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 MOTH POLLINATION
You can watch yucca moths pollinate flowers between dusk and midnight. The female gathers pollen from the flower anthers by using her specially adapted mouthparts, called palps. She forms the sticky pollen into a ball which she carries between her tentacles and her thorax (under her "chin" so to speak). The pollen ball is then "stuffed" or "combed" into the stigma of the various flowers she visits. The stigma is the receptive tip of the female pistil. Without this process, the yucca flower will not develop into the fruit or pod with seeds.

The flower's ovary contains eggs in three chambers called carpels, and each carpel is divided into two locules. When the female moth visits the flower, she backs up to the flower base and inserts her ovipositor to lay an egg in one or more of the six locules. By the time the egg hatches into a microscopic caterpillar, the yucca will have begun to develop a pod with little seeds, just as a fertilized apple blossom develops into a little apple. Many unanswered questions center around how the female moth knows a flower has not been visited before, and how many eggs she lays in how many flowers?

MALE AND FEMALE YUCCA MOTHS
Tegeticula yuccasella is the only true yucca moth found east of the Rocky Mountains and is responsible for pollinating all eastern yuccas and many western species as well. Moths emerge in late May and early June, presumably near a yucca plant. Males may appear a little earlier than the larger females. The smaller males have a blunt tail while the females bear a sword-like ovipositor (egg-laying blade). Males and females mate inside a flower after which the male plays no part in the pollination process.

MARK AND RECAPTURE
How far do yucca moths fly at night? And how big are the moth populations? Since we can't place radio collars on little moths, and we can't be certain of collecting all the moths in a locality, scientists have devised a technique for estimating the population by sampling and marking a small portion, and then resampling the population later. Simply, if you capture ten individuals and mark and release them yesterday, and today half of the individuals you catch are marked, then the estimated total population is twenty.

Ramsay has discovered that yucca moths stay very close to their home yucca clusters, and they remain active on the plants for less than a week.

Figure 2 image unavailable
Figure 2. Total number of moths and sex ratios (male : female) per capture day.