Rebecca Snell

University of Toronto, Faculty of Forestry


Summary of Master's thesis

This project considered the moth/yucca mutualism in a broader community context.

The relationship between yucca plants and yucca moths is one of the classic examples of an obligate pollination/seed predation mutualism. Female yucca moths are the sole pollinators of yuccas, however they also lay their eggs into the flowers, where their offspring eat a proportion of the developing seeds.

My thesis project examined how yuccas interact with three species of moths (yucca moths, cheater moths and Prodoxus moths) and how ants and aphids modify these interactions.

Yucca glauca



Formica ravida tending aphids on yucca bud


Female Prodoxus moth ovipositing into yucca flowering stalk


Ants, which are commonly found foraging upon yucca buds, fruit and aphid honeydew, may directly affect yuccas by consuming yucca buds/flowers/fruit or indirectly by disturbing one of the three species of moths. Cheater moths arrive later in the season, after pollination has already occurred. They lay their eggs directly into the developing fruit where their offspring also consume yucca seeds. Prodoxus moths also don’t pollinate yucca flowers, but lay their eggs into the green flowering stalk where their larvae consume stem tissue. Yuccas may benefit if ants lower herbivory from cheaters and Prodoxus. However, ants could have negative effects by eating buds and flowers, or by preventing yucca moths from pollinating.

The net result of ants was positive for yuccas. Ant-damaged buds had no impact on fruit production as yuccas abscise 90% of their flowers anyways. Ants had a small negative effect on pollinators, however this was filtered out by yuccas (via selective abscission of the flowers). Ants protected yuccas by reducing seed predation from cheaters and herbivory from Prodoxus. Aphids served to attract ants and indirectly protect the fruit from ant herbivory. When aphids were absent, ants were more likely to eat the fruit and damage seeds. The benefits of ants and aphids far outweighed their potential costs, and by reducing cheater larvae in the fruit, ants indirectly caused a 25% increase in the number of viable seeds per fruit.

The ferocious Formica ravida ant on a yucca fruit