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

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

C. V. RILEY

The unusual role of the yucca moth was discovered in 1876 by a Missouri entomologist named C. V. Riley. Charles Valentine Riley was an enthusiastic naturalist and writer with keen powers of observation in the field. Early in his career he was a writer for the Prairie Farmer, and later started the magazines American Entomologist and Insect Life. He became State Entomologist of Missouri from 1868 to 1877 and issued a series of state reports that were far ahead of his time for accuracy and insight into insects and pest problems. He also helped the French overcome problems with a grapevine insect that was ravaging European vineyards, and headed a Commission to study an outbreak of the Rocky Mountain Locust in Colorado and western Kansas. In those days, Riley lectured often at what are now the University of Missouri--Columbia, and Kansas State University, Kansas. He was appointed national entomologist (technically "Chief in Entomology") in the United States Department of Agriculture in 1878.

In his 13 years as national entomologist, Riley was a controversial man. Described as "restless", "ambitious", and "a great schemer", he built his division into what became a permanent Bureau of Entomology, today's Entomological Research Service. He strove constantly to make his work seem more important, and he made many political enemies both in the U.S.D.A. and in Congress. Yet his stature as a great entomologist was based on major entomological work both pure and applied.

Riley, with the assistance of entomologists Otto Lugger and Theodore Pergande, studied the life cycles of the thirteen and seventeen-year cicadas, the life history of blister beetles with unique larval forms, and the strange relationships of figs and fig wasps. Riley very much enjoyed travelling; however, his political enemies were able to curtail official travelling as "extravagance." In 1888, unable to go abroad himself, Riley sent Albert Koebele to an Australian entomological conference. The real mission however was to slip away to collect Australian pests of the cottony-cushion scale, an insect that was devastating citrus crops in California. Koebele brought back the Vedalia beetle which, after being cultured by Coquillet, proved to be an excellent predator and nearly miraculous biological control of the scale.

Riley's insight into the yucca moth as a pollinator required tedious field work, including much nighttime observation. This was conducted during his earlier days in Missouri, and mostly on the cultivated yuccas. Everyone knew bees pollinated many flowers, but the role of the small white moths sometimes found in the yucca flower was by no means obvious.

When Riley published the details of the yucca moth-yucca plant relationship in 1892, he had worked out its basic life history. He had answered the questions that could be asked at that time. Evolution by natural selection had been elaborated by Charles Darwin and the explanatory power of this theory was of great use in understanding insect diversity. Riley was an avid advocate of evolution and travelled to England to meet Darwin. Darwin was particularly interested in Riley's understanding of insect mimicry, a biological phenomenon only understood in the light of evolution. Riley also corresponded with Alfred Russell Wallace (co-discoverer of evolution by natural selection), Henry Bates (of Batesian mimicry), and Herbert Spencer.

However, the work of Gregor Mendel had not yet been recognized, and it would be over a half century before the fields of genetics and evolution developed to where additional questions in population genetics and population ecology could be asked. Riley's account of the yucca moth and yucca plant was so complete that further research only began again in the 1980s.

On September 14, 1895, three years after publishing the yucca moth relationship, C.V. Riley was speeding along on his bicycle when he hit a granite paving block that had fallen from a wagon. He broke his skull against the pavement and never recovered. He died at the age of 52, leaving behind his widow and five children. He had been a curator of insects at the United States Museum of Natural History. His 115,000 mounted insects formed the core of the U.S.N.M. collection which today is a major world research facility.