Yucca Moths and Yucca Plants: Discovery of “the Most Wonderful Case of Fertilisation”

The relationship between yucca plants (Yucca and Hesperoyucca spp.: Agavaceae) and yucca moths (Tegeticula and Parategeticula spp. [Lepidoptera: Prodoxidae]) is often cited as a classic example of insect–plant coevolution and, in particular, obligate mutualism (Powell 1992, Thompson 1994, Price 1996, Proctor et al. 1996, Pellmyr 2003). Female yucca moths exhibit morphological and behavioral adaptations that ensure pollination of yucca plants, which have highly modified flowers that reduce the possibility of self-pollination or passive pollen transfer by other insects (Fig. 1). The ovaries of the plants serve as a protected food source for the females’ offspring, which feed on seeds that develop as a result of the pollinating activity of the female moths. This relationship was first explored by the distinguished entomologist Charles V. Riley1 (Fig. 2) during the early 1870s, about a dozen years after the 1859 publication of the Origin of Species. At the time, Charles Darwin and Alfred Russel Wallace’s revolutionary theory of descent with modification was undergoing vigorous debate, in part because scientists questioned the significance of the role played by natural selection. In addition, most older naturalists (i.e., those of Darwin’s generation) clung to natural theology, which held that organisms were placed on earth for man’s benefit and that their countless and wondrous adaptations evidenced the Creator’s handiwork. In contrast, Darwinian theory, and natural selection in particular, removed all supernatural explanations in accounting for the diversity of life on earth and thus was problematic for many naturalists with strong religious beliefs (Mayr 1982, 1991; Ruse 1982; Bowler 1989; Moore 1993). This transitional phase in the scientific community’s conception of the natural world was articulated by Riley in his earliest publication on the yucca moth: Of late years, and more especially since the publication of Mr. Charles Darwin’s interesting work on the fertilization of Orchids,2 we have come to understand more and more the important part which insects play in the fertilization of plants; and the old idea, that color and perfume in flowers were intended for man’s especial pleasure, is giving way to the more natural and philosophic view that they are useful to the plants by attracting the needed insects. (Riley 1873a, p. 57) 1For biographical treatments of Riley, reputedly the most important entomologist of the 19th century (Sorensen 1995), see Smith and Smith (1996) and Sheppard and Weinzierl (2002). Fig. 1. Illustration by C. V. Riley (1892b) of Tegeticula yuccasella (Riley) ovipositing on Yucca filamentosa. In this rendering, Riley shows a portion of the petals (top) removed to demonstrate that the anthers point away from the stigmatic tube, making self-pollination highly unlikely.