Spring unfolds between March and June, starting with early flowers and bare trees budding. Then the trees get their leaves and a different set of flowers have their day. Now the yuccas have sent up new green stalks and it’s time for them to produce clusters of pale cream-colored flowers.
There’s a white moth that matures at the same time, ready to visit those flowers. The Yucca Moth would be easy to overlook. It is small and not brightly colored. But in nature, very cool things are happening all the time, even when we don’t notice.
On May 10th at Sheri Capehart Nature Preserve, several of us took a walk at sunset to see the yuccas and their moths and think about how their lives depend on each other. John and Grace Darling led this group of nature folks, and John told us about how the insect and the plant come together at just the right time, and how they allow each other to survive. The biological term that describes their relationship is “mutualism.”
There are many ways in which species of plants and animals interact in nature. For example, we all know that when one species benefits at the expense of another one, it is a parasite. In commensalism, one species benefits while the other one isn’t harmed but also does not benefit. As they move, the cattle stir up insects which the birds consume. The birds benefit, and the cattle really don’t care.
Maybe the most beautiful relationship is mutualism, in which each species benefits. We might think of friends or people we love, where we care for each other and bring each other joy. But that’s an example where both are humans. For the word “mutualism” to apply, the players have to be different species. They can’t see themselves in their partner or communicate like two humans. And yet, their lives revolve around each other.
Here’s how it works for moth and yucca. As the yucca produces flowers, the moths reach their adult form and fly to them. The flower is where males and females find each other and mate. Then when the female moth is ready, she gathers pollen from a flower and flies off to find a different flower where she can put pollen on the stigma, at the top of the female part of the flower. This is important, because the offspring are genetically healthier if the parent is not pollinated by its own flower.
The female moth then positions herself so that she can lay eggs in the ovary of the flower, at the base of the female structure. When the eggs hatch, the larvae feed on the developing seeds, but only a few. After this, they chew their way out of the seed pod and fall to the ground. There, they dig down and wait out the rest of the season before emerging the next year, ready to do it all again.
The moth larvae only eat yucca seeds, so the plant giving up a few of its seeds makes the moth’s survival possible. And nothing else pollinates the yucca, so if there were no moths, there would be no yuccas. Each is necessary for the other’s life.
At the preserve, we walked to a spot where Glen Rose Yuccas grow, and several were hung with clusters of blooms. After looking into a few of them, we began seeing the small, pale moths hanging onto the inside of the flower. Sometimes there was one moth, but one photo shows four of them in the same flower. A few were disturbed by our getting close and they flew off into the twilight.
A little later we walked to a second site where there were more flowering yuccas and their moths, and also some dried stalks from last year. We were looking for tunnels where the larvae had left the seed pod. I found one and John confirmed it, peeling away the outer coating and looking at the stacks of flat seeds. The larva had eaten a few and then exited, leaving the rest of the seeds to propagate more Glen Rose Yuccas.
And then it was full dark and time for us to walk the trail out of the preserve under a beautiful full moon.
(This post is a slightly modified version of what I wrote in a “Letter to Nature Folks” available at my website, Our Lives in Nature.)