Species account by Benjamin Felser, of Bowdoin College. Sky Island Borderlands Spring 2021 program.
Yucca moth larvae come into quite a luxuriant world. They emerge from their eggs into a warm, moist cavity stocked with all the yucca seeds they could possibly eat. They gorge themselves until pupation and worm (or caterpillar) their way into the soil until the time is right to emerge as a full-fledged moth. But how does this world of plenty arise for the little pupae?
To begin, the mutually beneficial relationship (a.k.a. mutualism) between these moths and yuccas first arose some 40 million years ago. Yucca (Yucca spp. and Hesperoyucca spp.) are bold plants which grow in rosettes with straight, thick leaves up to 4 ft long. Each leaf is smooth at the edges but ends in a sharp (correction: very sharp) spike to ward off clumsy humans and herbivores fiending for the water stored within them. They largely grow across the semiarid regions of the southwestern United States and across Mexico on slopes above the desert. Most Sonoran yucca stay below 6 feet, and the tallest part of the yucca is the flowering crown.
Their crowns tower 1-4 feet above the plant and flower in spring nights when their pollinating moths are most active. Yucca moth species (Tegeticula spp. and Parategiticula spp.) each evolved with one yucca in mind, causing their life cycles to be completely dependent on yucca flowers. Most yucca moths emerge from their cozy silk cocoons (again, luxury) after the rain and are drawn by smell to their specific yucca. Upon finding a suitable crown, they situate themselves deep into the flower, collecting up to 10% of their body weight in pollen with specialized mouth-tentacles before cutting a hole into the female part of the flower to lay their eggs. They then walk up the stalk of the flower to repeatedly–and I quote from the Sonoran Desert Museum–“slam dunk” the collected pollen into the flower.
It is this pollinator-plant dynamic which Charles Darwin called “the most remarkable example of fertilization ever published” in 1877. But increasing water scarcity threatens the yucca-moth mutualism as flowering is water-dependent. To remain key food sources for rodents, javelina various insects and more, cattle ranchers and urban centers have to be more conscious about water use; otherwise, moths may emerge before yuccas flower and the desert will lose its quintessential duo.
Cover photo credit: “Yucca Elata–Soaptree Yucca” from Southeastern Arizona Wildflowers and Plants.