom this week, none punctuated the hills more beautifully than yucca (aka soapweed). As always, nearly every yucca stem with actively blooming flowers hosted an abundance of yucca moths, the only pollinator of yucca plants. If you aren’t familiar with the incredible relationship between yucca moths and yucca plants, you can read about it in a previous post.
It sounds too good to be true; two species helping each other survive for millions of years – each getting as much as they give.
For more than 40 million years there has been a relationship between yucca plants and yucca moths. It’s a particularly important one because neither the yucca or the moth can survive without the other. The moth’s larvae depend on the seeds of the yucca plant for food, and the yucca plant can only be pollinated by the yucca moth.
In the central United States, soapweed yucca (Yucca glauca)- is pollinated by a moth known as Tegeticulla yuccasella. Each spring, adult moths emerge from underground cocoons and the males and females meet up with each other on yucca plants to mate. When a female is ready to lay eggs, she first goes to a yucca flower to collect pollen. Unlike most moth species, yucca moths have two short tentacles near their mouth that they use to scrape pollen from the anthers of the flower. As she collects the sticky pollen, the yucca moth packs it into a ball and sticks it under her head. She then flies off to another yucca flower.
When she arrives at the second yucca flower, usually one that has very recently opened, she goes straight to the bottom to find the ovary. She opens a small hole in the ovary and lays her eggs inside. Once the eggs are laid, she scrapes a small amount of pollen from her sticky ball with her tentacles, walks to the stigma of the flower, and packs the pollen into tiny depressions within the style. She may then return to the ovary of the same flower to lay more eggs or fly to another flower. Either way, before she leaves the flower, she marks it with a pheromone (a chemical other moths can sense). The scent marker will tell later visitors that they’re not the first to reach the flower, and they will either lay fewer eggs than the first moth, or none at all, depending upon how many moths have left their scent already. This helps moderate the number of larva that hatch within each flower, and prevents the plant from aborting the flower altogether, which it will do if too many eggs are laid.
When the eggs hatch, the larvae feed on yucca seeds within the fruit. Typically, there are more seeds than the larvae in a particular flower can eat (since the plant aborts flowers that are too heavily laden with eggs). When the larvae finish eating, they burrow out of the fruit – usually during rain events, interestingly – and burrow down into the ground to make their cocoon and wait until the next spring when the whole process plays out again.
As far as anyone knows, and it’s been studied since the 1870’s, no other species besides the moth pollinates yucca flowers. Similarly, yucca moth larvae don’t feed on anything other than yucca seeds. So each species depends upon each other for survival, and both benefit from the relationship.
It’s a beautiful thing.
If you’re interested in a much more detailed review of yucca moths, you might like this article by Olle Pellmyr.
If you want to read about some additional interelationships with yucca, the moth, and other insects, you might be interested in this article by Laura Hebert.
– thank you to Mary Ann Feist for help with this post.