Every month we showcase a relationship between two or more species or groups of species that can be considered a mutualism: a relationship where both members benefit. This month’s mutualism is between a genus of shrub (Yucca) and several specialized moths.
The mutualism between two genera of yucca moths and Yucca shrubs is considered to be one of the most apparent cases of coevolution between plants and insects. Coevolution is where two species reciprocally affect each other’s evolution (see my recent post on evolutionary arms races).
Yucca are a genus of perennial shrubs within the same family as asparagus and agave. Yucca are notable for their rosettes of tough, sword-shaped leaves and branched clusters of pale white flowers. Yucca are native to arid (hot and dry) parts of North and South America.
Yucca moths are two genera of small moths which inhabit the same regions. Yucca moths depend solely on Yucca shrubs for their food. Each spring, male and female yucca moths emerge from the soil out of their underground cocoons and fly to a nearby Yucca. The moths may have been in their cocoon for up to three years and yet time their emergence perfectly with the Yucca’s blooming! After mating, female yucca moths find the flowers and collect pollen from the stamens (male flower organs). They then fly to another Yucca and deposit the pollen into the stigma (female flower organ), fertilizing the ovules (plant eggs) which will develop into seeds. This is a remarkable adaptation; few insects “voluntarily” pollinate flowers, rather they pollinate by transferring pollen that incidentally attaches to their bodies while seeking nectar or some other reward.
After transferring pollen from one shrub to another, a female yucca moth cuts into the ovary of the flower and oviposits (lays) an egg. She may cut open and oviposit eggs multiple times into the flower before she moves on, which can build up scar tissue stressing the plant. Once the moth larvae hatch, they feed on the developing seeds fertilized by their mother. The larvae consume only a small proportion of the seeds, leaving many seeds intact so the plant can still reproduce.
Reciprocal specialization of the yucca moths and the Yucca has led to this relationship becoming obligate in nature; neither species can successfully reproduce without the other. No other insects pollinate Yucca. No other flowers host yucca moths.
This dependence on one another has led to a highly stable relationship. If the moths were to attempt to take advantage and lay too many eggs in each flower, seed production would decrease, reducing the amount of new Yucca plants. This in turn, would reduce the number of hosts for the moths. If the Yucca dropped their infected flowers and spent more energy on the non-infected flowers, there would be less moths and therefore less pollinators.
That isn’t to say that they don’t try to cheat one another. The Yucca have a great strategy for keeping the moths in check. Losing these seeds to moth larvae is a major cost for the plant. When yucca moths lay more than 14 eggs in one flower, the flower is selectively dropped by the plant before the larvae hatch. Not only does this reduce the cost to the plant that would have ensued from feeding too many moth larvae, but it also removes the young of moths that are too greedy. This selects for yucca moths that lay less eggs per flower, keeping the plant-moth balance in check.
For a goofy, but helpful visualization of the relationship check out this video below. After 2:00 the video goes off topic so feel free to skip that part. I would like to point out that yucca moths do not hold pollen underneath their chin, rather they have a special set of mouth appendages which collect, form, and carry balls of pollen.
If you’d like to read more about mutualisms check out the posts below! Stay hungry!