Moths and Joshua Trees Work Together

This Research in Action article was provided to LiveScience in partnership with the National Science Foundation.

A Joshua tree (Yucca brevifolia) stands in the Tikaboo Valley of Nevada, the site for a study of a highly specialized collaboration between the trees and the moths that help pollinate them.

In the valley, Yucca moths pollinate Joshua tree flowers even though the flowers contain no nectar. The effort is entirely an adaptation by the moth to help the tree propagate.

Why would the small grey moth want to help Joshua trees reproduce? Because Joshua trees actually return the favor, helping the Yucca moths reproduce. Female moths lay their eggs in immature Joshua tree seeds, which ultimately become a source of food for the moth caterpillars when they emerge.

The relationship is even more specialized by region. There are two varieties of the moth: a slightly larger Yucca moth species that exists in California and central Nevada and a smaller species that dominates southern Nevada and Arizona.

The Joshua trees are similarly different. The covering that protects immature seeds in the Joshua trees pollinated by the larger moths is substantially thicker than the covering on the seeds pollinated by the smaller moths.

Not only do the larger moths prefer the Joshua trees with the thicker seed coverings, they have evolved slightly longer ovipositors to enable them to successfully lay their eggs. Conversely, the smaller moths have shorter ovipositors, enabling them to successfully pollinate the trees that they visit.

Since Charles Darwin, scientists have wondered whether co-evolution encouraged greater diversity for species. The study in Tikaboo Valley seems to suggest that this may be true.

Willamette University biologist Christopher Irwin Smith, working with Olle Pellmyr at the University of Idaho and other colleagues, is studying the relationship between Yucca moths and Joshua trees and the implications for evolution and species diversity. You can read Smith's account of his work in the Behind the Scenes feature Some Trees and Insects Are Made for Each Other.

Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation. See the Research in Action archive.

- Josh Chamot, NSF

Image Credit: Christopher Smith, Willamette University