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

WHAT PREVENTS A CHEATER?

The most puzzling aspect of the yucca and moth relationship is what prevents a "cheater"? A cheater is a female moth that lays two or three times the number of eggs per flower, thus producing so many larvae that all of the seeds in the pod are consumed. While this would result in two to three times the number of moth offspring in the next generation, it would also decimate the seed production of the local yucca plants. Over time, if the yucca plants failed to produce variation needed to survive changing conditions or disperse to better areas via seeds, the local population--and the cheating yucca moth--would die off. This theory for controlling greedy moths is called "group selection" because the whole group is "selected against." Yet most biologists do not believe in group selection, in part because we do not see whole communities die out from heating organisms and because other factors directly limit "cheating."

From Figure x, it is obvious that some pods do indeed carry more than two or three larvae. It is not known whether this is due to one cheating moth laying many more eggs, or from a failure to deposit a chemical signal that eggs have been laid, and other moths each lay a few more additionally. But on average, over two-thirds of the seeds escape being eaten by larvae.

What might be discouraging moths from laying more eggs per flower? Kingsolver suggests that the yucca plant produces a huge array of flowers and then drops (or "dehisces") a large


Figure 6. Interior of yucca pods showing damage by yucca moths. Plenty of black disc-like seeds remain to produce future yucca plants..

number of the flowers including many that have been pollinated. This establishes a "lottery": if the moth gambles on a few big bets--laying lots of eggs at a few flowers--they may all be dropped and she produces no young. If the moth places smaller bets (fewer eggs) on many more flowers, there is a greater chance that a few will be among the flowers that are not dropped, and will produce some more moths. In this way, a plant might force moths to lay eggs "thin and wide."