At the end of every May, across the Midwest and Great Plains, yucca plants sprout a stalk of white flowers. Inside some of these flowers lurks a plain little white moth. These yucca plant and their yucca moths are the classic textbook example of "mutualism." And it was discovered over a century ago by a famous entomologist from Missouri.

MUTUALISM
Many organisms live together in relationships where one depends upon the other. Some, relationships, such as the coyote and rabbit, represent "predators" that feed upon "prey." Corn rootworms, corn stem borers, and corn earworms are, of course, all dependent upon their host corn plant. Parasites such as ticks and tapeworms feed on the surface or inside their hosts. And some creatures, such as the follicle mite, live in our eyebrow follicles without doing any harm at all. However, when two organisms have evolved a relationship where both benefit and neither is harmed, that is "mutualism."

Many mutualistic relationships have been documented. Wood termites contain one-celled protozoans that pre-digest wood cellulose in the termite gut. Without the protozoa, the termites starve with a belly full of useless wood fibers. And the protozoa need the moist environment of the termite gut, and the termite's ability to harvest and deliver wood fiber. The termite-protozoa relationship and the yucca-moth relationship are the most common and therefore classic cases of mutualism cited in textbooks.

COEVOLUTION
When an organism becomes totally dependent upon another, they may share the same biological fate. For instance, if a loon becomes extinct, a loon louse specialized to parasitize just this species will also become extinct. The protozoa and termites mentioned above have also locked their future survival together--they have "coevolved." When we discover a coevolved system, it provides scientists an opportunity to play with the system to discover how each organism is dependent upon the other, and to probe how the relationship evolved. For instance, we can clear protozoa from termites using antibiotics, observe how the microorganisms are transferred, and search for similar microorganisms in termite relatives.

But the yucca plant and yucca moth are the textbook case of coevolution. First, the yucca plant has no ability to reproduce seeds without the moth. Yuccas can propagate small rosettes around the parent plant, but these vegetative sprouts are copies of the parent. Over decades, the plant cannot move but a few feet, and there is no possibility for genetic variation. Without the moth, the whole flowering effort (expensive to the plant in energy terms) is a total waste. The only pollinator of the plant is the yucca moth; bees are not attracted and neither wind nor bees can pick up the sticky pollen.

The yucca moth is likewise dependent upon the yucca plant. There are no alternate host plants known for the yucca moth; the yucca moth caterpillars must eat yucca seeds or starve. Without the plant, the moths die off in one generation. Without the moth, the plant cannot reproduce variation or disperse; given any major climate changes, it too will go extinct. The system is therefore tightly coevolved.


Figure 1. Yucca moths hide inside yucca flowers during the daytime.

A "TRANSPARENT" SYSTEM
The yucca plant and yucca moth system is very "transparent" to study by biologists. The yucca plant remains in place; it does not rapidly disperse to new fields is not pollinated by other insects. The moth is likewise tied to the plant and does not venture into other flowers. The adult moth resides inside the yucca flowers, allowing an easy census of adult populations. And the active larval stages are all contained inside the developing yucca fruits where they can be easily sampled, and where the amount of food they eat is easily measured. Compared to wide-ranging insects with multiple host plants and hidden life cycles, the yucca-moth association is conveniently packaged for those researchers who know to ask the right questions.