Saturday, June 12, 2010

Yucca Moth

I was excited to find a yucca moth last week on the first blossom I checked.  Examining over 100 blossoms, I couldn't find another moth.  While the picture of the little white moth may not seem impressive, the back story is.

Many years ago, before Google first appeared- Remember research before Google? - I was searching information on yucca.  We had noticed it was usually present in cemeteries.  Having tried unsuccessfully to to get rid of a yucca in our yard, we thought it might have been planted in cemeteries in the past as a symbol of eternity.  I contacted an expert,  John Richard Schrock at Emporia State in Kansas and he told me the story of the yucca moth.
He mentioned that neither the moth or the yucca can survive without each other, a relationship called obligate mutalism.  I have been looking for this tiny white moth on our yucca plants ever since. The story of the relationship is briefly outlined in the NBII Website.
The relationship between the yucca moth (Genus: Tegeticula) and yucca plant (Genus: Yucca) is believed to have begun at least 40 million years ago and is one of the most cited examples of co-evolution. The yucca plant requires pollination by the yucca moth and moth larvae, in turn, require developing yucca seeds for food - a relationship known as an obligate mutualism. This is actually one of the few documented cases of active pollination because the moth purposefully places pollen on the plants' stigmas. The female yucca moth has specially adapted mouth-parts used for pollen handling. The moth drags its tentacles across the yucca's anthers and collects a large amount of pollen, which the moth then forms into a sticky ball and carries it between its tentacles and thorax. This pollen ball is very large - often made of nearly 10,000 grains of pollen - and can constitute up to 10% of the moth's weight. After collecting the pollen ball, the moth flies to a different yucca plant to deposit its eggs. The moth deposits its eggs in the flower's ovary and uses its tentacles to scrape the pollen ball onto the top of the yucca plant's stigma, pollinating the yucca plant. The moth then climbs to the new flower's anthers and collects pollen, and repeats the entire process.
This is just the beginning.  As Schrock describes it in the Kansas School Naturalist:
When the petal fall away, the seed pods develop, a sure sign that the moth had been there.  small gray-to-pinkish yucca moth caterpillars may be feeding anywhere in the core of the seed rows. If they burrow toward the end and run out of food, they bore sideways into another locule or across a carpal wall. Often an outside constriction in the pod reveals their internal consumption of seeds. 
When the larvae are mature, they excavate an exit burrow to the surface of the pod, although they may continue feeding for a time. When they completely chew through the surface, they leave an exit hole or scar.
From here the cycle becomes more complex.  A step by step description including lots of pictures is available at Waynesword
including this picture of a seed capsule of Yucca whipplei in October showing the larva of a yucca moth (Tegeticula maculata) inside its feeding cavity in one of the seed chambers.
The Kansas School Naturalist article describes colorful history of the discovery of this moth-yucca association.   
The unusual role of the yucca moth was discovered in 1876 by a Missouri entomologist named C. V. Riley.   He was a writer for the Prairie Farmer, and later started the magazines American Entomologist and Insect Life. He became State Entomologist of Missouri from 1868 to 1877, and became "Chief in Entomology") in the United States Department of Agriculture in 1878. 
Riley was an avid advocate of evolution and traveled to England to meet Darwin. Darwin was particularly interested in Riley's understanding of insect mimicry, a biological phenomenon only understood in the light of evolution. Riley also corresponded with Alfred Russell Wallace (co-discoverer of evolution by natural selection) and Henry Bates (of Batesian mimicry).
The whole story of the rest of the life cycle, including the history and pictures, is available at this link.  I would highly recommend it as entertaining reading.
Postscript from Richard Schrock:
"I live on Yucca Lane...it was by accident, no calculated move.  And I planted yuccas all along the Lane...people say "you grow yuccas" and I correct them: it is a yucca moth farm, perhaps 2000 head on a good year.   The tough part is keeping the ugly pods on through the summer so  the larvae can develop.  Most people want you to cut them off after they flower and that of course dooms the moths.
This has been a late year, with the male moths emerging before the yuccas were in bloom.


Note: Another obligate species found all over the yucca in early summer is halticotoma valida.

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