REPRODUCTIVE CONSEQUENCES OF INTERACTIONS BETWEEN YUCCA-GLAUCA (AGAVACEAE) AND TEGETICULA YUCCASELLA (LEPIDOPTERA) IN COLORADO

For over 100 years the association between Yucca (Agavaceae) and Teget icula (Lepidoptera: Incurvariidae) has been accepted as a quintessenti al example of an obligate mutualism. The yucca moth is purported to be the sole pollinator of Yucca, while Yucca flowers provide courtship a nd mating arenas, and Yucca seeds provide food for developing Tegeticu la larvae. We studied Yucca glauca in northern Colorado, comparing the reproductive ecology and breeding systems of Yucca in plains populati ons, the ''preferred'' habitat of Yucca, with ''marginal'' sites at th e edge of the local elevational distribution. Tegeticula are abundant at plains sites, and fruit set is significantly higher than in the foo thills, where fruit set is limited by the paucity of moths. The low fr equency of moths at high elevation, coupled with behaviors of adult fe male Tegeticula which lead to self-pollination, failure to pollinate, and periodic overloading of fruits with eggs, may help explain why Yuc ca glauca appears to maintain alternative pollinators. Some fruits lac k evidence of Tegeticula infestation, suggesting that larvae die befor e completing development, or that fertilization of Yucca sometimes occ urs without the intervention of Tegeticula. Biochemical analyses of ne ctar and observations of floral visitors revealed that it is highly li kely that the fly Pseudocalliope sp. nov. (Lauxaniidae), which congreg ates and mates on Yucca glauca blossoms, acts as a secondary pollinato r. Autogamy appears to occur infrequently in natural populations. We t herefore propose that the yucca-yucca moth symbiosis be viewed as a fa cultative mutualism.