Volume 41, Number 2,
June 1995:
The Yucca Plant and
the Yucca Moth

Text-only version

ISSUE HOME PAGE

ABOUT THIS ISSUE
- about KSN
- about the authors

IN THIS ISSUE
- introduction
- mutualism
- coevolution
- a "transparent" system
- yucca moth pollination
- male and female yucca moths
- mark and recapture
- C.V. Riley
- the yucca plant
- yucca flowers
- yucca products
- yucca pods and larval moths
- bailing out of the pods
- old pods
- what we do not know
- what prevents a cheater?
- how did the yucca and yucca moth relationship evolve?
- solving problems
- for additional information

SLIDESHOW
- View all images in this issue.

The Yucca Plant and the Yucca Moth
by Marylee Ramsay and John Richard Schrock

YUCCA PODS AND LARVAL MOTHS
By mid-June, the white yucca flowers have all dropped away. Many yucca stalks are barren, indicating that there were no yucca moths to present in the locality. But green yucca pods during the summer are a sure sign the moths were busy in the flowers, and most pods will contain a few of their larvae.

Six stacks of black coin-like seeds form rows inside the pods. You can split the green pods into thirds, each carpel holding two rows or locules of seeds. The small gray-to-pinkish yucca moth caterpillars may be feeding anywhere in the core of the seed rows. If they burrow toward the end and run out of food, they bore sideways into another locule or across a carpal wall. Often an outside constriction in the pod reveals their internal consumption of seeds.

Germination of yucca seeds reveals that nearly 100 percent are viable, so every seed eaten or left intact was/is a potential yucca plant. Figure x shows that while most pods hold two or three larvae, it is possible to find no larvae at all, or as many as six to twelve! Since there must be pollination to produce a pod, it is possible that egg laying was disturbed by storms, etc. or that some eggs fail to hatch and young larvae die. But the higher numbers of larvae also suggest that the moth has the potential to eat a larger share of the seeds without some control by the plant.


Figure 4. Total number of emergence holes according to the number of holes per pod sampled (from Ramsay).

BAILING OUT OF THE PODS
When the larvae are mature, they excavate an exit burrow to the surface of the pod, although they may continue feeding for a time. When they completely chew through the surface, they leave an exit hole or scar. Riley describes the larvae as descending to the surface on a silken thread, a common ability for many caterpillars. Davis reports that they just drop to the ground. In either case, the larvae begin crawling for some time and eventually burrow into the soil to form a cocoon and eventually pupate. It is assumed that they emerge the next year but Riley observed one larvae unchanged and still living in a cocoon after two and a half years.

OLD PODS
By August, the green pods begin to blacken and dry out. The caterpillars have long since left and the pods continue to dry and split. Over the next year, the shells act as "pepper shakers," rustling and sporadically scattering the black seeds. The wind that dislodges the seeds also increases the chances the flattened discs are carried some distance.

Figure 5. Yucca pods at end of summer.

WHAT WE DO NOT KNOW
In the fall when the larvae emerge from the pods and drop to the ground, where do the larvae go to pupate (change into an adult moth)? Do they pupate in the rosette of yucca leaves or away from the yucca plant? Do moths emerge the very next spring or do they emerge after two winters?

How does a female moth know if she is revisiting a flower she has already pollinated, or if it has been pollinated by another moth? When she lays eggs, is she leaving a pheromone, a scent that signals other moths that this flower has been "taken"?