By early June, the white flowered stalks of the yucca plant are dotting the Kansas landscape; a short species of yucca on the dry hillsides of western Kansas or the taller eastern yucca around our houses and graveyards. Few Kansans notice the small white moth inside many of those flowers nor do they know about the strange case of gambling that goes on each spring between the moth and the plant.
It was Charles Valentine Riley, who first realized there were no bees buzzing around the yucca flowers. Riley was state entomologist in Missouri and often lectured at the University of Missouri at Columbia and sometimes at Kansas State University too.
By 1876 he had figured out this moth-plant relationship. Only the yucca moth pollinated the plant; the yucca cannot produce seeds without the moth. And the moth only lived on the yucca plant; no plant, and the yucca moths would all die. It became a textbook case of mutualism where two species totally depend on each other.
CV Riley worked out the life cycle. The white moths hide inside the flowers in the daytime and come out after dark to mate and lay a few eggs at the base of each flower they pollinate. The yucca seed pods then grow through the summer. The little larvae eat some of the seeds in the pod before dropping to the ground in the fall. And there were plenty of seeds left over to produce young yucca plants. CV Riley moved on to become national entomologist in 1878, and this classic “just so” story was studied by most biology students.
A century later we began to ask more questions. One problem was: what keeps the moth from “cheating”? Female moths usually lay only 2 or 3 eggs per flower, but the pod that develops has enough seeds to feed a dozen moths. So why doesn’t a moth “cheat” and lay 10 or 12 eggs; it would have many more young and replace those moths that only laid 2-3 eggs. Ms. Marylee Ramsey, an ESU masters student who went on to become a high school biology teacher in Goddard, Kansas, decided to tackle the problem. She marked moths and counted emergence holes in the pods and found that some moths did try to cheat, laying up to eleven eggs per flower.
Why didn’t these cheaters eventually win? How could a yucca plant defend itself against the super moths? Ramsey had enough data to guess, and researchers in Colorado later confirmed, that the yucca plant forced the moths to gamble. The yucca randomly drops most of its flowers, perhaps 90 percent. The moths have no way of knowing which 10 percent will develop into seed pods if pollinated. Therefore the moths are forced to place bets. Like a casino gambler, the moth can place a few bets (that is a few eggs) on a lot of flowers, and probably have a few winners. Or it can place many bets (lots of eggs) in just a few flowers, and risk losing them all. That is probably how the plant controls the moth.
Now who would have expected all that gambling to be going on in Kansas.
If you want to learn more about the yucca plant and yucca moth, this issue is available free upon request from the Department of Biology at Emporia State University. For the Kansas School Naturalist, this is John Richard Schrock.